Dr. Fred Ortenberg
OZONE: SPACE VISION (Space monitoring of Earth Atmospheric Ozone)
Haifa, 2002
Ozone: Space Vision (Space monitoring of Earth Atmospheric Ozone)
Copyright Š 2002 By Dr. Fred Ortenberg, ASRI, Technion November 2002
Printed in Israel Design by Graphic Touch, Ltd (Haifa)
Asher Space Research Institute, Technion Technion City, Haifa, 32000, Israel http://www.technion.ac.il/ASRI
FOREWORD “Space Monitoring of Earth Atmospheric Ozone” was written as part of the activities of the Asher Space Research Institute (ASRI), Technion, Israel Institute of Technology, intended for public education. It grew as a result of ASRI involvement in the development of an ozone-meter on board the Gurwin-Techsat Technion satellite, launched in July 1998 and still active in space.
The main mission of the ASRI is to advance science, technology and education in all space-related fields. The ASRI operates with a broad national perspective. It fosters interdisciplinary work and collaboration between Israeli researchers from all universities and agencies as well as industry. The ASRI has also established collaborative projects with other countries.
The ASRI was established in 1984. Its members are professors in several Technion faculties, and it has a technical staff of scientific experts in a variety of space-related fields. ASRI is the leading space research center in Israel and is also involved in the development of space systems based on advanced and innovative technologies.
As part of its activities, ASRI developed a technology demonstration micro satellite called Gurwin-Techsat. This project covered the conception, design, development, construction, test, launch and operation of a low Earth orbit satellite carrying a number of experimental scientific payloads. The purpose of this program was to demonstrate that smaller, high technology satellites could be rapidly developed and implemented for a fraction of the cost of heavier and more complicated satellites, and still be able to perform valuable space research. This project was based on a unique collaboration between Israel’s leading industrial organizations and ASRI.
Being designed as a multi-purpose platform suitable for space research, GurwinTechsat was equipped with six different devices to carry out on-board scientific experiments. All of the experiments have been repeatedly run during more than four years of satellite flight history. Of these six payload devices, the operation of the ozone-meter was one of the most successful. The ozone-meter was intended for
measuring atmospheric ozone concentration. The ozone-meter flight test has enhanced the prospect of developing reliable, low-cost instruments for small satellites, necessary for carrying out such vital tasks as continuous ozone monitoring.
The study of ozone is important because of the effects of UV radiation on humans (e.g., skin cancer, eye cataracts, and deterioration of the immune system) and on agricultural systems (e.g., slower plant growth and reduced crop yield). Each spring, when the Sun rises over the Antarctic, chemical reactions involving man-made chlorine and bromine compounds occur in the stratosphere and destroy ozone, causing the "ozone hole". Total recovery of the ozone layer to levels observed before 1980 will take at least 50 years, and expected changes in climate, including a cooler stratosphere, could delay this process.
Here is the story: The World Meteorological Organization in Scientific Assessment of Ozone Depletion published in 1998 a truly "global" document, reflecting the thinking of the international scientific community. This document discussed the general questions most frequently asked by students, the general public, and leaders in industry and government: How can chlorofluorocarbons (CFCs) get to the stratosphere if they're heavier than air? What is the evidence that stratospheric ozone is destroyed by chlorine and bromine? Does most of the chlorine in the stratosphere come from human or natural sources? Can natural changes such as the Sun's output and volcanic eruptions be responsible for the observed changes in ozone? When did the Antarctic ozone hole first appear? Why has an ozone hole appeared over Antarctica when CFCs and Halons are released mainly in the Northern Hemisphere? Is there an ozone hole over the Arctic? Is the depletion of the ozone layer leading to an increase in ground-level ultraviolet radiation? Does ozone depletion cause climate change? How severe is the ozone depletion now?
Is the ozone layer expected to recover? If so, when?
In this book you can find answers to all the above questions. This book focuses on the observation of the stratospheric ozone layer from space, and humanity's attempts to protect it. A study of ozone variations in the Earth’s atmosphere is a very complicated dynamic phenomenon. Methods of ozone monitoring from space are also very complex. This book tries to present this very important and multifaceted technical subject as simply as possible, with minimal formulae and without recourse to complicated mathematics.
Funding of ASRI’s research and educational activities is partly met by different national and international organizations, but it is the Asher family’s support that provides the principal source of ASRI’s financial stability, including the publication of this book. We wish to express our thanks to the Asher family whose commitment has enabled the advancement of space research in Israel on a firm basis.
Professor Moshe Guelman Head, Asher Space Research Institute, Technion, Israel Institute of Technology Haifa, September 2002
Content
Introduction ...............................................................................................5 1. General Properties of Ozone .............................................................7 2. Optical Properties of Ozone ........................................................... 12 3. Ozone in the Atmosphere ................................................................ 16 4. Ozone Control, Physical Concepts and Methods ......................... 23 5. Solar Backscattering Ultraviolet Method ..................................... 31 6. The Emission Method ...................................................................... 39 7. Limb Methods .................................................................................. 44 8. Lidar Sounding of Atmospheric Ozone ......................................... 51 9. Monitoring Instruments Present State and Trends ..................... 53 10. The Antarctic Ozone Anomaly ....................................................... 62 11. Conclusion ........................................................................................ 73 12. A Certain Philosophical Epilogue .................................................. 75 13. Timeline of Atmosphere Ozone History ........................................ 78 Ozone Devoted References and Internet Web Sates .......................... 80 Glossary of Ozone and Space related terms ....................................... 81
Introduction The outcome of ozone depletion is one of the disasters facing humanity, alongside such threats as drastic changes in the world climate, exhaustion of soil and water resources, progressing deforestization and the uncontrolled expansion of deserts. Possibly, the destruction of the ozone layer over the Antarctic is a precursor to rigorous changes of the ozonosphere on a global scale.
Scientists, worldwide, are involved in studies of the complex processes going on in atmospheric ozone. Theoretical and experimental research of the ozonosphere is being conducted on an unprecedented scale. However, the problem is far from being unravelled; many important happenings related to the enigma are yet waiting to be resolved, especially those concerning the impact of natural and anthropogenic issues on the precious ozone layer. Consistent monitoring of the environment on a broad basis is an essential requisite for exhaustive analysis and far-reaching conclusions. Scientific research leading to credible forecasts of changes in the Earth's ozonosphere, both on a global and local scales, necessitates regular measurements of ozone concentrations and other characteristics with the help of existing devices, as well as new and emerging methods and means of ozone observation.
Atmospheric ozone is a product of dynamic equilibrium of numerous processes going on both in the atmosphere and beyond; that is why optical methods for remote sensing in the different layers of the atmosphere are becoming increasingly effective. Space based systems for the control of the ozone layer in the Earth's atmosphere have been developed and Research and Development in this area is continuing in many countries. The main objective of space monitoring is the quantitative determination of ozone in specified layers of the ozonosphere on a global scale, providing periodicity, precision and special resolution unattainable by other means.
How are space-based ozonometric devices equipped and arranged? What space methods are used for measuring ozone concentrations and what are their advantages over other methods? What perspectives are envisioned in the improvement of space-based apparatus?
What impact on the whole endeavour has been brought about by satellite data acquisition and analysis? How has our vision of the Earth's atmosphere changed as a result of ozone space research program implementation? What results have been obtained in this area during the last few years? How are Israeli scientists involved in operative ozonosphere monitoring?
This is the range of questions that will be answered in this book. However, before addressing sophisticated problems of global ozone monitoring with the help of space apparatus we will introduce some basic conceptions of physics, chemistry, meteorology and atmospheric optics, essential to the understanding of the phenomena. In the first chapters of the book, chemical and physical properties of ozone and basics of ozone spectroscopy will be discussed. After that, we will characterize our knowledge of the ozonosphere and relevant methods of research.
This interdisciplinary book has been written for students of the aerospace faculty, physics, chemistry, electrical and civil engineering as well as for readers interested in problem of ecology, space exploration, and science progress. The publication is intended for public education, its main purpose is raising the level of public awareness of issues dealing with global environmental changes. It includes an advanced insight into the Earth's ozonosphere. Satellite research methods of atmospheric ozone are outlined; basic principles and architecture of remote probing apparatus are described. This publication could become a basis for designing a course of lectures for students of different faculties associated with state-of-the-art achievements in space research and related high-tech disciplines.
First of all, the author would like to appreciate the continual and valuable support of Prof. M. Guelman during the preparation of this book. The author also would like to thank Drs. A. Shiryaev, A. Livne and O. Tublin for their assistance with the editing, proofreading and formatting of the book, as well as A. Volfovsky, B. Kimelman and D. Rosenberg for the graphics.
1. General Properties of Ozone The principal player in our story is ozone. A molecule of ozone O3 is a relatively stable combination comprising three atoms of oxygen. It is well known that the specific smell of air after a thunderstorm is explained by the presence of ozone in the atmosphere. This peculiar smell has been mentioned in many works of literature, beginning from ancient Iliad. The German chemist Christian Frederick Shenbein who is said to have discovered ozone in 1840 coined the new substance "ozone" (ozonesmelling). Later it was shown that ozone is a modification of oxygen, and that the ozone molecule is actually a trivalent atom (Fig. 1).
Fig. 1. Molecule Ozone Structure At room temperature ozone is a gas of a light-blue color; at lower temperatures it transforms into a liquid of indigo-light blue color, and has a boiling point of 119.9°C; in the solid state, ozone forms needle-like crystals of O3 and molecular oxygen O2; to coexist in all three states of matter is one of its exceptional features. Pure ozone in all these three states is of an explosive nature.
A molecule of ozone is nonlinear and has a triangular structure with an obtuse angle at the apex and equal inter-nuclei distances (Fig. 1). The process of ozone formation from oxygen can be depicted in the following way: Exothermic reaction → 2 O3 = 3 O2 + 68 Kcal ←Endothermic reaction The formation of ozone is accompanied by heat absorption, whereas decomposition is associated with release of heat. At normal temperature and pressure the reaction
proceeds very slowly. This is due to the important role played by atomic oxygen in the reaction: O2 + O + M = O3 + M,
(1)
where M is any particle necessary for the removal of energy from the molecule of ozone being formed.
At high temperatures ozone disintegrates, the equilibrium of the reaction shifts to the left in the direction of high concentration atomic oxygen formation; at low temperatures the equilibrium shifts to the right in the direction of ozone formation; however, atomic oxygen concentration at such temperatures is low, and, therefore, again, there is no output of ozone. This is why the most favorable conditions for the formation of ozone are relatively low temperatures and the presence of an additional quantity of unstable atomic oxygen. The source of such oxygen could be the dissociation of oxygen molecules due to the impact of a particle stream, electromagnetic radiation, electric discharge, etc. Essentially these principles underlie the operation of ozonizer, used to produce ozone for practical purposes. For example, the electrosynthesis of ozone in a barrier discharge is based on the dissociation of oxygen molecules under the influence of electric energy discharge in an electric gap. Atomic oxygen formed in the course of such dissociation, combines with an oxygen molecule in the presence of any particle (oxygen, nitrogen) and is converted into ozone, which, in its turn, reacts with oxygen atoms and is converted into molecular oxygen. This establishes mobile equilibrium in the formation and disintegration of ozone, limiting the yield of ozone in such ozonizers to 5-7%. We should note that this description of ozone synthesis is of a schematic character. In reality, obtaining ozone is accompanied by a number of additional chemical processes, and depends on such factors as temperature, humidity, supply rate of oxygen or air, as well as specific features of the apparatus being used.
A molecule of ozone is essentially stable, i.e. it does not dissociate on its own. Low concentration ozone, void of impurities, dissociates relatively slowly. However, as temperature rises and the quantity of admixed gases increases (e.g. NO, Cl2, Br2, I2 and others), under the influence of radiation and particle flow, ozone dissociation speed substantially increases. Thus ozone is unstable at the presence of admixed
gases, and this is one of its main properties. It is also a very strong oxidant (second to F). Its high activity as an oxidizer and its capacity to react with many substances greatly increases its applicability. In addition, it has a number of beneficial properties as a disinfectant and odorizing agent. The following applications of ozone are well established; purification and disinfections of drinking water, industrial water, and drainage, decolarization, neutralization of harmful and toxic substances, eliminating unpleasant odors, cleansing industrial exhausts, ozonizing in air conditioners, processing and storing foodstuffs and forage, sterilizing bandaging materials, as well as in therapy and disease prophylaxis.
Ozone in the atmosphere is generated mainly by processes accompanying the absorption of light: the photochemical reaction bringing about the formation of ozone consists of a sequence of happenings, beginning with the absorption of light by a molecule of oxygen and ending with the formation of stable molecules. This comprises primary and secondary events. The primary events include the initial act of light absorption by a molecule, bringing it to a state of excitation, followed by its destruction; and thus the end products of the primary events are two atoms of oxygen. Atoms and molecules are known to exist only in specific energetic states determined by laws of quantum mechanics. Thus, an atom of oxygen can exist in states designated 3P, 1D, 1S, where O(3P) is the normal state of the atom, whereas O(1D) and O(1S) are the excited states. The energy bonding atoms in an oxygen molecule comprises 5.115 eV. To "cleave" an oxygen molecule, a light quantum is necessary which must have energy equal to the bonding energy of atoms in a molecule. While absorbing such a quantum, an oxygen molecule dissociates into two normal atoms. Under the influence of light having a lower wavelength (respectively larger quantum energy) the molecule O2 will be dissociated into excited oxygen atoms. The threshold wavelengths of the radiation absorbed in the course of molecular oxygen dissociation, can be presented as: O2→ O(3P) + O(3P) - 242.4 nm, O2→ O(3P) + O(1D) - 175.0 nm, O2→ O(3P) + O(1S) - 133.2 nm.
Thus, irradiation of gaseous oxygen by ultraviolet radiation, can generate substantial quantities of highly concentrated atomic oxygen. In addition to the appearance of oxygen atoms, as a result of irradiation, there also emerge excited molecules of oxygen. All these active particles take part in secondary events (analogous to reactions described by formula (1) producing the end product-ozone). The principle described is effected in photochemical ozonators, in which the dissociation of oxygen is brought about by ultraviolet radiation, generated by a special discharge lamp. Due to the reversible character of reaction (1), formation of ozone runs alongside with its destruction. Ozone concentration reached in such ozonizers, does not exceed 1-2% in volume. We should remind the reader that the amount of ozone in the atmosphere is extremely low ("traces" in volumetric estimates 10-6-10-5 %).
In this book we will frequently deal with values showing ozone concentration in mixtures of gases. In addition to the volumetric percentage mentioned above, atmospheric chemistry employs the term volumetric concentration expressed as the number of parts per million (ppm), which is equivalent to 1cm3 of ozone in 1m3 of air. The partial pressure of a gas depends on the fraction of this gas in a mixture of gases. Thus, on the Earth's surface where the pressure is 1 ATM, an ozone concentration of 1ppm corresponds to a pressure of 10-6 ATM. The usage of volumetric units is really handy because there is a direct connection between the volume (or partial pressure of ozone) and the number of its molecules; for instance, if the amount of ozone in the air constitutes 1ppm, then, on an average, every millionth molecule would be a molecule of ozone. When the amount of gas in the air is minute, it is appropriate to use the number of molecules in a unit of volume for defining concentration. In normal conditions the number of molecules in 1cm3 of air is 2.69 . 1019. An ozone concentration equal to 1ppm would be equivalent to 2.69 . 10
13
cm-3. Sometimes
ozone concentration is described in units of mass (micrograms) per unit of air volume (usually, microgram/ m3). For translating millionths of a part into ozone density, one can use the following relationship: 1ppm = 2140 Âľg/m3. The term "ozone density" is also used to denote the thickness of the ozone layer contained in a 1km-thick layer of the atmosphere referred to normal pressure and temperature.
In addition to the above-mentioned unit, the term "total column amount" is widely used in metrology. Total ozone X denotes the amount of ozone in a vertical column of the atmosphere; it is numerically equal to the thickness of the ozone-gas layer in this column in normal conditions, and is expressed in ATMâ‹…cm. The value X=10-3 ATMâ‹…cm. is often called the Dobson unit after the English scientist who conducted research in the field of atmospheric ozone and in 1924 created one of the first optical devices for measuring Total Ozone (TO). At this point it is appropriate to emphasize the very small content of ozone in the atmosphere. If all Earth's atmospheric ozone were concentrated in a single layer on the Earth's surface, the thickness of such a layer of pure ozone would be only 3mm. At the same time, the total ozone mass in the atmosphere is 3.109 ton.
Having revealed some basic properties of such an exotic substance as ozone, we will mention its toxic impact on humans, animals and vegetation. Not going into details, we will just point out that at certain concentrations, ozone is capable of poisoning a human being, causing death of experimental mice, birds; it has a destructive impact on forests and plants even at low concentrations observed in natural surroundings. This "bad ozone" is a powerful photochemical oxidant that damages rubber, plastic, and all plant and animal life. It also reacts with hydrocarbons from automobile exhaust and evaporated gasoline to form secondary organic pollutant such as aldehydes and ketones. Nevertheless, the term ozonizing is often referred to as a beneficial process for purification or refreshing.
2. Optical Properties of Ozone The energy of a molecule can be depicted as the sum of three parts - electronic, vibrational and rotational energy. It is well known that energy states change in a discrete manner. Sets of energetic states are of a specific (individual) nature for every molecule. Transitions of molecules from a certain energy state to another are accompanied by the absorption or radiation of a quantum of electromagnetic energy. Spectra, originating with such transitions, depend on the molecular constants of the radiating or absorbing molecule and are a sort of visiting card ("signature") of the specific molecule. In ozone, transitions from one electronic state to another occur on instants of radiation or absorption of light in the visible, ultraviolet (UV) and so-called vacuum UV (lower than 200.0nm) ranges of the spectrum. Every electronic transition is accompanied by relatively small changes in the energy of vibrational-rotational states of the molecule; due to this, the electronic, vibrational, and rotational spectrum of the molecule is essentially a system of bands closely spaced out. If a molecule, as a result of light absorption, reaches an excited state, possessing adequate energy to destroy the weak link in a molecule, the latter will dissociate. In an ozone molecule the energy of the (O-O2) bond is equal to 1.05 eV; the rupture of this bond leads to the disintegration of ozone into molecular and atomic oxygen. The most important ozone absorption bands are located in the 200.0-300.0 nm wavelength range (Fig. 2). The capacity of a gas to absorb light is characterized quantitatively by the coefficient of absorption k(ν) in the Lambert - Beer law I(ν, x) = I (ν, 0) • 10 - k(ν) x
(2)
Here, I (ν, 0) is the intensity of a monochromatic beam, of a frequency ν, entering the window of a vessel of length x, filled with gas at a given pressure; I(ν, x)- the intensity of light after passing through the gas in the vessel. Sometimes the base e of a natural logarithm is used instead of the figure 10 in (2). The value of the coefficient of absorption k(ν) measured in cm-1 in Hartley's bands of absorption for ozone is calculated according to formula (3); this is shown in Fig. 2 as a dependence of the coefficient k(ν) on the incident radiation wavelength. Just as many other bands of absorption in molecular spectroscopy, these bands are named after scientists who
discovered them. From formula (2) we can see that in the point of maximum absorption k=135 cm-1, at a thickness of the ozone layer equal to 0.3cm, the factor I(ν, 0)/I(ν, x) would be equal to 10.40 ! This means that the Earth's layer of ozone will weaken the incident radiation on this wavelength 1040 times; i.e. it will absorb practically all of it.
Fig. 2. Absorption in Ozone bands At wavelengths longer than 300.0 nm adjacent to Hartley's bands, weaker absorption bands of Haggins and Shalon-Lefevr can also be observed (Fig. 2). Coefficients of absorption in these bands are several orders of magnitude lower than in Hartley's bands. Some closely spaced bands in these systems have easily discernible sharp maximums and minimums. Further, in the visible part of the spectrum, there is Shappuis'es broadband, with which the blue color of ozone is associated. Very strong absorption of ozone can also be observed in the range of vacuum ultraviolet (100.0200.0 nm). Together with the absorption in Hartley's bands, this absorption brings to an abrupt end the Sun's spectrum on the Earth's surface at wavelengths less than 290.0 nm; this is very important in terms of protecting life on our planet from short wave irradiation. It should be noted that values of coefficients of absorption change substantially with temperature (the coefficients of absorption referred to in the graphs were measured at 0°C).
The bands relevant to the vibrational-rotational transitions in an ozone molecule are located in the infrared part of the spectrum (3-15µm). Absorption coefficients in these
bands vary greatly. There are several bands of high absorption (4.75; 9.57 and 14.2µm) among which, the λ=9.57µm band is the most interesting one, consisting of a number of closely spaced spectral lines. This band is situated in a "window" free of absorption by water vapor and carbon dioxide and, therefore, its role in atmospheric dynamics is exceptionally important. Furthermore, rotational ozone spectra, just as in most cases of nonlinear polyatomic molecules, are observed in the microwave spectral band (1-10cm).
Such a detailed description of the absorption spectra is required for the two following reasons: first of all, investigation of absorption spectra not only allows the identification of a specific gas in a mixture of gases, but also the means for measuring quantitative values. That is why this method is widely used in ozonometry and will be described in the following sections; the second reason is that when light is absorbed by ozone, there ensues a chemical transformation, and chemical changes can be brought about only by light absorbed by a molecule.
The initial stage of photochemical reaction is molecule dissociation. The end products of the photochemical reaction can differ with the absorption bands in which photodissociation takes place. Specifically, when ozone is disintegrated by light into molecular and atomic oxygen, an atom and molecule of oxygen can be found to be both in ground and excited states, depending on the energy of the absorbed quantum (wavelength of the absorbed light). We have already mentioned quantum states of an oxygen atom 3P, 1D, 1S. Analogous symbols are used in molecular spectroscopy to designate electronic energy states of an oxygen molecule in the ground state (3Σ-g) and excited states (1∆g, 1Σ+g, 3Σ+u, 3Σ-u). To understand the secondary processes occurring after the decomposition of ozone into an atom and molecule of oxygen, it is very important to know in what energy states they appear as a result of the initial absorption of light.
The reactive capacity of atoms and molecules in an excited state is substantially different from their reactive capacity in the ground state. As an example, we will consider the process of photodissociation in the excited state relevant to the Shappui
bands. When red light (~600.0nm) is absorbed, the initial process ends up in the decomposition of ozone into an atom and molecule of ozone in ground states: O3 → O(3P) + O2 (3Σ) A secondary process of atomic oxygen and ozone interaction follows this O(3P) + O3 → 2 O2 Consequently, the absorption of one light quantum led to the destruction of two ozone molecules. In this case, it is said that in the Shappui bands, the quantum yield of ozone (O3) disintegration is equal to 2. One of the products of photo-disintegration, as a result of excitation in the Haggins-bands, is an excited oxygen molecule, e.g. O2(1Σ): O3 → O(3P) + O2 (1Σ), followed by the processes O(3P) + O3 → 2 O2 O2 (1Σ) + O3 → 2 O2 + O(3P) O(3P) + O3 → 2 O2 In accordance with this mechanism, the total yield of O3-disintegration is equal to 4. In the same way, it can be shown that the quantum yield of ozone - photo dissociation in the case of excitation in Hartley's bands can be equal to 6.
Two important conclusions can be made on the basis of the above-mentioned interaction of light and ozone: 1) molecules of ozone absorb light in a broad band from vacuum ultraviolet up to the microwave frequencies, the most intensive absorption taking place on wavelengths shorter than 300.0nm; 2) when ozone absorbs light in either the ultraviolet or the visible light bands of the spectrum, a molecule of oxygen is created as a result of the photochemical destruction of ozone.
3. Ozone in the Atmosphere Due to its peculiar properties, atmospheric ozone is a regulator of radiated energy flow reaching the Earth's surface. The evolution of ozone on Earth can be described in the following way. Conversion of methane, water and ammonia in the initial atmosphere of the Earth into a "broth" of organic compounds, in which life originated, occurred in the presence of intensive ultraviolet radiation. However, the latter is very dangerous to the sensitive equilibrium of chemical reactions in live cells, and probably, the first organisms survived only because they evolved under a layer of water, thick enough to protect them from ultraviolet light. As a result of photosynthetic disintegration of the water molecule, the Earth's atmosphere obtained its free oxygen. Only after the advent of oxygen and ozone in succession, did the intensity of ultraviolet radiation at the Earth's surface decrease to a level, allowing live organisms to abandon the protection provided by water, and to begin settling on land. Prolonged existence of life on land became possible, thanks to the ozone layer, a guardian, which in itself, was a product of life.
Fig. 3. Vertical structure of Earth atmosphere and ozonosphere In the process of evolution, oxygen and nitrogen became the basic components of the atmosphere. Fig. 3 depicts the Earth's atmosphere profile. One can see that pressure
diminishes with height smoothly and monotonously, and does not display any specific structure. At the same time, interaction of radiation with atmospheric matter brings about the development of a substantially complex thermal configuration. In terms of temperature changes with height, the atmosphere can be considered as divided into several layers. The area of the atmosphere adjacent to the Earth's surface (the troposphere) is characterized by a lowering of the air's temperature with height equal to 6.5째C/km. In the next layer (the stratosphere) the temperature somewhat increases (approximately by 1째C per km) due to the absorption by ozone of ultraviolet (UV) solar radiation. In the mesosphere, the temperature consistently decreases with height (2-3째C/km). Higher up stretches the thermosphere, in which air temperature again increases with height, due to the absorption of the short-wave UV solar radiation by molecular oxygen, accompanied by dissociation of the latter. The boundaries between the mentioned layers are called tropo-, strato- and mezo-pauses. Proportions of gas components in the atmosphere are different in different layers. The mean dependence of partial ozone pressure on the height above the tropics is depicted in Fig. 3 (bold line). From the graph we can see that ozone distribution looks like a two-layered pie, corresponding to two layers of the atmosphere. The concentration of ozone in the troposphere is low (1-4mPa) and its spread with altitude increases relatively smoothly, but in the stratosphere it grows sharply, reaching a peak value, and then rapidly decreasing. Usually, when the height of the ozone layer is mentioned, it refers to the zone of its maximum concentration. The height of the ozone layer depends on the latitude of the locality and of the season. The limiting positions of ozone layer on the Equator and the Pole, and the seasonal layer maximal and minimal positions from summer to winter are shown in Fig. 3 by two lower dotted lines.
Here, one can also see the height of ozone concentration maximum in the polar region. Characteristic of the vertical Ozone Profile (OP) in the atmosphere is its instability with time. Because the build-up of ozone proceeds mainly as a result of photochemical reactions in the stratosphere, the mass of ozone is concentrated in the latter (about 85-89% of the Total Ozone (TO) in the atmosphere). The layer of enhanced concentration in the stratosphere serves as a shield, preventing the ultraviolet spectral "wing" of the Sun's radiation to reach the Earth's surface. TO content in a column of atmospheric air varies greatly with latitude. Thus, e.g., the TO
content above the Earth's polar regions is approximately twice as large as over the equator. In addition, TO undergoes daily, seasonal, perennial and long-term variations; the latter are associated with the cyclic nature of solar activity. The thickness of the ozone layer, observed in nature, varies substantially: from 70 to 760 Dobson Units. Variations of ozone concentration round the clock could amount to 25% of its average value. Both TO and OP are determined by the concentration of ozone in each layer of the atmosphere and depend on the process intensity of build-up and destruction.
The above described mechanism of ozone formation and destruction was based on the assumption that the process is initiated via the absorption of UV radiation by oxygen, whereas ozone destruction occurs under the influence of sunlight in the visible and UV spectral ranges, as a result of collisions with oxygen atoms. Computation for such an (oxygen) atmosphere showed vertical profiles much the same as those in reality, although actual concentration proved to be substantially higher. It was revealed that in the stratosphere approximately 80% of the ozone build-up in sunlight is eventually destroyed through mechanisms, taking into account interaction of ozone with many of the "small" atmospheric components. Hundreds of reactions between ozone and the other gases- components of the atmosphere were investigated; it was shown that a major role in the destruction of ozone (in the catalytic cycle) is played by nitrogen oxide: NO + O3 → NO2 + O2, NO2 + O → NO + O2. Such reactions can be displayed for other substances such as clorine: Cl + O3 → ClO + O2 ClO + O → Cl + O2 The combined effect of each pair of reactions brings about the disappearance of ozone and atomic oxygen, whereas nitrogen oxide and atomic oxygen are consistently reduced, i.e. each molecule or atom of these substances is responsible for the destruction of a large number of ozone molecules. When the above mentioned reactions are included in the ozone layer model, the calculated values become
substantially more realistic. Now we understand why implantation of contaminating substances into the stratosphere is considered to be so significant. Actually, a single molecule of a contaminating substance can initiate a sequence of reactions, bringing about the dissipation of many ozone molecules.
At this point it seems appropriate to answer the question: in essence, why is the ozone shield so vital? It is well known that ultraviolet radiation in small doses increases the generation of vitamin D in humans and animals, thereby enhancing assimilation of phosphorus, and bone formation. Medical and prophylactic effects of ultraviolet radiation are well known. Life on Earth has adjusted itself to solar radiation, transparent to ozone (~290.0nm), and is very sensitive to shorter wavelengths. The depletion of the ozone layer leads to an increase of UV radiation reaching the Earth's surface, and a change in the vertical profile of ozone alters atmosphere heating and, consequently, the climate. UV radiation decomposes the chromatin of the cell nucleus and deters cell reproduction; it also damages the DNA molecule that contains genetic code. Superfluous UV, associated with ozone layer depletion can bring about growth of skin cancer and could decrease the effectiveness of the human immune system. Even a small decrease in the total thickness of the ozone shield (e.g. for inhabitants of mountainous areas) would substantially increase the probability of these diseases; a two-fold thickness decrease would be threatening to the Earth's genofund.
Higher doses of UV radiation have a direct impact on the health of people and lead to the increase of infectious diseases, impairment of eyes (e.g. cataract), probability increase of skin cancer. UV radiation limits the growth of some plants; larger doses of radiation can also prompt the lowering of agricultural output. UV radiation also has a negative impact on water organisms, specifically on phyto- and zooplankton, fish roe. Radiation also affects non-biological objects: it causes destruction of many sorts of plastic materials, enhances dangers in the aftermath of air pollution in cities and industrial regions, etc. At the same time, an increase of ozone concentration has been distinctly observed at heights of up to 10 km, especially above industrial locations of Europe and the USA. We have already mentioned the toxicological impact of increased ozone content on the biosphere. A negative effect of ozone on humans has been observed even in the stratosphere, during flights of modern aircraft. At specific
altitudes, concentrated ozone, detrimental to the health of the crew and passengers, is pumped into the cabin together with stratospheric air.
To assess possible scales of ozonosphere distortion, we will consider the sources from which substances, catalyzing the destruction of ozone, enter the atmosphere. The main source of nitrogen oxide (NO) is N2O, which forms in the course of bacterial processes on the Earth's surface. Gradually penetrating the stratosphere, nitrogen dioxide reacts with atomic oxygen (which appears during ozone photolysis, or, even higher - during molecular oxygen photolysis) with the formation of NO. Another direct source of nitrogen oxides in the stratosphere is high-flying aircraft. . Several researchers suspected that the reactive nitrogen compounds from the supersonic transport exhaust might accelerate the natural chemical destruction of ozone, causing ozone levels to drop.
The main natural source of chlorine in the stratosphere is methyl chloride formed from algae, which provides but a part of all the chloride currently transported through the tropopause. In addition to methyl chloride, there are other natural sources of chlorine, transported into the stratosphere-nitrous acid in volcanic emissions, chlorides in sea compounds. Anthropogenic sources include perchlorate in hard fuels used in rockets. Nevertheless, in comparison with the chlorofluorocarbons (CFC), all these sources contribute a relatively small share.
At the beginning of the 70s it was decisively established that CFC's used in refrigerators and aerosol containers became a substantial constituent of the atmosphere. There is no effective mechanism of destroying these extremely stable substances in the lower atmosphere, and they are transferred into the stratosphere. The most important of chlorofluorocarbons-freons, CFCl, CF2Cl2, etc.- absorb ultraviolet radiation; as a result, photochemical reactions with the release of Cl are effected.
Without the breakdown of manufactured chlorofluorocarbons, there would be almost no chlorine in the stratosphere. CFC-12 concentrations were less than 100 part per trillion by volume when they were first measured in the 1960s. Between 1975 and 1987, concentrations more than doubled from less than 200 parts per trillion by
volume to more than 400 parts per trillion by volume. The amount of chlorine in stratosphere increased by a factor of 2 to3.
In parallel with the catalytic destruction of ozone, NO and Cl take part in reactions with molecules and radicals containing oxygen and hydrogen; the end products of these reactions (specifically HNO3, HCl) are conveyed into the troposphere and then flushed by rain. If these reactions providing an effective exit of NO and Cl out of the atmosphere, are not taken into account in model computations, the impact of nitrogen oxide and chlorine on the ozone layer could be overestimated. The fact that ozone emerges as a result of photochemical processes implies that its build-up depends on the intensivity of sunlight. Many instances were observed when variations in TO correlated with the Sun's activity. As a result of the latter, there could also be an increase in the amount of nitrogen oxides, leading to the decrease of ozone.
Seasonal variations of ozone concentration brought about by changes of atmosphere circulation are extensive in the higher altitudes (Fig. 3). Natural temporary variations could be quite large in comparison with values brought about by expected anthropogenic changes. For instance, at an average global content of ozone in the Earth's atmosphere equal to 297 Dobson's Units (D.U.), average monthly values of TO are subject to three-fold seasonal and territorial variations during the year. All these factors up till now did not allow the registration or reliable assessment of the impact of human activities on the ozone layer, although there is no doubt that this influence is substantial. The first evaluations were based on a simplified model, which includes only several reactions. More complicated models are being developed, and there is a growing feeling that present scales of emissions do not justify the credibility of those preliminary assessments, which predict a catastrophic outcome.
Nevertheless, the ozone layer may prove to be extremely sensitive to different influences. Large amounts of nitrogen oxides could enter the atmosphere when the Earth passes through meteoric showers. It has been calculated that the Tungus meteorite which landed in Siberia in 1908 brought about the cumulation of 30 million tons of nitrogen oxide at an altitude of 10-100km, and this is about 5 times as much as the total amount of nitrogen in all the stratosphere. Such a huge amount of nitrogen
oxides affected the ozone layer; this was substantiated by the decrease of atmosphere transparency to radiation in the UV range, which during 1909-1911slowly returned to its initial state. An explosion of supernovae, close to the solar system, could also cause a similar effect.
Nuclear explosions also pose a threat to the ozone layer. A small decrease of ozone in the beginning of the sixties was caused by an increase of nitrogen oxides, due to armament testing. After nuclear tests came to an end, TO again increased. This proved that nuclear explosions are capable of influencing TO through photochemical reactions of the nitrogen cycle, leading to the destruction of ozone.
4. Ozone Control, Physical Concepts and Methods A valid question evolves: how was the information about the ozonosphere, its structure and variability obtained? What methods and means can be used to determine ozone concentration in the "thick" of the atmosphere in different geographical regions of our globe? Clearly, the laws and patterns of ozone distribution in the atmosphere are not just fruits of scientists' imagination; they are the product of countless measurements of ozone content. Such measurements have been conducted systematically beginning from the thirties, both on the ground, and with the help of equipment positioned on high-flying airplanes, scientific rockets and balloons. Measuring methods, in terms of instrumentation interaction with atmospheric ozone, can be divided into three groups: ♌
taking air samples from specific parts of the atmosphere and their successive laboratory analysis;
♌
contact measuring, whereby the instrument interacts with the air in situ and during the metrical process;
♌
remote methods, based on measurement interpretation of different parameters, characterizing electromagnetic radiation caused by the presence of ozone in the atmosphere.
The development of associated apparatus and its application in scientific research grew into an independent area of scientific knowledge - ozonometry. We will not overburden the reader with the description of devices, meant for ozone control from earth, and in airborne, rocket, and balloon conditions, keeping in mind, too, that the reader will get acquainted with similar space-based equipment. To give the reader an idea about the scale of ozone control, we will just note that the world net of groundbased ozonometric stations alone, a decade ago, exceeded seventy. Apparatus based on the latter provides a large part of climatological information about atmospheric ozone. Remote methods having high selectivity, sensitivity, and precision are preferred when mass measurements of ozone are necessary for providing instant global depiction of ozonosphere conditions. The practicality of installing optical instrumentation aboard satellites gave new impetus to remote methods. Measurements
in space began in August 1967 on board the OGO-4 satellite and were followed in the USA on Nimbus-3,-4,-6,-7, Tiros-4, Atmospheric Explore-5, and on board former Soviet Union satellites of the Meteor and Cosmos series, as well as orbital stations. A large selection of ozonometric devices was developed and tested for the acquisition of data relevant to the time- and space- state of the ozonosphere, over vast territories, with good periodicity.
We will examine, in more detail, remote methods of probing with the help of satellites. These methods are of the passive and active types. Passive methods are used on board satellites for measurements of the spectral distribution of outgoing electromagnetic radiation of the Earth, so as to determine the composition of the latter, on the basis of the data obtained. Active methods are applied for determining atmospheric parameters, using spectroscopic measurements of electromagnetic radiation transmitted from a satellite and reflected by the atmosphere. To understand the processes going on in the atmosphere, we shall analyze the interaction of electromagnetic radiation with the substance, in an arbitrary uniform layer of thickness l. Let the lower-lying layers' radiation reach the boundary of the abovementioned layer with intensity I (ν, 0).
The absorption spectrum, i.e. dark lines or stripes against a bright backdrop of incoming radiation is observed in rays, outgoing from the layer, if the decrease of light flux because of absorption is more than the contribution of the layer's own thermal radiation (spontaneous and compelled). If medium domination prevails, then the emission spectrum will be observed, i.e. bright stripes and lines on a weaker backdrop of incoming radiation. The character of ray propagation in a medium largely depends on the optical thickness along the distance passed by the ray l, generally defined as τ (ν, l) = ∫ k(ν, x) dx, where k(ν, x) -volumetric spectral absorption coefficient of the medium. For a homogenous medium τ (ν) = k(ν) l. If k(ν) l ≤ 1, then the medium is considered optically thin; if k(ν) l > 1, the medium is said to be optically thick.
In the case of cold medium (no thermal radiation of layer molecules) radiation propagation in a layer follows the previously mentioned Beer-Lambert Law. For an optically thin medium and a continuous external source spectrum, the spectral absorption coefficient graph of a substance is "reproduced" without distortion in the spectrum of radiation after the latter has penetrated the layer. Moreover, the decrease of radiation intensity is proportional to the optical thickness, and, therefore, to the number of absorbing molecules in the layer. The maximum in the absorption band becomes less evident with the increase of optical thickness. A similar situation evolves in the case of a hot (self-radiating) medium, when no radiation from an external source enters the layer. If the medium is optically thin in the frequency range of interest, then the intensity of the radiation observed will be characterized by a distribution, proportional to the spectral form of radiating component lines. As the optical thickness of the hot medium grows, so the spectral distribution of the radiation being observed begins to deviate from the spectral form of the radiation component line, and, expanding, reaches saturation. Because absorption lines are usually "stronger" than lines of excitation, and are prone to easier observation, absorption spectra are used in remote probing of atmospheric components (including the "smaller" ones).
The Earth's atmosphere can be envisaged as consisting of layers having approximately the same density of matter, and insignificantly changing temperature and pressure with layer height. Transport of radiation from the lower layers to the upper ones is essentially a process much more complicated than that considered above for radiation propagation in a uniform layer. However, the simplest case allows us to come to important conclusions relevant to radiation behavior in such a complex system as the Earth's atmosphere. Estimations have shown that separate layers, and the atmosphere as a whole, are optically thick media in respect to practically all substances present in the atmosphere. That is why in the outgoing Earth radiation the absorption band of the component being studied, has a deformed shape.
Radiation of external sources interacts with substances in the atmosphere and forms the outgoing radiation; these sources are, specifically the Earth, the lower layers of the atmosphere in respect to the higher layers, and also celestial bodies (the Sun, the
Moon and the stars). The thermal radiation of the Earth and the lower layers of the atmosphere depend on the temperature and the radiation properties of its surface, the temperature of the surrounding air and also on the cloudiness. For some average temperature of the Earth's surface (~290K) the Earth's maximal radiation comes from within the waveband 10-12 µm. The Earth's own thermal radiation exceeds radiation of all the other sources beginning with λ = 4 µm.
Fig. 4. Spectrum of Earth Thermal IR-radiance recorded from space: a - Desert Sahara; b - Mediterranean Sea; c - Antarctic Region Thermal radiation spectra registered on board the "Nimbus-4" satellite from an altitude of 800km are shown in Fig. 4. (The satellite was launched by the USA in 1974). The spectral distribution curve (a) was obtained above a hot, sandy surface at a temperature of ~50°C. In the waveband 12.8 - 13,7µm there is a clear manifestation of thermal flux absorption by atmospheric carbon dioxide CO2. Abrupt, shallow declivities at wavelengths 16 - 25 and 6.7 - 8 µm, are the result of absorption by water vapor, H20.The ozone absorption band close to λ = 9.6 µm is clearly identified,
whereas close to λ = 7.7 µm there is a relatively weak absorption band of methane CH4.
In the spectral radiation curve (b) obtained above the Mediterranean Sea, where the temperature of the water is ~20°C, absorption bands relevant to gases mentioned above, are less distinct, whereas above Antarctica (curve c) in the region of absorption bands of CO2 and O3 the radiation flux exceeds the level relevant to the thermal emission of the Antarctic surface. The spectral distribution curves clearly demonstrate the process of thermal radiation transport from the Earth's surface into the upper layers of the atmosphere. Above the hot regions of the Earth mainly the thermal radiation of the Earth cover and the lower layers of the atmosphere build up the outgoing radiation. This radiation interacts with the atmosphere throughout its depth and is substantially weakened in spectral areas enclosing absorption bands of atmospheric gases. Characteristic of a cold Earth surface and low levels of the atmosphere, is that the outgoing radiation depends mainly on the radiation of the upper layers, where there is but a low presence of absorbing atmospheric gases and, therefore, an insignificant weakening of penetrating radiation flows.
Fig. 5. Solar spectra and absorption bands of atmospheric gases
Sunrays interacting with the Earth's atmosphere and its surface produce the shortwave part of the outgoing radiation. Fig. 5 shows the energy spectrum of solar radiation before it enters the atmosphere and at sea level, in the absence of clouds. The energy spectrum of blackbody radiation at Sun surface temperature ~5900 K is also shown in Fig. 5. We can see that as the energy spectrum of solar radiation penetrates the atmosphere, it changes significantly: there is a general weakening, and abrupt declivities appear; these are a result of selective absorption by atmospheric gases water vapor, carbon dioxide, oxygen, ozone and others. Specifically, absorption of solar radiation by ozone is observed in all the previously mentioned bands of its molecules. The general weakening of the solar radiation flux depends on the wavelength insignificantly; it is brought about by the scattering of electromagnetic waves caused by optical irregularities of the atmosphere (due to density fluctuations) and by solid and liquid particles suspended in the atmosphere, which shape aerosol and water-vapor clouds. The short-wave part of the outgoing radiation is composed of solar radiation, scattered by the atmosphere and reflected from the surface of the Earth and clouds.
There are two methods of identifying atmospheric gas components on the basis of outgoing spectral radiation characteristics. The first method demands a solution of the direct radiation transportation problem, making use of data relevant to the distribution of density and temperature with altitude, as well as characteristics describing the interaction of electromagnetic waves and matter. The solution of this problem yields a spectral distribution of the outgoing Earth radiation. By changing the distribution of the unknown component quantity, relative to altitude, and also its total amount, one can, in principle, reach a state when the calculated and measured spectral distributions in the outgoing radiation will be in good agreement. Here two problems evolve. The first one is the lack of certainty: is the density distribution of the unknown component quantity (versus altitude) the only possible distribution? The second problem lies with large computational volumes required by numerous possible variants of density distribution.
This necessitates the development of inverse ways for solving the problem (the second method) - determination of the unknown component in the atmosphere according to the spectral characteristics of the outgoing radiation, and avoiding numerous intermediate calculations. The mathematical basis of these methods is the integral equation of the following type I(ν) = ∫ G(ν, x) φ(x) dx, where I(ν) is the dependence of the outgoing radiation intensity on frequency ν; φ(x) the unknown density distribution of substance vs altitude; G(ν, x) - the core of the integral equation, characterizing the contribution to the outgoing radiation of the amount of substance found in a layer of atmosphere at an altitude x. This equation is known as Fredholme's integral equation of the first order, and is notable for the absence of a single answer. In this sense the solution to the equation is relevant to the class of incorrect problems, demanding the involvement of statistical methods to assess the correctness of solutions, by acquisition of data, characterizing the atmosphere and the parameter being determined.
There are three basic passive methods of remote ozone layer sensing: method of atmospheric emission, based on measuring the inherent radiation of the Earth and its atmosphere, measurements of Solar Backscattered Ultra Violet (SBUV) radiation, and the absorption method involving the measurement of atmosphere transparency in the following direction: satellite - radiation source (the Sun or the stars). Radiation measurements in these methods are conducted in the nadir direction, or at different angles to nadir. Depending on the direction of observation relative to nadir, these measurements are conditionally divided into limb and nadir measurements.
Fig. 6. Methods of Ozone measurements from space Charts illustrating these measurements are shown in Fig. 6. In the case SBUV radiation, the outgoing radiation of the atmosphere is measured in the vertical nadir direction (1) or at different angles to the vertical line (2). In the case of limb absorption measurements lines of observation are pointed to the Earth's horizon (3). Due to the immense length of the ray's path in the atmosphere, these measurements allow, the determination of other "small" gas components in the upper layers of the atmosphere, in addition to ozone. Nadir (4) and limb (5) measurements of the atmosphere inherent thermal radiation are carried out on the night Earth side. In the case of the limb measurements of SBUV radiation (6), lines of observation are pointed near the Earth's horizon.
5. Solar Backscattering Ultraviolet Method It has been mentioned above that the short-wave part of solar radiation undergoes substantial scattering, caused by molecules and density fluctuations. Radiation scattering follows Rayleigh's Law, according to which the intensity of scattered radiation is inversely proportional to the wavelength in the 4th power, and is distributed in such a way that in the direction of propagation, it is twice as intense as in the transverse direction. The content of ozone, even at a maximum of its distribution vs height, is substantially smaller than the concentration of the main components in the atmosphere. That is why the absorption of sunrays in the ozone bands is a gradual process, which increases as the rays move "in-depth" into the atmosphere. Radiation penetrating into the atmosphere brings about a considerable flux of scattered Ultraviolet Radiation in the opposite direction. Surely, the penetration depth of solar rays into the atmosphere depends on the absorption properties of ozone on a given wavelength. In Hartley's main absorption band, solar radiation is totally absorbed in the ozone layer, and the intensity of the outgoing UVradiation in this band of the spectrum depends on the scattering of solar radiation in the upper layers of the atmosphere and its absorption by ozone.
In the area of weaker absorption (the Higgins band) the Sun's radiation reaches the ground after having been thinned down. The outgoing radiation in this part of the spectrum depends on solar radiation scattered by the atmosphere, and reflected from the Earth's surface and it’s clouding. Designing instrumentation for measuring backscattered UV solar radiation should take into consideration such an important factor as the choice of a specific number of spectral ranges in the ozone absorption bands for conducting measurements.
To accomplish this objective, one can use methods of mathematical modeling to solve the direct problem of determining the intensity of scattered UV solar radiation. The following data has to be assigned for this purpose: standard pressure distribution with altitude, vertical distribution of ozone concentration found out in the course of on-theground and rocket measurements, coefficients of Rayleigh's ozone scattering and absorption, coefficients of aerosol scattering, the albedo of the underlying surface, the
Sun's zenith angle, etc. The so-called weighting functions constitute the results of such computations. These functions characterize the contribution of radiation scattered from different levels of the atmosphere, in respect to the total radiation on a given wavelength, outgoing beyond the upper borders of the atmosphere, in the nadir direction. Calculated weighting functions show that, in the waveband 0.25-0.3Âľm, UV radiation from the Sun is absorbed in the ozone layer before reaching the Earth's surface; each of the wavelengths in this band has a certain depth of scattering and a strongly manifested, though relatively narrow, layer of effective scattering. The "effective layer" for shorter wavelength located on more higher atmosphere layers. This part of the spectrum is used to determine the vertical distribution of ozone (the so-called Ozone Profile). UV-radiation in the 0.3-0.34Âľm band penetrates into the lower layers of the atmosphere. On these wavelengths, scattering envelopes practically all the dense layers of the atmosphere from the Earth's surface to altitudes of 60km. This waveband is used to determine the total content of ozone in the atmosphere (the so-
o
erg / cm2 . A . srad
called Total Ozone).
10 1
1 3398
2
10 0 10-1 10-2 10-3
2557 2500 3000 3500 o Wavelength, A
Fig. 7. Backscattered ultraviolet radiation (1), Extraterrestrial Solar irradiance (2) The total radiation scattered by all the layers determines the intensity of UV solar radiation scattered by the Earth's atmosphere. The end result of the method is to describe the distribution of ozone in the atmosphere on the basis of spectral measurement data, obtained from a satellite. The mathematical premise for solving the inverse problem - reduction of ozone to its initial content, on the basis of spectrometric data - is Fredholm's integral equation of the first kind (mentioned above). In Fig. 7 we can see a typical dependence of the intensity of UV solar
radiation, scattered by the Earth's atmosphere in the absorption band of ozone; (measurements were conducted on board the Nimbus-4 satellite). What attracts attention is the extensive range of scattered radiation intensity, which varies by a factor of 104, from small values in the short-wave band to large values in the long-wave range, whereas the intensity of direct UV solar radiation increases smoothly with the growth of wavelength. Estimations have shown that a number of factors (variations in ozone profile concentrations, albedo of the Earth's surface and the clouds, Sun zenith distance) taken as a whole, bring about changes in the intensity of scattered radiation limited to one order (ten times). Hence, the total variation of atmosphere scattered UV-radiation in the given band of ozone absorption is equivalent to five orders of magnitude. To establish ozone content in the atmosphere, with a precision of 3-5% (level of natural variations of ozone), the precision of relative intensity measurements of scattered radiation should be better than 1% (using the above-mentioned wavelengths.) The simplest way of meeting flux measurement precision requirements is to use an UV spectrometer with sequential spectrum scanning and a single radiation receiver. Here it seems appropriate to remind the reader that assessing the vertical ozone profile requires all the abovementioned spectral channels, including the long-wave bands, whereas for total ozone assessments, long-wave channels alone can provide adequate information. Henceforth, we will use the terms UV OP-spectrometer and UV TOspectrometer to designate apparatus used for determining the vertical ozone profile and total ozone content, respectively. The probability of flux changes by a factor of 105 leads to serious problems in the design of electronic and optical devices for the UV OP-spectrometer. To provide a signal-to-noise ratio exceeding 100, the electronic channel must have highly linear characteristics in all the range of varying input signals. A change of radiation intensity in the ozone absorption band equivalent to 104 times (at a measurement precision of at least 1%) in all the spectral channels, requires a drastic rejection of radiation, (dispersed on the elements of the optical system), reaching the receiver; this rejection should provide a signal as low as 10-6 of the flux input. At the same time, the contribution of dispersed radiation (not distributed in terms of wavelengths) to any spectral channel constitutes a value less than 1% of the
flux being measured. Such a degree of dispersed radiation rejection can be achieved with the help of a double monochromator. In UV OP -spectrometers, developed in the USA and other countries, the dispersion of light, according to the wavelengths is effected by diffraction gratings reflecting a fraction (10-3) of the falling radiation flux. In these spectrometers a double monochromator with two diffraction gratings is used. 1
2 3 4
5 13 12 6 7
11 10 9
8
Fig. 8. Simplified schema of TOMS In the UV TO- spectrometer, due to the relatively small change of radiation intensity in the spectrum-scanning interval, there is no need for a double chromator. The UV TCO- spectrometer, developed in the USA was coined TOMS (Total Ozone's Mapping Spectrometer). It has six spectral channels on the following wavelengths (µm): 0.3125; 0.3175; 0.3312; 0.3398; 0.36 and 0.38. TOMS observes the ozone layer in a direction perpendicular to the satellite's orbit plane, with a 3° pitch in the angle range of ± 52.5°, relative to nadir. The first two pairs of spectral channels are used to determine TCO; two long wavelength channels (in which ozone absorption is practically nonexistent) are required for albedo control of the underlying surface. A simplified optomechanical diagram of the TOMS spectrometer is shown in Fig. 8. Here elements 7 and 9 are a subassembly for scanning the spectrometer view of sight across the Earth's surface; the rest of the elements, with the exception of the light flux modulator 10, and the directing mirror 11, constitute the optomechanical UVspectrometer of the series spectrum scanning type.
The following is a description of the spectrum scanning principle. The light flux 8, after passing the input slot of the monochromator 13, and having been reflected from the collimating mirror 1, is converted into a parallel beam, which throws light on the diffraction gratings 12. The radiation, diverged in terms of wavelength, is reflected from the collimating mirror and focused on a stationary array 2 of the output slots. Light beams, each having one of the mentioned wavelengths, pass through the defined slots. The corresponding slots are positioned on a selector-disc 3 in such a way, that when a pair of slots are aligned, the others are overlapped (shut). At the same time, beams of specific wavelengths pass through the output optics and are focused on the cathode surface of a photomultiplier 5. Shifting of the selector-disc 3 with the help of a step motor 6 allows the sequential measurements of signal intensity on each of the six allocated wavelengths. The Solar Backscatter Ultraviolet (SBUV) spectrally scanning radiometer is based also on abovementioned optical schematic diagram (Fig. 8). The SBUV-2 nadirviewing sensor measures the spectral solar irradiance and spectral scene radiance (backscatter solar energy). Sensor can operate in sweep mode (continuous scan over range 160-400nm) and discrete mode (measures from 252.0-339.8nm in 12 discrete bands with 1nm bandwidth). The instrument makes measurements from which the vertical distribution off atmospheric ozone can be determined to an absolute accuracy of 5%. In 1978 a set of UV spectrometers was stationed on the NASA satellite Nimbus-7 for TO mapping and OP measurements above the satellite footprint areas: This set (TOMS and SBUV) allows TO mapping during a 24 hour period over the whole of the Earth's surface and at any time of the year. One exception: measurements in winter over the polar caps are not feasible - during the winter the Sun does not shine over these areas. Since the satellite launching, massive data has been obtained, relevant to the Earth's ozone layer, and, specifically, over the South Pole, in the "ozone hole" area. The Shuttle Solar Backscatter Ultraviolet (SSBUV) is also instrument using UV backscatter in nadir to measure vertical profiles of ozone in the stratosphere and in the lower mesosphere in spectral range from 200 to 405nm. The SSBUV design is based
upon the above described technology. The objective is to fly the SSBUV payload on numerous Shuttle missions to provide complementary calibration data for long-term satellite ozone data sets. The first flight with SSBUV instrumentation occurred on October 1989 on the Shuttle Atlantis (STS-34). Throughout this Shuttle flight coincident observations were taken with the SBUV on Nimbus-7 satellite and SBUV2 on NOAA-9 and NOAA-11 satellites. The similar experiments with SSBUV were continued on following Shuttle flights of Atlantis, Discovery, Columbia, Endeavour until 1996.
Other countries for measuring TO and OP are also using UV spectrometers. International efforts allow the monitoring of ozone over our common home - the Earth. An early example of International Cooperation is the agreement between the Goddard Center (NASA, USA) and the former USSR, according to which TOMS was installed on board the space apparatus of the Meteor-3 type. The launching of such an ecological orbital patrol system led to investigations, which solved numerous puzzles of the ozonosphere. TOMS has helped revolutionize our understanding of the Earth's ozone layer and associated systems.
QuikTOMS is the latest mission to carry the TOMS instrument. Its diagrammatic drawing and real view on the testbed are presented on Fig. 9 and Fig. 10 correspondingly. QuikTOMS will follow in the footsteps of previous TOMS instrument based satellites like Earth Probe, Nimbus 7, Meteor 3, and ADEOS. QuikTOMS is a secondary payload, it shares its delivery system (Taurus rocket) with the Orbview-4 mission. QuikTOMS uses innovative MicroStar satellite platform, which supports payloads up to 68 kg and provides a three-to five-year mission life. The instrument records daily global measurements of the Ozone, Aerosols, Erythemal UV exposure, and Reflectivity. QuikTOMS main objectives are: ♦
Determination of long term change in global total ozone level;
♦
Understanding the processes related to the "ozone hole" formation and local anomalies in the equatorial region;
♦
Improved understanding of processes that govern the generation, depletion, and distribution of global total ozone;
Unfortunately, the much waited for NASA QuikTOMS launch on Sept. 21, 2001 has ended in failure following problems with the Taurus vehicle's second stage. Another such instrument is slated for launch in 2003 on the Earth Observing System Aura satellite.
Fig. 9. QuikTOMS view of the instrument diagram with the component call-outs [23].
Fig. 10. QuikTOMS during ground testing [23]The Global Ozone Monitoring Experiment (GOME) was launched on April, 1995 on board the second European
Remote Sensing Satellite (ERS-2). This instrument can measure a range of atmospheric trace constituents, with the emphasis on global ozone distributions. GOME is a nadir-viewing spectrometer that measures the solar radiation scattered by the. The field of view may be varied in size from 320 km x 40 km to 960 km x 80 km. GOME can provide complete coverage of the globe at the equator in approximately three days. The schematic of the spectrometer optics is shown in Fig. 11. As an instrument, GOME can be described as a double spectrograph, which predisperses light at a prism and then produces a spectrum using a set of holographic gratings. A combination of this optical arrangement and the use of four individual linear detector arrays (each with 1024 detector pixels) enable the simultaneous observation of the Earth's back-scattered spectrum between 240 and 790 nm (extending from the ultraviolet into the visible parts of the spectrum). The spectral resolution of GOME is moderate: between 240 and 400 nm it is approximately 0.2 nm; between 400 and 790 nm it is approximately 0.4 nm. The GOME employs a mirror mechanism, which scans across the satellite track with a maximum scan angle that can be varied from ground control. A similar instrument will be flown on the EUMETSAT Metop series of satellites.
Fig. 11. GOME Spectrometer optics [25]
6. The Emission Method In contrast to the Solar Backscattered Ultraviolet, the Emission Method involves the study of the material content of the atmosphere through its inherent (thermal) radiation. As was mentioned above, ozone has strong vibrational bands on wavelengths of 4.7; 9.6; and 14.2 µm. The 4.7 µm band is intensively covered by water vapour bands, and in addition, in this band, reflected radiation still has a considerable impact on the Earth's outgoing radiation. The 14.2 µm band is overlapped by absorption bands of carbon dioxides and water vapour. That is why the 4.7 and 14.2 µm bands cannot be readily used for determining ozone content in the atmosphere. In terms of such an approach, the most suitable is the 9.6 µm ozone band.
In the late sixties and early seventies, in the USA, continuous spectra of outgoing radiation were identified in a wide band of wavelengths (from 5 to 25 µm), which allowed appropriate spectral zones to be allotted for probing the Earth's atmosphere. Consequently, spectral measurements were conducted using the IR spectrainterferometer IRIS. The spectra registered over different areas of the Earth's surface, shown in Fig. 4, clearly bring out the 9.6 µm ozone absorption band. The TO was roughly approximated by measuring the depth of the declivity in the outgoing radiation on the specific wavelength. In the spectrograms, radiation absorption by water vapour can be easily observed, and should be taken into account in TO computations. In general, the retrieval of TO is accomplished on the basis of the abovementioned formalism of Fredholme's integral equation of the first order, using data, relevant to the ingredient content of the atmosphere, the vertical distribution temperature and humidity.
Fig. 12. Simplified schema of infrared interferometer IRIS Apparatus IRIS is based on Michelsons' interferometer circuit, with a linear movement of the mirror. The functioning of such an interferometer is explained in Fig. 12. The atmospheric radiation flux, after being reflected from mirror 1, is directed through window 2 onto a light-splitting plate 3, on which an amplitudinal division takes place, and the reflected and propagating parts of the flux are divided in a proportion 1:1. The radiation fluxes, reflected from the movable mirror 5 and the fixed one 4, after secondary separation on the plate 3 are directed to the converging mirror 6 and are focused on the receiver 7 placed in the focus of mirror 6. These fluxes appear at the radiation receiver with a time lag equal to the doubled value of the distance deviation between the light separation plate 3 and mirrors 4 and 5. The intensity of monochromatic radiation entering the light-sensitive surface of the receiver, depending on the phase difference of the fluxes will change according to cosine law from zero value to fourfold the intensity of unit flux.
The dependence of the receiver signal value on the position of the movable mirror is called the interferogram of the flux being studied. With a uniform motion of the movable mirror, the interferogram is actually a cosinusoid. In the case of a continuous spectrum the interferogram is a complex time function. Thus, the output of the interferometer receiver tract is not a spectrum, but is, in essence, some time function, which, with the help of integral Fourier transform, should be converted into a spectrum.
To attach the measurements to the wavelength master scale, the following elements are incorporated into the interferometer: a high-precision channel for measuring the displacement of the movable mirrors 9 and 10, components of the interferometer 3,4 and 5, the interferometer filter 11, a convex lens 12, and a radiation receiver 13. The named channel works on the principle of an interferometer, so that any movement of the mirror at the output of the receiver 13, brings about pulses of voltage having delays equivalent to λ/2, where λ-the wavelength of the monochromatic source 8. The mirror 1 is meant to compensate the shift in the field of view of the device, located on the surface of the Earth, which occurs with the movement of the satellite. For this purpose, the mirror 1 performs periodical angular movements, so as to aim the device observation line at a specified footprint zone, during interferogram measurements. In such a way, the distortion effect brought about by the changing state of the atmosphere along the footprint line can be eliminated.
Fourier transform spectrometry is a labour-consuming indirect method for generating spectra. As to the development of the interferometer itself (especially for space-based apparatus) - the task is a formidable one in terms of scientific and technological complexity. To mention just one problem: the configuration of the interferometer should be resistant to vibration and deformations caused by temperature gradients. So what is so attractive about Fourier Spectroscopy? The main advantage of the method is that it provides a higher effectiveness in light flux energy consumption, in comparison with direct spectra measurements (e.g. with the help of a scanning spectrometer, utilizing diffraction grating and one receiver.). In the latter case, measurements are conducted sequentially along spectrum stretches equal to the spectral resolution of the device, whereas the larger part of the spectral interval being studied does not participate in the measurements. We should note that doubling the time for measuring a single spectral element is equivalent to taking two independent measurements and then averaging them. If the error has a stochastic character, then such averaging will bring about a √2-fold increase in precision. If the time taken for measuring the specific element increases N times, then the precision with which the flux intensity is measured increases √N times. All the spectral components of the flux simultaneously take part in the structuring of the interferogram. This means that with a
certain time for spectrum measurement, the same spectral resolution, and N - the number of elements being resolved, the precision of flux intensity measurements on a given wavelength in an interferometer, is √N times higher than that of a spectrometer. In the interferometer, one radiation receiver, so to say, takes upon itself the simultaneous measurement of intensity in each of the spectral stretches.
The mass and power consumption of the Fourier- spectrometer IRIS was small; it provided spectra in the 5 to 25 µm wavebands, recording one interferogram in 11 sec; the spacial resolution was 100km. The spectrophotometer HIRS was developed in the USA for operative measurements of ozone and other parameters of the atmosphere. It is an instrument for high-resolution radiation measurements in the IR band. HIRS is a 20-channel filter spectral device. The channels are in the 0,7 to 15 µm wavebands and one of them is the central band on a wavelength of 9.6 µm. The spectral data of the apparatus allow the retrieval of temperature and humidity dependence on altitude; this data is essential for determining TO on the basis of data in the 9.6 µm channel.
The architecture of HIRS can be depicted as an optical system of a telescopic type with a scanning mirror at the input end, meant to shift the sighting line in the limits of the apparatus observation angle. The diameter of the telescope is 15 cm. The light flux at the output of the telescope is divided by a dichroic splitter into long wave (longer than 6.4µm) and short wave (lower than 6.4µm) parts. The latter is repeatedly split so as to single out a flux in the visible part of the light spectrum. The field of view of the apparatus is formed with the help of two diaphragms - for the short wave and long wave channels. After passing through the diaphragms, the fluxes reach the bandpass filters attached to a rotating wheel; as the latter rotates, the respective spectral bands sequentially reach receivers of the respective radiation fluxes. Two cryo-cooled solidstate receivers are used in the apparatus: one in the long-wave channels, on the basis of the triple compound HgCdTe, and the other using InSb in the short-wave channels. In the visible band, a silicon receiver not requiring cryo-cooling is used. The field of view of the telescope is 1.8°, the scanning angle is ± 49.5°, and time of scanning within this angle is 6.4 sec, time for measuring the spectrum - 0.1 sec.
The HIRS spectrometers based on operative meteorological satellites are capable of producing a global map of TO, with a 30-km geometrical resolution, during 8 hours. It should be noted that above the Earth's polar ice-caps, TO data precision using this method is low, especially during the winter period; this is explained by the low temperature of the Earth's covering and the lower layers of the atmosphere, in the vicinity of the Poles.
7. Limb Methods Radiation flux measurements conducted from satellites in the direction of the Earth's disk brim are called limb evaluations. There are several limb methods for determining ozone content (and other components of the atmosphere): the sorption method which infers the measurement of atmosphere transparency in the absorption band of the component in question, during the instant of satellite setting into the shade, and emerging from the latter; solar backscattered UV (SBUV) method used in the direction of the horizon; the emission method. The main advantages of limb methods for atmosphere probing are: higher sensitivity (60 to 70 times) to the "small" gas components, brought about by the longer route of the ray in the atmosphere; high vertical resolution (1-3km) in the altitudinal distribution of atmospheric components; absence of influence on the part of low layer atmosphere heat radiation, as well as Earth covers, on measurements.
A schematic of solar ray paths in the case of sorption method measurements is shown in Fig. 6 (symbol 3). Because of the remoteness of the Sun, the radiation flux can be roughly perceived as a series of parallel beams in the direction of observation beyond the Sun at different positions of the satellite in orbit - in the form of parallel lines through the atmosphere at different distances from the Earth. When the satellite sets into shade, the lines of observation come closer to the surface of the Earth, when the satellite emerges out of the shade these lines move away from the surface of the Earth. The point of smallest withdrawal of observation lines from the surface of the Earth is called the targeted spot. The normal drawn (perpendicular) from the latter to the Earth crosses the terminator - the line separating the illuminated part of the surface from that in the shadow.
The essence of the method is in the measuring of the transparency function of the atmosphere at different altitudes in the absorption bands of the components being identified. A measurement of direct solar radiation is taken when the satellite enters the shadow or emerges from it, and also away from it (in conditions when there is no influence of the atmosphere). Spectrometric data is used to determine the attenuation of solar radiation in the atmospheric column along the trajectory of the beam. The
determined total content of the absorbing substance in the atmospheric column takes into account Rayleigh's absorption and losses due to aerosols. The reciprocal problem (determining the vertical distribution of the component's density) can be solved, using the above-mentioned data for different altitudes. The model of the atmosphere is depicted as a layered spherical structure. The mathematical problem in this case is considered to be of the correct type, since the vertical distribution being obtained is the only one possible. The density of the absorbing substance is determined close to the targeted spots; the altitudinal density distribution is retrieved in the vicinity of the terminator. The vertical resolution is determined by the field of sight of the apparatus according to the tilt, whereas the horizontal resolution by the inadequacy of the mathematical model relevant to the real atmosphere, and comprises approximately 150km.
The Stratosphere Aerosol and Gas Experiment using the sorption method is based on a spectrometer called SAGE. The instrument vertically scans the limb of the atmosphere during satellite sunsets and sunrises. The device comprises a tracking mirror with a two-coordinate electromechanical drive, Sun direction sensors, and a telescope responding to radiation reflected by the mirror, a diffraction grating spectrometer, and radiation receivers. The diameter of the telescope is 5.1cm, spectrometer field of view- 0.5′(elevation) and 2.5′ (azimuth); precision of Solar disk brim targeting - 0.5′; the diffraction grating includes 1200 lines/mm; radiation receivers are of the silicon photodiode type. The instrument has seven spectral channels in the 0.38 to 1.02 µm band; this allows the identification of the "small" gas components of the atmosphere (H2O, NO2) and aerosols, in addition to the measurement of ozone content in Shappuis absorption band. The altitudinal concentration distribution of ozone, nitrogen dioxide and water vapour can be retrieved on the basis of spectrometric data, with a vertical resolution of 1-3 km, a horizontal resolution of about 150km, and a precision of 10%.
SAGE was launched first aboard the Application Explorer Mission spacecraft and provided ozone measurements using the solar occultation technique until 1981. SAGE II began operation with the launch of Earth Radiation Budget Satellite in 1984 and its
observations are making important contributions to studies of the Antarctic ozone hole.
Occasionally, for investigating ozone with the help of the sorption method, relatively simple apparatus, usually used in rocket and stratospheric measurements, is positioned on a satellite. Specifically, these are spectrophotometers with a wide view of sight, comprising assemblies of several small telescopes, the optical axes of which are parallel, and each one of which is supplied with a narrow-band filter having a certain central wavelength. A broad-angled device is placed before the telescope objective so as to restrict irrelevant light fluxes. Vacuum diodes having stable characteristics were usually used as radiation receivers. In the spectrophotometer two or three spectral channels work in the ozone absorption zone (0.26-0.3Âľm) and one (basic) channel beyond this band (on wavelengths 0.34-0.4Âľm.).
Processing of spectrometric data is based on the method of differential absorption of UV radiation, which presumes that radiation absorption in an atmospheric column is described by Beer-Lambert's law. To determine the quantity of absorbed substance the ratio of radiation intensities on two different wavelengths has to be calculated; one of these wavelengths is located in the absorption band of the substance, whereas the other is beyond this band, but relatively close to it. A rough estimation (not taking into account Rayleigh and aerosol attenuation) shows that the logarithm of the ratio mentioned, is proportional to the amount of absorbing substance in the atmospheric column.
The construction of the spectrophotometers described is simple, but they have a substantial deficiency - the altitudinal distribution of substance density retrieved according to the spectrometric data has a relatively small vertical resolution (worse than 5 -10m). This can be explained in the following way: due to the large angular size of the Sun (33′) large volumes of the atmosphere take part in the formation of the radiation flux entering the photometer; thus, for example, the Sun's diameter observed from a height of 800km is equal to 20km in the region of the targeted spot. Despite a number of advantages of the sorption method, we should mention its limited scope for measuring the Earth's atmosphere. In the case of typical subpolar orbits, altitudinal
distribution of ozone and other "small" gas components can be determined over some areas of the Earth's surface in the eightieth Northern and Southern latitudes, where footprint lines intersect with the terminator. Possibilities of improving the informative capacities of the method are related to perspectives of utilizing stars having stable UV radiation, as outer sources irradiating the Earth's atmosphere.
The prospects of the SBUV method (observations in the direction of the horizon, marked 6 in Fig. 6) are great, in terms of covering the Earth's atmosphere. In contrast to nadir methods, here the radiation scattered by the atmosphere is measured in a direction close to normal in respect to falling solar rays.
The emission method in limb measurements is effected in the Infrared (IR) and Microwave ranges of the spectrum. Investigations of the Earth's atmosphere with the help of LIMS (Limb IR Monitoring of the Stratosphere) to be the most successful; this instrument was installed on the satellite Nimbus-7. The apparatus was divided into two distinct modules - electronics and radiometer sections. The radiometer block in itself consists of two parts - a solid-state cryo-assembly and optical-mechanical assembly, both of which are attached to a common plate installed on the satellite. The solid-state cryo-assembly includes a receiver subassembly and a cryogenic device with two stages of cooling, the first of which uses solid-state ammonia (NH3) and provides a temperature of 152K for all the elements of the receiver assembly, with the exception of the detector itself; the second stage uses solid methane (CH4), and provides a temperature of 63K necessary for the detector functioning. Photoresistors made of a triple mixture (Cadmium-Mercury-Tellurium) serve as the sensitivity elements of the detector.
13
12
11
10 9
7
8 6
5
4
2 3 1
Fig. 13. Optical schema of LIMS apparatus The optical-mechanical assembly consists of the optical elements of the telescope and electric actuators of the scanning mirror and a radiation interrupter. A simplified optical scheme of the device is shown in Fig. 13. Items 2-6 are parts of the telescope with a scanning mirror 1. Shown in the figure are: 2,4 – off-axis parabolic mirrors, 3 – modulator, 5 – aligning plane mirror, 6 - Cadmium –Tellurium lens. Items 7-13 are part of the radiation receiver assembly. The slit 7 and window 8 having a temperature of 152K separate the radiation receiver assembly from the telescope, which has the temperature of the satellite container. After passing the system of parabolic mirrors 9 and a focusing lens 10 the radiation reaches the band filters 11 which slits 12 determined the field of view of the different spectral channels. Separated in space according to the wavelengths the radiation flux arrives at a strip of independent radiation receivers 13. Spectral bands of LIMS and a list of "small' gas components of the atmosphere are listed in the table: Channel
Radiating gas
Spectral band, μm
1 2 3 4 5 6
NO2 H2O O3 HNO3 CO2 CO2
6.14-6.4 6.42-7.3 8.77-10.8 10.9-11.9 13.25-17.3 14.9-15.8
Limb scanning is accomplished in 12 sec., beginning from the altitude 160 km and down to 40 km lower than the Sun disk brim. During the next 12 sec. the radiometer's line of observation returns to its initial position. During these 24 sec. of satellite orbit, the observation instrumentation covers 140 km of the atmospheric extent along the limb; the vertical and horizontal spatial resolution of the apparatus comprises 2-4 and 20-30 km, respectively. The objective of the LIMS experiment was to map the vertical profiles of temperature and the concentration of ozone, water vapour, nitrogen dioxide, and nitric acid in the lower to middle stratosphere range, with extension to the stratopause for water vapour and into the lower mesosphere for temperature and ozone.
Infrared radiances from the LIMS instrument were stored on a Radiance Archive Tape (RAT). Archive data was used to derive a series of two products: ♌ Inverted Profile Archival Tape. This data set contains corrected IR radiance profiles, inverted daily profiles of temperature, and mixing ratios for ozone, water vapour, nitric acid, and nitrogen dioxide, all as a function of pressure. In addition, earth location, time, cloud-top, and housekeeping information are included. ♌ Map Archival Tape. This data set contains daily global maps of 6 atmospheric parameters (temperature, ozone, nitrogen dioxide, water vapour, nitric acid, and geopotential height) derived from radiance measurements.
The Shuttle Ozone Limb Sounding Experiment (SOLSE) flied on the Space Shuttle (STS-87, 1997) to demonstrate that vertical ozone profiles can be measured using light scattered at the earth's "limb" (i.e. the horizon at very high altitudes). The objective of the SOLSE is to determine the altitude distribution of ozone in an attempt to understand its behavior so that quantitative changes in the composition of our atmosphere can be predicted. SOLSE is intended to perform ozone distribution that a nadir instrument can achieve. This will be performed using Charged Couple Device (CCD) technology to eliminate moving parts in a simpler, low cost ozone mapping instrument. Instrumentation includes an Ultraviolet (UV) spectrograph with a CCD
array detector and visible light cameras, calibration lamp, optics and baffling. On orbit a crewmembers activated SOLSE which perform limb and Earth viewing observations. Limb observations focus on the region 20 km to 50km altitude above the horizon for the Earth's surface. SOLSE Earth viewing observations enable to correlate the data with other nadir viewing, ozone instruments.
The use of the microwave frequencies in limb observations reduces limitations related to the lower altitudes of atmosphere probing. Such opportunities are explained by lower attenuation of microwaves in aerosols and water vapour (clouds) in comparison with other frequency bands. Earth research with the help of limb methods and development of space based apparatus continue to play a significant role in unraveling the mysteries of ozone in the atmosphere.
8. Lidar Sounding of Atmospheric Ozone Nowadays LIDAR has become the synonym of laser radar (locator). It was the advent of the laser that turned optical radars into such effective instrumentation. Recent accomplishments in the fields of laser technology, computerization, and methods of processing signals allowed the development of satellite lidar systems for measuring concentrations of "small" gas components and ozone in the first place. Active remote sensing with the help of lidars unveils some unique opportunities, unattainable by passive systems. One such possibility - improvement of vertical resolution in nadir sounding, thanks to short pulse duration and precise master time attachments of reflected signals.
Another peculiarity of lidar systems is relevant to their spectral characteristics. Stable laser sources emitting close to some lines of absorption are perfect in meeting the demands necessary, to determine the molecular components of the atmosphere, using the method of lidar differential absorption, considered to be one of the simplest and reliable techniques in optical atmospheric research. The method has much in common with the method of differential absorption of UV radiation described above. It is based on the independent reception of lidar signals on two close wavelengths, one of which is in the absorption band of the substance, and the other is not.
Backscattered radiation is used to assess the relative attenuation in the substance with high distance resolution along the sounding track. Since practically direct calculations of substance density are used in this case, the precision of the lidar method is substantially worse than in the case, when reciprocal problems are solved on the basis of passive spectral nadir measurements.
Fig. 14. Structure of the Lidar system In Fig. 14 we can see an arrangement of a differential absorbing lidar, designed in the USA for remote sounding of ozone in the lower atmosphere. The lidar consists of two large subsystems - for radiating and for reception. The radiating subsystem includes a pumping laser on a crystal of Yttrium-Aluminium Granate with implanted ions of Neodymium 2 and dye-based lasers 1; the receiving subsystem comprises a telescope 3, radiation receiver block 4 and an information processing unit 5. One of the dyelasers with a frequency multiplier works on a wavelength of 0.286 µm, which is readily absorbed by ozone; the other emits on a wavelength of 0.3 µm used as a master-frequency. Dye-lasers on wavelengths of 0.6 and 0.582 µm also emit on basic harmonics. For sounding the aerosol profile a basic harmonic of a pumping laser (λ=1.064 µm) is used. Reception of backscattered laser radiation is effected by a telescope having an input diameter of 30 cm. Photoelectric multipliers are used as radiation receivers. The field of view of the receiving subsystem is 5′.
The differential absorption lidar system installed on the DC-8 aircraft was repeatedly used to measure the ozone profiles in the vicinity of the ozone hole over Antarctica. As early as 1987 ozone profiles were identified for altitudes of 10 to 20 km above sea level, inside the polar vortex and beyond it. The vertical resolution was 500m.
9. Monitoring Instruments Present State and Trends Concerns about the atmospheric ozone layer during the past thirty years have led to the development and application of several satellite-borne sensors for observing the global distribution of ozone in the stratosphere. Amongst the different techniques (solar backscattered experiments, limb-scanning apparatus, atmospheric emission methods, etc.) measurements from space of Solar Backscattered Ultra Violet (SBUV) radiation in the nadir direction is more widespread. The satellite measurements of backscattered solar ultraviolet radiation has been used to produce data sets of global total ozone and ozone profiles from TOMS and SBUV instruments on Nimbus, NOAA, Meteor satellites, Space Shuttle, GOME on board the ERS satellite. In spite of all their advantages, these instruments are complicated optic-electronic devices, bulky, heavy (about 50 kg) and great power consuming (>50W), and they can be installed and operated only within large space complexes (several tons). Expenses borne on such ozone experiments amount to dozens of million dollars.
Main instruments intended for measurements of the atmosphere ozone from space are presented in the following table. There are devices both developed, in flight operation and future planning. The name of spacecraft, on which the instrument is allocated, its launch date, and some remarks about instrument performance and experience also present in this concluding table. Table Survey of major spaceborne instruments for ozone monitoring Instrument BUV (Backscatter Ultraviolet Spectrometer)
Platform
Nimbus-4, AE-E (Atmosphere Explorer-E) SSH (Infrared Spectrometer), DSMP (Defense SSH-2 (Infrared Temperature and Meteorological Moisture Sounder) Satellite Platform) series SBUV (Solar Backscatter Nimbus-7 Ultraviolet) / TOMS (Total Ozone Mapping Spectrometer) SAGE-1 (Stratospheric Aerosol AEM-2 (Application and Gas Experiment) Explorer Mission-2)
Launch date
Comment
Apr. 8 1970 Nov. 20 1975
2 Ebert-Fastie-Type monochromators AE-E reentered on June 10, 1981
Sep 11, 1976
Starting with the F1 satellite
Oct. 24, 1978
Nadir viewing Ebert-Fastie spectrometer of TOMS. Swath width of 2700km (scanning). TOMS failed in May 1993. SBUV failed in 1990 The instrument is a sun photometer. Operations continued until Nov. 1981
Feb. 18, 1979
UVSP (Ultraviolet Spectrometer and Polarimeter)
SMM (Solar Maximum Mission)
Feb. 14, 1980
UV ozone Experiment, Airglow Instrument, Solar UV Monitor
SME (Solar Mesosphere Explorer) EXOS-C (Exospheric Observations-C) NOAA-9 (National Oceanic and Atmospheric Administration-9), NOAA-11 Space Transport Systems: STS-34, -41, -43, -45, -56, -62, -66 and STS-72 Meteor 3-6 (Russia)
Oct. 6, 1981
UARS (Upper Atmospheric Research Satellite)
Sep, 13, 1991
Sun occultation method (HALOE) Heterodyne limb sounder (MLS) Atmospheric Infrared Emission (CLAES) Atmospheric Emission limb-sounding instrument (ISAMS)
SPOT-3 (Systeme Pour l`Observation de la Terre) ERS-2 (European Remote-Sensing Satellite-2) Priroda
Sep.26, 1993
SPOT-3 entered safehold Nov, 97; solar occultation through the Earth's atmospheric limb
Apr. 21, 1995
Differential optical absorption spectroscopy) measurement concept
Apr, 23. 1996
Module of the MIR station
July 2, 1996
Operational as of 2000
UV spectrometer
SBUV-2 (Solar Backscatter Ultraviolet-2)
SSBUV (Shuttle Solar Backscatter Ultraviolet)
TOMS HALOE (Halogen Occultation Experiment), MLS (Microwave Limb Sounder), CLAES (Cryogenic Limb Array Etalon Spectrometer), ISAMS (Improved Stratospheric and Mesospheric Sounder) POAM-II (Polar Ozone and Aerosol Measurement-II) GOME (Global Ozone Monitoring Experiment) Ozon-M TOMS TOMS, RIS (Retroreflector in Space) MAHRSI (Middle Atmospheric High Resolution Spectrograph Investigation)
TOMS-EP (Earth Probe) ADEOS (Advanced Earth Observation Sattellite) CRISTA-SPAS-2 (Cryogenic Infrared Spectrometer and Telescopes for the Atmosphere) SPOT-4
Dec. 12,1984,
Occultation measurements make possible a mapping of ozone concentrations at altitudes of 5075 km until March 1989 All three instruments are Ebert-Fastie grating spectrometer for mesosphere ozone study. The satellite was operational until April 1989 Nadir Observation of backscattered UV to obtain ozone profiles in altitude of 25-60 km. The mission ended in 1987 Satellite service ended Aug. 1995,
Sep. 24, 1988
Satellite service ended Sep. 1994
Oct. 19, 1989
Coincident observations with SBUV-2 on NOAA-9 and NOAA-11
Feb. 14, 1984
Aug. 15, 1991 TOMS operation until Dec. 1994
Aug. 17, 1996 ADEOS failed on June 30, 1997
Aug. 7-19, 1997
Platform took place on Shuttle flight STS-85
POAM-III (Polar Ozone and Aerosol Measurement-III) OLME (Ozone Layer Monitoring FASat-Bravo Experiment) (Fuerza Aerea Satellite) OM-2 (Ozone Meter-2) Techsat/Gurwin II
Mar. 24, 1998 Operational
OSIRIS (Optical Spectrograph and Infrared Imaging System) TOMS-5
ODIN QuikTOMS
July 10, 1998
Total column ozone measurements
July 10, 1998
Total ozone, ozone profile measurements
Feb. 20, 2001
Detection of aerosols and trace gases Launcher fails in Sep. 2001
GOMOS (Global Ozone Monitoring by Occultation of Stars), SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Cartography) SAGE-III (Stratospheric Aerosol and Gas Experiment III) GLI (Global Imager), ILAS-II (Improved Limb Atmospheric Spectrometer-II)
Envisat (Environmental Satellite)
2002
UV/Visible/Near-infrared limb viewing grating spectrometer; Star occultation measurement method, differential optical absorption spectroscopy and backscatter UV of solar and lunar radiation
Meteor-3M-1
Dec. 2001
Self calibrating solar and lunar occultation
ADEOS-II 2002 (Advanced Earth Observation Sattellite-II) ACE-FTS (Atmospheric SCISAT-1/ACE 2002 Chemistry Experiment-Fourier (Science Satellite-1/ Transform Spectrometer) Atmospheric Chemistry Experiment) HIRDLS (High Resolution Aura mission 2003 Dynamics Limb Sounder), EOS/CHEM (Earth MLS (Microwave Limb Sounder), Observing System / TES (Tropospheric Emission Chemistry) Spectrometer), OMI (Ozone Monitoring Instrument) EPIC (Earth Polichromatic Triana 2004 Imaging Camera) GOME-2 (Global Ozone MetOp-1 2005 Monitoring Experiment-2), IASI (Meteorology (Improved Atmospheric Sounder Operational-1) Interferometer) ODUS (Ozone Dynamics GCOM-A1 (Global planned 2006 Ultraviolet Spectrometer), Change Observation SOFIS (Solar –Occultation Mission-A1) Fourier Transform Spectrometer for Inclined-orbit Satellite) OMPS (Ozone Mapping and NPOESS (National planned 2006 Profiler Suite) Polar-orbiting Operational Environmental Satellite System)
Observation from the near UV to the near IR. Solar occultation measurement of polar stratospheric ozone Instruments is a classical sweeping Michelson interferometer looking at the sun through the atmosphere
Infrared limb sounder, Measurements of limb thermal emission, Infrared imaging Fourier transform, Hyperspectral capabilities
View from Lagrangian point on sunlit Earth from sunrise to sunset at constant scattering angle Ozone total amounts and profiles , Fourier Transform nadir-viewing imaging interferometer operating in the thermal infrared spectrum Hight precision nadir-viewing grating spectrometer, Solar occultation measurements with high vertical resolution Instrument includes the nadir wide-field sensor and limb-viewing sensor suite
The current tendency of lowering project costs, on one hand, and miniaturizing space equipment, on the other hand, has led to the development of microsatellite technology and the respective devices. Due to their simplicity and low cost, small satellites are becoming very attractive for Earth observation experiments. However, small satellites have substantial limitations in terms of incorporating sophisticated, specific payloads. These restrictions call for a new class of advanced instrumentation and systems, specifically designed for application in small satellites. For example, NASA is developing an innovative, relatively lightweight, low-cost ozone instrument for ′s
next-generation objectives in Earth science. This compact ozone-monitoring instrument optimized for combined absorptive and refractive stellar occultation techniques. Spectral measurements as the star sets deeper into the atmosphere are diagnostic the atmospheric composition, and constituent profiles may be determined from the relative transmission (i.e., the ratio of occulted to unocculted spectra). As a result, extinctive occultation measurements are self-calibrating and ideal for long term trend monitoring. Instrument capable flying on a variety of spacecraft platforms for ozone and climate studies in the 21st century.
The Solar Backscattered Ultra Violet radiation measurement series followed the launch in July 1998 of the Israeli TechSat microsatellite with the Ozone Meter on board. This new instrument also corresponds to the above-mentioned cost effective methodologies. Small mass, low power consumption and acceptable dimensions of this instrument allowed its incorporation into the microsatellite. The instrument is designed to measure the vertical distribution of ozone and total ozone content in the column beneath the satellite.
Fig. 15. Small Ozonometer that successfully operates on the Israeli Techsat microsatellite The operating principle of the Ozonometer is based on a filterwheel-photometer that measures the SBUV radiance in the 252.0-340.0 nm spectral region in 7 specific, narrow (1nm) wavebands. The instrument's optical head with weight only 1.7kg (Fig. 15) is mounted so as to look in the nadir direction with a maximum view of
12°x3º. As the satellite moves in its sunsynchronous, retrograde orbit, the instantaneous field of view traces swaths on the ground approximately 170km wide. The statistical method was applied to obtain ozone data from both the measured radiation and a priori information.
Successful flight tests of this "Small SBUV" Ozone Meter: displayed that it can capture large scale variations of ozone over a broad range of atmospheric conditions (0.3-100mb) with errors ranging from 10 to 20% for derived profile, and from 5 to 10% for total ozone. Comparison of the Ozone Meter with other space based ozone instruments shows that instrument has very small weight and size, low power consumption, and its measurement capabilities (accuracy, spatial resolution, etc.) approach those of instruments that are more complicated in design.
The FASAT-Bravo microsatellite was launched for the Chilean Air Force. It is a space science and technology demonstration mission. The primary science instrument is the Ozone UV Backscatter Instruments (OUBI). The instrument relies on measurement of UV backscatter in 4 bands over the sunlit part of the orbit. The ratios of backscatter in bands are a guide to ozone levels in the upper atmosphere, and the photodiode channels permit regular in-orbit calibration. The instrument is constructed using low cost techniques based on existing SSTL camera hardware, and simple dye based filters are employed and applied to conventional array CCD's. The two cameras are tuned to 380 and 313nm and cover an area of 560x400km each at a ground sampling distance of 1.4km. By processing the differences between the two images, an instantaneous ozone concentration images can be collected. The instrument objective power is 0.5 W for continuous operation. OUBI was carefully calibrated at NASA facilities and its measurement results compare closely with the data from the TOMS-EP mission.
Fig. 16. Earth-Satellite-Earth laser beam absorption experiments Advanced Earth Observing Satellite (ADEOS) is the international space platform dedicated to Earth environmental research developed and managed by the National Space Development Agency of Japan (NASDA). The objective of ADEOS is to contribute to elucidation of phenomena of the earth system through integrated observation using a number of sensors. One from main observed parameters is threedimensional distribution of atmospheric ozone. . Measurements of ozone, CO2, CH4, CFC-12, etc. are carried out using infrared pulsed lasers on the ground and retroreflector in space observation concept (Fig. 16). The retroreflector located on the satellite face panel is used in long-path (Earth-Satellite-Earth) laser absorption experiment. RIS has a corner-cube structure with an effective diameter of 50 cm, reflectivity of 0.8, and divergence of reflected beam 60urad. ADEOS was launched successfully on August 17, 1996. It provided a large volume of data containing valuable information about our environment atmosphere, ocean and land for about 10 months until it suddenly got out of control because of the structural damage in its solar array paddle.
The Optical Spectrograph and Infrared Imager System, known as Osiris, is flying on the Swedish-led Odin small satellite, launched on Feb. 20, 2001, and is providing scientists with unique set of data on ozone depletion. Osiris produces daily maps that detail vertical ozone concentrations above Earth’s surface at 1.5 km intervals.
Comparison Osiris’ data with readings taken by other spacecraft and ground-based instruments confirms high accuracy of Osiris ozone concentration measurements.
The principal goal of the Atmospheric Chemistry Experiment Mission of the Canadian Space Agency is planned in mid-2002 to measure and to understand the chemical and dynamical processes that control the distribution of ozone in the upper troposphere and stratosphere. A comprehensive set of simultaneous measurements of trace gases, thin clouds, aerosols and temperature will be made from low earth orbit. Measurements will be taken during solar occultation as the sun's light passes through the various layers of the atmosphere. The space instrument is a UV and visible array detector spectrometer designed to measure the attenuation of the solar beam through the atmosphere as viewed from the spacecraft at sunrise and sunset and make measurements of solar radiation scattered back into space from the surface of the earth and the earth's atmosphere. From these measurements in the wavelength range of 2851000 nm at a resolution of 1-2 nm, scientific information about the aerosol and ozone profiles of the atmosphere can be deduced. The concentrations of more than 30 molecules will be measured as a function of altitude.
Science Instruments Triana is the first Earth observing mission to travel to Lagrange1, or L1 (the neutral gravity point between the Sun and the Earth). From L1, Triana will have a continuous view of the Sun-lit side of the Earth at a distance of 1.5 million kilometers. In order to obtain the same coverage with current Earth-observing satellites in low Earth orbits and geostationary orbits, scientists must manipulate, calibrate, and correlate data from four or more independent satellites. The full view of the Sun-lit disk of the Earth, afforded by the L1 location, has tremendous potential for Earth science. One from Triana tasks is research ozone hole evolution using ozone concentration data.
Fig. 17a. EPIC – Earth Polychromatic Imaging Camera [26]
Fig. 17b. Triana Observatory [26] The Triana contains the science instruments, Spacecraft Bus, and subsystems required to operate the mission and process the data that it sends and receives (Fig. 17a). Its Earth Polychromatic Imaging Camera (EPIC) views the entire sunlit Earth, from sunrise to sunset, in 10 different wavelengths ranging from the Ultraviolet to the near infrared (see Fig. 17b). The use of 10 precision filters and an accurately calibrated camera permit us to measure key science products that are of interest to the science community, the educational institutions, and to the general public. Triana will get total daily coverage using the 317, 325, and 340 nm channels, measure ozone levels across
the globe with a resolution of 8 km, and monitor the changes in ozone levels through the course of each day. In addition, Triana has the 605 nm channel (Chappuis ozone band) that can be combined with the UV channels to produce good data on ozone give coverage at high solar zenith angles.
10.The Antarctic Ozone Anomaly Our readers probably expect to find answers in this booklet to exciting questions relevant to the so-called ozone hole. We shall not disappoint them, and will, without delay, consider basic experimental data, confirming the emergence of an ozone deficit, in the South Polar Region during the last twenty years; we will also discuss the main hypotheses concerning the origin of this phenomenon.
A drastic fall in the total ozone content (disclosed in the Antarctic after 1979) has been registered annually in October, i.e. during the Antarctic spring, and some fluctuations in the effect have been noticed. Ozone specialists did not expect this phenomenon and, initially, attempts were made to explain it by inadequacy of measurement precision. However, regular measurements of ozone that followed, both from satellites and ground based networks, left no doubt about the objectivity of observations. The expanse with an anomalous low concentration of ozone is approximately equal to the area experiencing polar night. The total ozone content in October (annually, beginning from 1979) decreased, and in some places the drop had reached 50% of the nominal value. At the same time, the boundaries of the ozone hole shifted and spread beyond the perimeter of the continent in the direction of the populated areas of Australia, South America, and Africa. Like an infection that grows more and more virulent, the continent-size hole in Earth's ozone layer keeps getting bigger and bigger. Its size steadily increased, covering a polar area of several million square kilometers. Some years saw a relative maximum of the total ozone (TO) content in the center of the ozone hole, surrounded by an area having lower concentration of ozone, while other years did not evidence any substantial drop of concentration during the spring. The dynamics of the hole during October of each year shows that the hole is, in essence, a huge vortex in the atmosphere. During the months following October the ozone layer is gradually reinstated and returns to its normal condition. Thus, speaking about the ozone hole, one implicates a large area over the Antarctic where the concentration of ozone is diminishing. In same period, for the Northern Hemisphere a negative anomaly of the TO at the end of the winter months
also was observed. Less dramatic, but still significant, depletion of ozone level has been recorded around the globe.
Fig. 18. Daily estimates of Antarctic ozone hole area Image on Fig. 18 shows how change the size and depth of Southern Hemisphere ozone hole. The ozone hole area is defined as the size of the region with total ozone below 220 Dobson units. The daily ozone hole area estimates for last years 1999 and 2000 (solid lines) compare with the entire climatology (grey shaded) and with the climatological mean (white curve). The maximum hole size observed in 2000 is approximately 28 million square kilometers, while in 1999, the maximum was 25 million square kilometers.
How drastically could the formation of the ozone hole influence our planet? Should the hole appear in the equatorial region there would emerge a biological effect caused by the decrease of protection against ultraviolet radiation; what detrimental effect could this have on live organisms? In the vicinity of the pole, after the polar night, when the hole appears, the sun's rays reach the Earth's surface under a very low angle, and are, therefore, less dangerous. It seems that there is no immediate threat to personnel, taking turns at scientific stations. However, the appearance of the ozone hole gave an impetus to research dealing with the impact of ultraviolet radiation on
the inhabitants of the ocean and plankton in the Antarctic region. Observations also showed that there was a strong correlation between the drop of ozone concentration, especially significant in the 10 to 25 km layer, and the lowering of temperature in this layer. Such interference in atmospheric thermal processes can lead to climatic changes.
The ecological situation brought about by short term depletion of the ozone layer in the Antarctic would not have been so dramatic, had our knowledge been adequate to understand the laws, regulating the formation and disappearance of the ozone hole, and thus allowing us to forecast the sequence of expected events. Regrettably, the annual advent of a spring minimum in the Antarctic ozone content has not yet been unequivocally explained. The absence of a common understanding of events in the Antarctic gives rise to alarm and anxiety of the population. Until the reasons behind the formation of the ozone hole will have been fully understood, it will be impossible to predict the consequences, both in terms of existence of biological objects on Earth and the latter's climate. Let us consider the probable mechanisms of ozone formation.
According to one of them, ozone depletion is brought about by the increase of nitrogen oxides, in its turn, influenced by 11 years cyclic solar activity (sunspot cycles). During the maximum of solar activity, a 30-60% growth of nitrogen oxide concentration in the mesosphere was observed in the southern hemisphere. Later on, a transfer of these oxides into the lower levels of the stratosphere was observed during the polar winter. As we know, photochemical reactions of the "nitrogen" cycle together with the contribution of the nitrogen oxides cause the destruction of ozone, and that leads to the formation of the ozone hole. Such a mechanism can realistically explain the shaping of the ozone hole. However, variation in UV radiation with sunspot cycles contributes to ozone production only account for 2-4% of the total variation in ozone concentrations. There are at least three questions, which remain unanswered in the framework of this mechanism. In first, why the ozone hole didn't change with solar activity cyclic, but vary in size year after year? Second, why didn't an ozone hole appear during previous 11-year cycles of solar activity? Third, why did the ozone hole evolve only in the Southern hemisphere?
Another possible mechanism associates the formation of the ozone hole with a "chloride" cycle of anthropogenic origin. One of the photochemical reactions involving chloride, bringing about the destruction of ozone, was examined in one of the previous parts of this booklet. The mechanism, related to reactions of the chloride cycle, presumes an input of chlorine compounds into the polar stratosphere via atmospheric circulation; compounds destroying ozone are "fed" into the atmosphere from the Earth's surface by millions of aerosol packages, domestic refrigerators, and emissions of chemical plants, etc. Despite the fact that anthropogenic activities have not, as yet, caused a substantial decrease of total ozone content in the atmosphere, freons could be responsible for the destruction of ozone over the Antarctic - such is the opinion of a large group of scientists. Here there is also an unanswered question: why didn't the mechanism driven by anthropogenic activity manifest itself so strongly in the Northern hemisphere, where the supply of chloride, bromide and other compounds, destroying ozone, is far more intensive?
Finally, a third possible mechanism - the so-called dynamic mechanism, tries to explain the formation of the ozone hole purely by circulation processes in the stratosphere and the mesosphere, and a horizontal redistribution of ozone. Ozone loss is accelerated over the frozen continent because the Antarctic stratosphere contains cloud particles not normally present in warmer climes. These icy particles have a critical effect on the chlorine and bromine pollution floating in the stratosphere. Normally, the chlorine and bromine are largely locked into "safe" compounds that cannot harm ozone, but the ice particles transform them into destructive chemicals that can break apart ozone molecules with amazing efficiency. With very little known about the Antarctic ozone losses, atmospheric researchers could not tell which theory was correct.
Despite uncertainty about the Antarctic phenomenon's cause, scientists firmly believed halocarbons would eventually deplete the global ozone shield. Their certainty and the jarring unexpectedness of the ozone hole's appearance motivated countries to act. In September 1987, diplomats from around the world met in Montreal and forged a treaty unprecedented in the history of international negotiations. Environmental ministers from 24 nations, representing most of the industrialized world, agreed to set sharp
limits on the use of CFCs and Halons. According to the treaty, by mid-1989 countries would freeze their production and use of halocarbons at 1986 levels. Then over the next ten years, they would cut CFC production and use in half. The Montreal Protocol provides a dramatic example of science in the service of humankind. By quickly piecing together the ozone puzzle, atmospheric researchers revealed the true danger of halocarbons, allowing world leaders to take decisive action to protect the ozone layer.
30 25 Area of North America
Size (million sq km)
20 15 Area of Antarctica
10
Ozone values <220 DU
5
Average within period 7.09-13.10 Vertical lines are minimum & maximum area
19 8 19 1 8 19 2 8 19 3 8 19 4 8 19 5 8 19 6 8 19 7 8 19 8 8 19 9 9 19 0 9 19 1 9 19 2 9 19 3 9 19 4 9 19 5 9 19 6 9 19 7 9 19 8 9 20 9 0 20 0 01
0
Year
Fig. 19. Average area of Antarctic ozone hole during 1981-2001 years Southern Hemisphere Winter Summary of the observed ozone variation during 19802001 years is presented on Fig. 19. The area covered by extremely low total ozone values of less than 220 Dobson Units, is defined as the "ozone hole area" (prior to the springtime period in Antarctica, when ozone depletion occurs, the normal ozone reading is around 275 Dobson units). For each year average in September-October (07.09-13.10) areas of Antarctic ozone hole are shown together with estimation of maximum and minimum areas in same period. For some last years (2000-2001) the size of the ozone hole remained large in September and early October, but rapidly decreased in size and ended in mid-November, the earliest in the last 10 years. In other words, last years the ozone hole was of record size, but it formed very early and then collapsed quickly (Fig. 18). Average October anomalies up to 40 percent below the 1979-1986 base period were observed over the South Atlantic Ocean and Antarctica,
with negative anomalies of more than 10 percent also observed over southern South America. Vertical profiles of ozone amounts over the South Pole, at the end of September and early October for last years, showed complete destruction of ozone in the 15-20 km region, similar to values observed during other recent years. Total column ozone over the South Pole reached a minimum reading of 100 Dobson units on Sept. 28, 2001. The minimum total ozone value of 98 Dobson Units, observed on Sep. 29, 2000, was not as low as the 90 DU value observed in 1999, or the record low value of 86 DU observed in 1993. Lower stratosphere temperatures over the Antarctic region in these years were again low. Temperatures lower than -78째C occurred over a large region, and were sufficiently low for formation of polar stratospheric clouds, and for enhanced ozone destruction to proceed. The rate of decline in stratospheric ozone at midlatitudes has slowed during the 1990s. The fact that ozone depletion appears to have stabilized supports the conclusion that international actions are working well to reduce the use and release of ozone depleting substances.
2001 satellite data show the area of this year's Antarctic ozone hole peaked at about 26 million square kilometers - roughly the size of North America - making the hole similar in size to those of the past three years. Researchers have observed a levelingoff of the hole size and predict a slow recovery. Over the past several years the annual ozone hole over Antarctica has remained about the same in both its size and in the thickness of the ozone layer. This is consistent with human-produced chlorine compounds that destroy ozone reaching their peak concentrations in the atmosphere, leveling off, and now beginning a very slow decline. However, chemicals already in the atmosphere are expected to continue ozone depletion for years to come. The severity of the ozone depletion within the hole reached about the same levels as the past few years and the highly depleted region filled about three-fourths of the Antarctic polar vortex. In 2001 the vortex has been more stable and somewhat colder than average. Year-to-year fluctuations in the geographical size of the polar vortex and the size of the region with low temperatures will alter the size of the ozone hole over the next decade during the period that levels of ozone-destroying chemicals in the atmosphere begin a slow decline In the near future - barring unusual events such as explosive volcanic eruptions - the severity of the ozone hole will likely remain similar to what has been seen in recent years, with year-to-year differences associated with
meteorological variability. Over the longer term (30-50 years) the severity of the ozone hole in Antarctica is expected to decrease as chlorine levels in the atmosphere decline. Recovery could be expected with international adherence to the Montreal Protocol and its amendments banning and/or limiting substances that deplete the ozone layer. Changing atmospheric conditions and natural ozone variability complicate the task of detecting the start of the ozone layer recovery. Only over the middle latitudes in both the Northern and Southern Hemispheres has the ozone decline recently slowed. Based on an analysis of 10 years of South Pole ozone vertical profile measurements, scientists-optimists estimated that recovery of the Antarctic ozone hole might be conclusively detected as early as the year 2008. A full explanation of ozone and temperature anomalies must include all aspects of ozone photochemistry and meteorological dynamics. Continued monitoring and measurements including total ozone and its vertical profile are essential toward this end.
Fig. 20. February and March average area of Arctic low zone (<300 DU) of each year from 1979 to 2001 It has already been mentioned that initial signs of ozone concentration decrease in the Northern hemisphere have been observed. It should be noted that atmospheric activity in the stratosphere of the two hemispheres is substantially different. In the Northern hemisphere the average temperature is higher, and exchange between the polar region
and the mid-latitudes is more extensive. The destruction of the polar vortex occurs earlier in the Northern polar zone and this poses limitations to the effectiveness of photochemical reactions taking place in the vortex at low temperatures. Due to the weaker circulation of the vortex in arctic latitudes, in comparison with the stable circulation of the vortex surrounding the Antarctic, the amount of impurities entering the Northern subpolar region with the air currents, is less than that entering the Southern areas.
Fig. 20 shows the average area, during February and March for each year since 1979, of low ozone (lower than 300 DU). For 2001, the average area of low total ozone was smaller than for the previous year, and among the smallest values of all the years. For the Northern Hemisphere winter and spring of 2000-2001, total ozone values observed over the Arctic region were substantially higher than average. The conditions in the Arctic region in year 2001 are in contrast to conditions during 2000, when total ozone in the Arctic region was below the average. Lower stratosphere ozone destruction is strong when meteorological conditions of a strong polar vortex and cold polar temperatures prevail. High total ozone values in the Arctic region in the winter and spring of 2000-2001 are attributed to meteorological conditions which were not favorable for ozone destruction, even with the continued presence of ozone destroying chemicals in the stratosphere. Total ozone declined over mid-latitudes of the Northern Hemisphere at the rate of about 2 to 4 percent per decade from 1979 to 1993. In recent years the strong rate of decline of Northern Hemisphere total ozone has not continued, but current stratospheric ozone amounts continue to be below the amounts measured before the early 1980s.
New research confirms that giant atmospheric waves, called "planetary-scale waves," or "long waves," warm the stratosphere and act to heat polar air. Strong planetary waves form in the Northern hemisphere by such land features as high mountains (Himalayan plateau), they warm the Arctic stratosphere and suppress ozone destruction. Land shapes in the Southern hemisphere also produce planetary waves, but they tend to be weaker because there are fewer tall mountain ranges and more open ocean around Antarctica. The scientists long-ago recognized this connection but it have only now quantified by satellite and meteorological data linking planetary
waves to bursts of warming registered in the Arctic. The planetary waves can warm the polar region on 5o- 10o C and polar stratosphere temperatures in this case typically lie in range -73o to -63o C. Of course, as soon as the waves have dissipated, the polar region begins to cool down again. Polar stratospheric clouds form when temperatures in the stratosphere become extremely cold (below -78째 C). Polar stratospheric clouds are trouble for ozone since tiny ice crystals and droplets within the clouds provide surfaces where chlorofluorocarbons are converted into ozone-destroying molecules. Due to planetary waves the polar stratospheric clouds are common in Antarctica, but a rare sight in the Arctic.
Fig. 21. Antarctic polar stratospheric clouds those are dangerous for ozone [27] Indeed, planetary waves in the Northern hemisphere don't always heat the stratosphere enough to prevent substantial ozone destruction. In 1997, for example, the waves were weak because of capricious weather. That triggered a rare springtime ozone hole over the Arctic. Scientists are concerned that according the climate changes we would expect lower ozone values across the Arctic during this century. On the other hand the chlorine and other ozone destroying chemicals in the lower atmosphere, for example, peaked in 1994 and have since declined. Computer simulations show that ozonedestroying pollutants in the high stratosphere could return to pre-1980 levels in 30 to 50 years. Because climate change occurs on similar time scales, it's difficult to say which trend would dominate: the cooling of the stratosphere, which would encourage an Arctic ozone hole, or the decline of ozone-destroying molecules, which would suppress it. With the aid of Earth-watching satellites and ever-improving computer climate models, scientists hope to unravel the puzzle of Arctic ozone before it becomes a problem. After all, one planetary ozone hole is more than enough!
According to last (October, 2002) joint report, which was prepared by United Nations Organization (Environment Department) and the World Meteorological Organization, the level of ozone-depleting chlorine in the atmosphere was declining because of the ban on the use of chlorofluorocarbons (CFCs) in fridges and air conditioners, agreed under the Montreal Protocol. Under this global protocol, developing countries committed themselves to halving consumption and production of CFCs by 2005 and achieving an 85 per cent cut by 2007. The report asserts that after the ban was adopted, the atmospheric level of chlorine continued to rise, peaking in 2000; since then, the level had stabilized and was now declining, albeit slowly. Scientists note that the hole in the ozone layer was first detected nearly 30 years ago, when the hole size was equal to Australian continent; then hole grew three times the size of Australia. The researchers declare that now the hole over Antarctica is about to start shrinking and will close by 2050. They predict that the hole in the ozone layer would contract steadily from about 2005 and disappear by mid-century, although the ozone would be vulnerable for a decade.
This optimistic estimation of the ozone layer evolution is agreed with top story on Antarctic holeâ&#x20AC;&#x2122;s progression. Scientists from NASA and the Commerce Department's National Oceanic and Atmospheric Administration (NOAA) have confirmed the ozone hole over the Antarctic this September is not only much smaller than it was in 2000 and 2001, but has split into two separate "holes" (see Fig. 22).
Estimates for the last two weeks of the size of the Antarctic Ozone Hole (the region with total column ozone below 220 Dobson Units), from the NASA Earth Probe Total Ozone Mapping Spectrometer (EPTOMS) and the NOAA-16 Solar Backscatter Ultraviolet instrument (SBUV/2), are around 15 million square kilometers. The researchers stressed the smaller hole is due to this year's peculiar stratospheric weather patterns and that a single year's unusual pattern does not make a long-term trend. Moreover, the data are not conclusive that the ozone layer is recovering. This year warmer-than-normal temperatures around the edge of the polar vortex that forms annually in the stratosphere over Antarctica are responsible for the smaller ozone loss.
Fig. 22. Comparison of the first split ozone hole on record and the Antarctic ozone hole at the same time one year earlier [28] In 2001, the ozone layer thinning over Antarctica reached 26.5 million square kilometers, larger than the size of the entire North American continent. Due to higher Antarctic winter temperatures, the 2002 hole seems to be about 40% smaller.
So, humanity shares the alarm and anxiety related to changes of atmospheric ozone concentration. The quest for a viable answer to the question posed by nature gave rise to a spectrum of opinions concerning the mechanism leading to the formation of the ozone hole, and the outcome facing our planet: beginning with total complacency and ending with predictions of ozone catastrophe. What actually lies between these extreme points of view - objective truth or a new problem? Further research should lead to the correct answer.
11.Conclusion A variety of satellite methods can be used to obtain substantial masses of information relevant to ozonosphere characteristics, necessary for understanding a sophisticated complex of dynamic, photochemical, and radiation processes, analyzing natural and anthropogenic disturbances, as well as for the investigation of temporary changes in the state of the ozonosphere. Of course, shifting only the ozonometric apparatus to space will not in itself solve the ecological problem of ozone; however it can substantially help in enhancing data adequacy and improving the quality of ozonosphere observation. In comparison with on-the-ground observations, remote methods of satellite meteorology provide the coverage of vast expanses of the Earth's atmosphere with a periodicity and spatial resolution not attainable by other means. In addition, remote methods do not have any influence on the characteristics of the atmospheric volume being investigated, even in the case of lidar methods (if the radiated laser power does not exceed a certain critical value).
In the previous chapters a whole series of satellite methods for ozone control was considered and their high effectiveness was demonstrated. The following question could arise: why so many methods of measurement, and such a diversity of instrumentation? The crux of the matter is that each of the methods described does not only have indisputable advantages, but also specific drawbacks. These were characterized in the course of apparatus description. For example, measurements, using certain methods can be conducted at a specific time, only above the terminator. Some of the methods cannot be used for control, over Polar Regions during specific periods of time. Still other methods allow the assessment of TCO only or substantially smoothed vertical distribution of ozone; Lidar-based apparatus requires excessive power supply on board the satellite, etc.
One of the principal criteria characterizing the performance of instrumentation is its concentration measurement precision. According to existing estimations, investigation and control of the ozone layer by space monitoring systems demands a precision of TCO better than several percent. Monitoring on a continuous basis, first and foremost, requires a thorough estimation of on-board measuring equipment quality. That is why
in the course of optimal long-term satellite monitoring system development, a special effort is being made to investigate the limitations, pertaining to the different methods of ozonosphere probing. Great attention is also being paid to the impact of methodical and instrumental errors, and their influence on remote measuring of ozone. Performance of apparatus in a harsh environment is also being consistently upgraded.
Satellite monitoring system with automatic means of data acquisition, processing and transmitting of observation data are essential for ozonosphere control; they should allow the detection of global or local changes in the ozone layer if and when they occur. Nonetheless, the main purpose of space monitoring of atmospheric ozone is investigating the formation, transportation and destruction of ozone, and research in the area of global meteorological, cosmic, anthropogenic impacts on the ozone layer. All this should lead to the creation of an ozonosphere behavior model, which could be used for long-term prediction of changes.
12.A Certain Philosophical Epilogue The remote sensing techniques for atmospheric ozone evaluation are based on comparison of observed radiance with the calculated radiance values of a modeled atmosphere. The method is based on measuring the outcoming radiation on a selected set of wavelengths and finding a solution of the radiative equation in terms of ozone concentration. Amongst the different techniques (solar backscattered experiments, limb-scanning apparatus, atmospheric emission methods, etc.) measurements from space of Solar Backscattered Ultra Violet (SBUV) radiation is more widespread. An inverse solution of the radiative integral transfer equation is possible due to significant narrowing of the weighting functions and the use of several of the wavelengths, backscattered at various levels in the atmosphere. Ozone values are derived from the ratio of backscattered Earth radiance to incoming solar irradiance. Ozone data are determinated from these measured albedos using the statistical inversion technique. The differences between the calculated albedos and measured albedos are then used to provide new profile values that are more consistent with both measured and calculated albedos. The computation of theoretical albedo, using successive iterations of the radiative transfer equation, requires the following a priori information: ♦ ozone absorption coefficient αλ as a function of temperature; ♦ Rayleigh scattering coefficient β λ ; ♦ standard temperature profiles; ♦ standard ozone profiles; ♦ surface pressure at the lower boundary of the atmosphere; ♦ solar zenith angle θ.
The ozone climatology used to construct the a priori information was obtained from the best available satellite, rocket, airplane and balloon ozone sound measurements. From possible mathematical solutions it is necessary to select the particular solution most likely representing physical reality. Programs developed for processing measurements, performed by the above-mentioned devices, provide adequate means for solving the problem and the construction of real profile sets and total ozone content maps, on the basis of statistical analysis of climatological data. The
information accumulated by the space-based instruments, during many years of operation, is extremely precise and sound. Results of ozone profile and total ozone measurements filed in archives are a good basis for control of the atmospheric ozone evolution, also for calibration and validation of novel space instruments being developed for ozone monitoring.
It is known, that the very existence of the objects, available for observations, as well as of the observer himself, imposes some limitations both on the feasible set of the laws of nature, and on their realization. These limitations find their expression in the so-called Anthropic Principle, which can be briefly formulated as follows: The Universe exists just to enable man's living in it.
Nowadays, the progress in natural sciences made it clear, that the physical peculiarities of the Nature (laws, constants) are adjusted in such a way that makes the life possible. Lately it was noticed that both the physical conditions, and the constants of the Universe are created such as to be prerequisite for the life in our Solar system.
Yet the exact wording of the Anthropic Principle can be found in the works of the naturalists of all preceding epochs, even though in a less precise and definite ways, tinted according to the scientific conceptions of their time. Galileo, for instance, poses a question, what could happen, if the Moon had been created on the orbit closer or, conversely, more distant from the Earth. In the former case, it would cause the very strong tides, fatal to the mankind. In the latter case, the Moon, under the solar perturbations, would eventually abandon the Earth, what in its turn would lead to the deathly consequences for the life, due to the stagnant water, lack of the mixing of the salt and fresh water, etc. In other words, the people by no means can see the Moon on the orbit substantially different from the actual one. Thus the Moon's orbit, tuned just to be "correct for life", is a good illustration of the Anthropic Principle.
As another classical example of this principle, the existence of the vital ozone layer can be given. The processes in the Earth atmosphere, that enabled the creation of the ozone layer, were considered before. Simultaneous action of a number of physical and chemical laws resulted in the ozone layer arisen in the stratosphere, which role is to
control the Earth's heat balance and absorb the short-wave radiation of the Sun, lethal for the life. The protective level of the ozone layer is provided by joint action of a number of factors, viz.: the contents of the solar irradiation with respect to the magnitude and the frequency; specific concentrations of the atmosphere basic constituents; the presence of the trace gazes; physical and chemical properties of the ozone and oxygen molecules; the dynamics of the atmosphere, etc. Whatever the variation of the present conditions, it would cause the change of the ozonosphere characteristics and hence, its protective and heat-regulation properties, which, in the long run, can lead to the extermination of the life on the Earth. Consequently, in accordance with the laws, acting in the nature, this unique, life-preserving layer had emerged, to become a spectacular example of the Anthropic Principle in action.
As we've just mentioned, the tiny changes of the trace gazes abundance, when accumulated, can bring about the deterioration of the ozone layer. The protective layer proved to be very sensitive to the change of any of those conditions that prompted its creation, in compliance with the Anthropic Principle. It is pitiful, that it was the mankind itself, who provoked the ozone depletion in the atmosphere; but it is inspiring, that this phenomenon was duly noticed and explained, and the relevant international activity was institutionalized and set going. Thus, there is a hope, that the process of the ozone layer deterioration would be stopped.
13.Timeline of Atmosphere Ozone History 600 million years ago: Formation of the ozone layer. Ozone atmospheric concentration ran the amount required to shield Earth from biologically lethal UV radiation with wavelengths 200-300nm. The ozone presence enabled organisms to develop, to come out of the ocean and to live on the land. 1840: The German chemist Christian Frederick Shenbein discovered the new substance "ozone". 1920: Beginning of study of the ozone concentration in the atmosphere by ground based instrumentation. 1920: Invention of the class of chemicals known as clorofluocarbons (CFCs), which contain chlorine, flouorine, and carbon atoms in a stable structure. 1924: English scientist G. M. B. Dobson creats the earliest optical devices (photoelectric spectrophotometer) for measuring total ozone in atmosphere. 1930: British physicist Sydney Chapman described reactions of ozone dissociation into molecular oxygen and atomic oxygen, and recombination of free oxygen atoms with ozone. 1973: First identification of the atmospheric ozone destroying pollutants. 1973: Two scientists from the University of California: F. Sherwood Rowland and Mario Molina, first discovered that man-made chlorofluorocarbons (CFCs) could play a major role in the destruction of stratospheric life-protecting ozone. 1978: United States government bans CFCs used in aerosol spray cans. 1978 to 1993: Nimbus-7 longest duration of any satellite mission to measure atmospheric
ozone
by three instruments:
TOMS
(Total
Ozone Mapping
Spectrometer), SBUV (Solar Backscatter Ultraviolet), and LIMS (Limb Infrared Monitor of the Stratosphere). 1980: British team, which had measured ozone levels over the Antarctic coast, first began noticing the phenomenon of ozone abundances dropped below normal at spring over the ice-covered continent. 1984: Ozone loss of 40% is detected over Antarctica during austral spring. 1985: Satellite images show existence of an Antarctic ozone hole.
1987: Montreal Protocol, stating that there would be a 50% cut back in CFC productions by 2000, is signed. 1987: Antarctic studies find chlorine to be primary cause of ozone depletion, ozone concentrations above Antarctica fell to half their normal levels. 1988: Ozone losses of 1.7 to 3% are measured over Northern Hemisphere. 1990: International delegates meet in London to strengthen the Montreal Protocol and agree to a complete phaseout of CFCs by 2000. 1991: Upper Atmospheric Research Satellite (UARS), launched from the Space Shuttle, contains several instruments to measure important trace gases and meteorological quantities in the earth's stratosphere and mesosphere. 1991 to 1994: Russian meteorological satellite Meteor-3 with TOMS on the board was launched and operate with success. 1991: Mount Pinatubo eruptions increasing natural levels of atmospheric chlorine. 1991: Airborne Arctic Stratospheric Expedition (AASE I) studies northern vortex. 1992: Record levels of ClO, 1.5 parts per billion, are measured over Bangor, Maine. Ozone depletion rates of up to 20% are found in the Northern Hemisphere. Maximum losses of 40 to 45% discovered over Russia. 1992: Parties to the Montreal Protocol meet in Geneva and agree on a 75% reduction in CFCs by 1994 and overall phaseout by January of 1996. Production grace period, to supply CFCs for essential purposes and the needs of developing countries, is extended to 2006. 1995: Professor Paul Crutzen, Professor Mario Molina, and Professor F. Sherwood Rowland receive the Nobel Prize in Chemistry. Their pioneering science research motivated by a desire to understand nature â&#x20AC;&#x201C; leads to practical results of immense societal benefit that could not have been anticipated when the research first began. 1999 to 2001: Antarctic ozone hole's size and ozone layer thickness has stabilized. The concentrations in the stratosphere of the ozone destroying gases, curtailed under international agreements, are only now reaching their peak, due to their long persistence in atmosphere.
Ozone Devoted References and Internet Web Sates 1. 2. 3. 4. 5. 6.
7.
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Atmospheric Ozone. Edit by C. S. Zerofos and A.Ghazi., Reidel, Hingham, Mass., 1985. Ozone Measuring Instruments for the Stratosphere. Edit by W. B. Grant. Vol.1, Optical Society of America, Washington, D. C., 1989. H.J.Kramer. Observation of the Earth and Its Environment. Survey of Missions and Sensors. Springer, Berlin, 4-th edition, pp.1500, 2002. K.Y.Kondratyev, C.A.Varotsos. Atmospheric Ozone Variability. Praxis Publ. Ltd, 624 pages, 2000. A.P.Cracknell. Remote Sensing and Climate Change. Praxis Publ. Ltd, 336 pages, 2001. M.Guelman, F.Ortenberg, A.Shiryaev, R.Weller. Microsatellites for Science and Technology: Gurvin-Techsat In-flight Experiments Results. Small satellites for Earth Observation, Digest of the 3-rd International Symposium of IAA, Berlin, pp.67-71, 2001. M. Guelman, F. Ortenberg, B. Wolfson. Flight Tests of the Novel TechSat Satellite Ozone Meter. Proceedings of the 40th Israel Annual Conference on Aerospace Sciences, pp. 299- 309, 2000. http://chemistry.beloit.edu/Ozone/index.html http://chemistry.beloit.edu/Ozone/pages/reference.html http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/ATM_CHEM/ozone_measurements.html http://earthobservatory.nasa.gov/Library/Ozone/ozone_4.html http://jwocky.gsfc.nasa.gov/ http://oea.larc.nasa.gov/PAIS/HALOE-Ozone.html http://spacelink.nasa.gov/NASA.Projects/Earth.Science/Atmosphere/Ozone.Studies/ http://www.al.noaa.gov/WWWHD/pubdocs/Assessment98.html http://www.ciesin.org/TG/OZ/cfcozn.html http://www.faqs.org/faqs/ozone-depletion/uv/ http://www.esa.int/export/esaCP/Pr_16_2000_p_EN.html http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/earthsci/ozone.htm http://www.solcomhouse.com/OzoneHole.htm http://www.technion.ac.il/ASRI http://wrabbit.gsfc.nasa.gov/ http://toms.gsfc.nasa.gov/ http://auc.dfd.dlr.de/GOME/ http://earth.esrin.esa.it/ http://triana.gsfc.nasa.gov/home/ http://science.nasa.gov/ http://www.gsfc.nasa.gov/topstory/20020926ozonehole.html
Glossary of Ozone and Space related terms A absorption - The process by which radiant energy is absorbed and converted into other forms of energy. A substance that absorbs energy may also be a medium of refraction, diffraction, or scattering. absorption coefficient - A measure of the amount of normally incident radiant energy absorbed through a unit distance or by a unit mass of absorbing medium action spectrum - Relative effectiveness of radiation in generating a certain biological response over a range of wavelengths, as erythema (sunburn), changes in plant growth, or changes in molecular DNA. The action spectrum for DNA respresents the probability of DNA damage by UV radiation at various wavelengths. Such DNA damage can lead to skin cancer. aerosol â&#x20AC;&#x201C; (1) Small droplet or particle suspended in the atmosphere, typically containing sulfur. Aerosols are emitted naturally (e.g., in volcanic eruptions) and as the result of human activities (e.g., by burning fossil fuels. aerosol â&#x20AC;&#x201C; (2) A product that relies on a pressurized gas to propel substances out of a container. Now most aerosol products use propellants that do not deplete the ozone layer, such as hydrocarbons and compressed gases. albedo - Portion of incident electromagnetic radiation that is reflected by by a body to the amount incident upon it, often expressed as a percentage; e.g., the albedo of Earth is 34%. The concept is identical with reflectance; however, albedo is more commonly used in astronomy and meteorology and reflectance in physics. Albedo is sometimes used to mean the flux of the reflected radiation; e.g., the Earth albedo is 0.64 calorie per square centimeter. The albedo is to be distinguished from the spectral reflectance, which refers to one specific wavelength (monochromatic radiation). aldehyde - Organic chemical compound derived from the oxidation of primary alcohols, having the common group CHO. Used in manufacturing of dyes, resins, and organic acids. Atmosphere secondary organic pollutant. anaerobic - Capable of living in the absence of free oxygen. anthropogenic - Involving the impact of man on the natural environment.
anticyclone - Extensive wind system, of high barometric pressure, that circulates clockwise in the Northern hemisphere and counterclockwise in the Southern hemisphere. atmosphere - Envelope of air surrounding Earth, which is retained by Earth's gravitational attraction. attitude - Orientation of a satellite relative to its direction of movement.
B backscatter ultraviolet (BUV) technique - One of several remote sensing techniques used for measuring atmospheric trace gases by satellite. Measurements are made of solar ultraviolet (UV) light entering the atmosphere (the irradiance) at a particular wavelength and of the solar UV that is either reflected from the surface or scattered back from the atmosphere (the radiance) at the same wavelength. biosphere - Portion of Earth and its atmosphere that supports life, including the living organisms within it. blackbody - An ideal emitter that radiates energy at the maximum possible rate per unit area at each wavelength for any given temperature. The spectral distribution of blackbody radiation is described by Planck law and by the related radiation laws. bromine (Br) - Deep red, corrosive, nonmetallic, liquid halogen that gives off an irritating reddish brown vapor. Element of halons, used in pesticides and fire extinguishers.
C calibration - Systematic adjustment by comparison to a standard, such as the graduated scale of a measuring instrument. May be used in algorithms and models to remove geometric and radiometric distortions in the data. carbon dioxide (CO2) - Odorless, colorless, incombustible, nontoxic gas that is produced during respiration, decomposition of organic material, and combustion. Important "greenhouse" gas that contributes to global warming by allowing solar radiation to pass through the atmosphere and trapping radiant heat reflected from Earth's surface. carbon monoxide (CO) - Poisonous, odorless, colorless gas, produced by incomplete combustion of gasoline and diesel fuels.
carbon oxides - Compounds containing carbon and oxygen. Chapman Reactions - Stratospheric process described by Chapman in which ozone dissociates into molecular oxygen and atomic oxygen, and the resulting free oxygen atoms recombine with ozone to form molecular oxygen. chlorine (Cl) - Heavy, greenish-yellow, irritating gas with a pungent odor. Capable of reacting with almost all other elements. Catalyst for ozone destruction. chlorine monoxide (ClO) - Intermediate product of chlorine interaction with ozone. chlorofluorocarbons (CFCs) - Group of inert, nontoxic, nonflammable compounds made up of chlorine, fluorine, and carbon; used in cooling, foam insulation and cleaning agents as well as aerosol propellants. CFCs are very stable in the troposphere. They are broken down by strong ultraviolet light in the stratosphere and release chlorine atoms that then deplete the ozone layer. CFCs are commonly used as refrigerants, solvents, and foam blowing agents. Two primary chlorofluorocarbons that photolyze at high altitudes to release chlorine atoms are CFC-11 (CFCl3), that known as trichlorofluorocarbon, used in aerosols and CFC-12 (CF2Cl2) known as dichlorofluoromethane, used in air conditioning systems as a refrigerant. chopper disk - Slotted disk that is rotated by an electrical motor. During rotation, the detector views the target and reference source alternately. The known radiance from the reference source and the amplitude of the incoming signal enable estimation of the target's radiance. climatological ozone profiles - Twenty-three standard profiles derived from a combination of SBUV measurements taken at altitudes greater than 16-mbar and low altitude balloon radiosonde data. Yearly averages were developed for three latitude bands: low (15째), mid (45째), and high (75째). climatological temperature profiles - Standard atmosphere temperature profiles. convective - The transfer of heat through motion within the atmosphere, especially upward directed motion.
D deoxyribonucleic acid (DNA) - Self replicating nucleic acid that contains genetic code within the cell. The primary structure consists of two long nucleotide chains that are joined by hydrogen bonds and twisted together to form a double helix. depolarizer - Device that removes the effects of light polarization.
diffuser plate - Plate used for capturing incoming solar radiation for measurement and intercepting radiation from a mercury-argon calibration lamp. dissociation - The separation of a complex molecule into constituents by collision with a second body, or by absorption of a photon. Dobson spectrophotometer - The earliest instrument that is used to determine ozone content of the atmosphere from ground station measurements and modern versions continue to provide data. It compares solar energy at two wavelengths in the absorption band of ozone by permitting the two radiations to fall alternately onto a photocell. The stronger radiation is then attenuated by an optical wedge until the photometer's photoelectric system indicates equality of incident radiation. The ratio of radiation intensity is obtained by this process, and the ozone content of the atmosphere is computed from the ratio. Dobson Unit (DU) - Unit of measurement of total ozone equal to 2.69 x 1016 molecules per square centimeter. An equivalent amount of ozone, at 1 atmosphere and 273째 K, would form a layer 0.001 cm thick. Named in honor of the British physicist G. M. B. Dobson. If 100 DU of ozone were brought to the Earth's surface, it would form a layer 1 millimeter thick. In the tropics, ozone levels are typically between 250 and 300 DU year-round. In temperate regions, seasonal variations can produce large swings in ozone levels. For instance, measurements in St. Petersburg, Russia have recorded ozone levels as high as 475 DU and as low as 300 DU. These variations occur even in the absence of ozone depletion, but they are well understood. Ozone depletion refers to reductions in ozone below normal levels after accounting for seasonal cycles and other natural effects.
E Ebert-Fastie monochromatic spectrometer - Instrument used to measure energy intensity within the ultraviolet region of the electromagnetic spectrum. electromagnetic spectrum (EMS) - Entire range of electromagnetic radiation ranging from gamma rays, less than 0.03 nanometers, to radio waves, greater than 30 centimeters. exit slits - Array of holes within a chopper disk that serve as fixed exits during wavelength calibration.
exosphere - Region of the atmosphere beyond 400 km that fades into interplanetary space.
F Fourier transform spectrometer - Spectrometer that consists of a collimator and beamsplitter, which divides the source beam into two parallel beams with equal amplitudes.
G global warming - Rise in global temperature caused by increased amounts of atmospheric gases that trap heat in Earth's atmosphere by absorbing longwave radiation. Global Warming Potential (GWP) - a number that refers to the amount of global warming caused by a substance. The GWP is the ratio of the warming caused by a substance to the warming caused by a similar mass of carbon dioxide. Thus, the GWP of CO2 is defined to be 1.0. Ozone depletion substances CFC-12 has a GWP of 8,500, while CFC-11 has a GWP of 5,000. GOME (Global Ozone Monitoring Experiment) - the first European passive remote sensing instrument operating in the ultraviolet, visible, and near infrared wavelength regions whose primary objective is the determination of the amounts and distributions of atmospheric trace constituents. grating - Surface with parallel grooves or slits that enable diffraction of incoming light into optical spectra. greenhouse effect - The phenomenon in which outgoing infrared radiation that would normally exit from a planet's atmosphere but instead is trapped or reflected because of the presence of the atmosphere and its components. The best scientific estimates to date suggest that increasing amounts of greenhouse gases are resulting in higher temperatures worldwide. These greenhouse gases are water vapor, carbon dioxide, ozone, nitrous oxide, methane, and chlorofluorocarbons (CFCs).
H halon - Compound formed when a halogen, such as fluorine (F) or bromine (Br) attaches to a carbon atom. The halons are used as fire extinguishing agents, both in
built-in systems and in handheld portable fire extinguishers. They couse ozone depletion because they contain bromine. hydrogen chloride (HCl) - Important chlorine-containing compound formed from the breakdown of chlorofluorocarbons. Also produced by volcanic eruptions. Less reactive than chlorine. hydrogen fluoride (HF) - Important fluorine-containing compound formed from the breakdown of chlorofluorocarbons. Also a product of volcanic eruption. hydrosphere - Aqueous envelope of Earth, including oceans, lakes, soil moisture, ground water, and atmospheric water vapor.
I instantaneous field of view (IFOV) - Ground or target area viewed by a sensor at a given point in time. infrared radiation - Electromagnetic radiation having a wavelength slightly longer than visible red light, from 750 nanometers to 1 millimeter. Its lower limit is bounded by visible radiation, and its upper limit by microwave radiation. Most energy emitted by Earth and its atmosphere is at infrared wavelength. The triatomic gases, such as water vapor, carbon dioxide, and ozone, absorb infrared radiation and play important roles in propagating infrared radiation in the atmosphere. Abbreviated IR; also called "longwave radiation." irradiance - Radiant flux per unit area of a surface.
K ketones - Organic compounds in which the carbon atoms of two hydrocarbon radicals are linked to a carbonyl group. Generally represented by the formula R(CO)R1, where R1 and R may be the same.
L Lambert-Beer Law - A relationship describing the rate of decrease of flux density of a plane-parallel beam of monochromatic radiation as it penetrates a medium that both scatters and absorbs at that wavelength. lidar (light detection and ranging) - A technique for active remote sensing in which a light source is used to probe the atmosphere. Laser light fired at the atmosphere is
reflected back by the atmospheric molecules to a detector and the attenuation (reduction) of this light provides information on atmospheric particles and molecules. limb emission technique - This is one of several remote sensing techniques for measuring atmospheric trace gases by satellite. Also called limb sounding technique. Instruments based upon the limb emission technique infer trace gas amounts (such as ozone) from measurements of longwave radiation (infrared or microwave) thermally emitted in the atmosphere along the line of sight of the instrument. lithosphere - Solid mass of Earth composed of rock, soil, and sediment.
M mercury-argon calibration lamp - Lamp that produces radiation centered at 253.7 nm, which is then diffused from a diffuser plate. Radiation measurements are made at multiple wavelengths and possible shifts are noted. mesopause - Transitional atmospheric region between the mesosphere and thermosphere. mesosphere - Region of the atmosphere, between approximately 50 to 100 km, in which temperature decreases with altitude. Meteor-3 - Third in a series of weather satellites launched by the former Soviet Union. Launched in August 1991 with a payload that included a Total Ozone Mapping Spectrometer (TOMS). methane - Simple combustible hydrocarbon. The major component of natural gas. microwave radiometer - Sensor that measures the intensity of microwave radiation (0.3 cm-30 cm) within a specific field of view. Mie scattering - Developed by Gustav Mie in 1908, this is a complete mathematicalphysical theory of the scattering of electromagnetic radiation by spherical particles. In contrast to Rayleigh scattering, the Mie theory embraces all possible ratios of diameter to wavelength, particularly the atmospheric scattering caused by large particles such as dust, pollen, smoke, and water droplets. More prevalent in the lower atmosphere, from 0 to 5 km. mixing ratio - Relative number of molecules of a specific type in a given volume of air. monochromator - Spectrometer that operates within a narrow range of the electromagnetic spectrum.
Montreal Protocol (MO) - The international treaty governing the protection of stratospheric ozone. The Montreal Protocol on substances that deplete the ozone layer and its amendments control the phaseout of ozone depleting substances production and use. Under the MP, several international organizations report on the science of ozone depletion, implement projects to help move away from depleting substances, and provide a forum for policy discussions. In addition, the Multilateral Fund provides resources to developing nations to promote the transition to ozone-safe technologies.
N nadir - Point directly beneath a satellite, opposite the satellite zenith. nanometer - A distance of one billionth of a meter. The nanometer, or nm, is a common unit used to describe wavelengths of light or other electromagnetic radiation such as UV. For example, green light has wavelengths of about 500-550 nm, while violet light has wavelengths of about 400-450 nm. Nimbus-7 - Polar orbiting satellite launched on October 24, 1978, as a research and development satellite to enable multidisciplinary studies of pollution, oceanography, and meteorology. The following instruments were onboard: coastal zone color scanner (CZCS), earth radiation budget (ERB), limb infrared monitor of the stratosphere (LIMS), stratospheric aerosol measurement II (SAM II), stratospheric and mesospheric sounder (SAMS), solar backscatter ultraviolet explorer total ozone mapping spectrometer (SBUV TOMS), scanning multichannel microwave radiometer (SMMR), and temperature humidity infrared radiometer (THIR). nitrogen - A colorless, odorless, nonmetallic element that occurs as a diatomic gas and constitutes nearly 80% of the atmosphere by volume. nitrous oxide (N2O) - Colorless gas, naturally produced through bacteriological decomposition of organic matter. Also produced anthropogenically and used as a mild anesthetic.
O occultation technique - One of several remote sensing techniques for measuring atmospheric trace gases by satellite. Occultation instruments measure solar, lunar, and even stellar radiation directly though the limb of the atmosphere during satellite Sun, Moon, and star rise and set events (depending on which celestial radiator is being used
by the satellite instrument). By measuring the amount of absorption of radiation through the atmosphere at different wavelengths (e.g., UV, visible, infrared), occultation instruments can infer the vertical profiles of various trace constituents, including ozone. optical spectrum - Portion of the electromagnetic spectrum, from 0.30 to 15 micrometers, that can be reflected and refracted with mirrors and lenses. oxygen - A nonmetallic element that occurs as a diatomic gas and constitutes 21% of air by volume, essential for plant and animal respiration, and required for almost all combustion. ozone - Gaseous compound of three oxygen atoms that is generated by a photo-electro process and has a distinct electrical or disinfectant odor. “ozone” – Greek word meaning “smell”, a reference to ozone‘s distinctively pungent odor. ozone absorption coefficients - Variable parameter inputs required for albedo calculations. Albedo measurements across an entrance slit vary according to ozone concentrations and temperature. Therefore, an integral of measurements is used in albedo calculations. ozone layer, ozone shield - thin ozone layer containing the bulk of atmospheric ozone. Nearly 90% of the Earth's ozone is in the stratosphere and is referred to as the ozone layer. This protective layer absorbs harmful, deadly solar ultraviolet radiation. ozone depletion - Loss of ozone through natural breakdown and anthropogenically produced chemical reactions. Ozone depletion refers to reductions in ozone below normal levels after accounting for seasonal cycles and other natural effects. ozone hole – Popularly known term for region of rapid, dramatic ozone depletion over Antarctica during the polar spring. Confined to south of 55° latitude. Disperses soon after temperatures rise above -80° C. ozone profile - The amounts of ozone at different levels in the atmosphere represented in a plot of altitude versus ozone amount (measured typically in number density or partial pressure). ozonesondes - Balloon-borne instruments used to determine ozone profile measurements.
P pair values - Ratio of the albedo value at a longer ozone-insensitive wavelength to the albedo value at a shorter ozone-sensitive wavelength. Used in the computation of ozone. photochemical reaction - A chemical reaction that involves either the absorption or emission of radiation. photodissociation - Dissociation (splitting) of a molecule by absorption of a photon. photolysis - Dissociation process driven by the Sun's radiation. photometer - An instrument for measuring the intensity of light or the relative intensity of a pair of lights. If the instrument is designed to measure the intensity of light as a function of wavelength, it is called a spectrophotometer. photomultiplier tube (PMT) - Photoemissive detector consisting of a photocathode and fused silica window that work together to multiply an incoming electron beam. photosynthesis - Chemical process driven by solar energy in which CO2 and H2O, in the presence of chlorophyll, are converted to oxygen and carbohydrates. Oxygen and water vapor are released in the process. planetary wave - A type of atmospheric wave with a wavelength upward of 10,000 km. These waves are mostly generated by large-scale surface topography like the Rocky Mountains and the Himalayas-Tibet complex or by land-sea boundaries. Such geographically forced planetary waves do not propagate horizontally but instead are stationary. The fact that they are stationary is related to the fact that the topographical forcing occurs at the same locations. Planetary waves often propagate upward from the troposphere into the stratosphere. POAM (Polar Ozone and Aerosol Measurement) - Solar occultation devices that are designed to measure the vertical distribution and overall stratospheric abundances of ozone, water vapor, nitrogen dioxide, and various aerosols. Instruments were launched aboard the SPOT-3 satellite in September 1993 and SPOT-4 satellite in March 1998. polarization - Uniform and nonrandom elliptical, circular, or linear variation of a wave, characteristic in light or other radiation. polar stratospheric clouds - High, thin clouds composed of nitric acid and water that form in the coldest regions of the stratosphere when temperatures drop below -80째C. Ice crystal surfaces within these clouds are efficient in converting inert chlorine reservoirs, such as ClONO2 and HCl, into reactive chlorine compounds.
polar vortex - Wind region around the North or South pole. The southern vortex is a well formed circular to oblong mass of extremely cold, stagnant air, held in place by the ocean surrounding the Antarctic land mass and a strong westerly circulation pattern produced by the coriolis effect. The northern vortex is not as distinct because the Arctic is a frozen ocean surrounded by rugged land masses, which cause the circulating winds to encounter a variety of temperatures. Precambrian - Of or pertaining to the earliest geologic period of history, approximately 600 million years ago, when Earth atmosphere protective ozone layer was forming.
R radiosonde - Balloon borne instrument used to measure and transmit meteorological data. Rayleigh scattering - The scattering of light by a body with a particle diameter less than 0.03 micrometers Dominant form of light scattering in the upper atmosphere, which produces the blue color of the sky. It is caused by atmospheric particulates that have very small diameters relative to the wavelength of the light, such as dust particles or atmospheric gases like nitrogen and oxygen. reflective spectrum - Portion of the optical spectrum, from approximately 0.38 to 15 micrometers, that defines the direct solar radiation used in remote sensing. reflectivity - Ratio of intensity of the total radiation reflected from a surface to the total radiation incident on the surface. RDCF â&#x20AC;&#x201C; (Radiometric Calibration and Development Facility) provides calibration support for nearly all BUV space-based instruments.
S satellite zenith angle - Angle between the position of a satellite and the zenith, which is the point directly over the observed target. SBUV (Solar Backscatter Ultraviolet) â&#x20AC;&#x201C; method and instrument for measurements the vertical distribution of ozone in the atmosphere. SAGE (Stratosphere Aerosol and Gas Experiment) â&#x20AC;&#x201C; Instrument employing a solar occultation technique to define the amount of ozone and other trace gases by measuring sunlight that comes trough the atmosphere at different altitudes.
seasonal cycle - The annual cyclical pattern in any atmospheric variable, whether temperature or trace gas concentration, caused by the seasons. Also called an annual cycle. signal processor - Processor located within the electronics system, that consists of multiple voltage to frequency converters that are responsible for converting an incoming signal from optical to digital. solar cycle - Periodic change in sunspot activity. One cycle is approximately 11.1 years. solar vector - Direction of an incoming solar radiation beam. Used in conjunction with the position of a spectrometer's diffuser plate to calculate albedo. solar zenith angle - Angle between the position of the Sun and the zenith, which is the point directly over an observed target. SOLSE (Shuttle Ozone Limb Sounding Experiment) - Space Shuttle (STS-87, October 1997) instrument that vertical ozone profiles can be measured using light scattered at the Earth's "limb". spectrometer - Instrument used to determine the distribution of energy within a spectrum of wavelengths. stratopause - Transition layer between the stratosphere and mesosphere. Marks the maximum altitudinal temperature increase within the stratosphere. It occurs at an atmosphere height of approximately 50 km; however this depends on latitude. stratosphere - Portion of the atmosphere between the tropopause, at approximately 8 to 15 km, and 50 km in altitude, depending upon latitude, season, and weather. sulfur dioxide (SO2) - Chemical compound that absorbs radiation of the same wavelength absorbed by ozone. Product of large volcanic eruptions. sulfuric acid (H2SO4) - Heavy, corrosive, oily acid. Vigorous oxidizing agent. Ozone concentrations may be affected by reactions on the surface of sulfuric acid clouds, resulting from major volcanic eruptions. sunspot - Relatively dark, sharply defined region on the Sun associated with an intense magnetic field. surface pressure - Pressure at an observation point on Earth's surface.
T thermopause - Transition layer between the thermosphere and exosphere, located at approximately 600 km in altitude. thermosphere - Region of the atmosphere in which temperature increases with altitude. Located at approximately 100 to 400 km. TOMS (Total Ozone Mapping Spectrometer) - instruments measure the total amount of ozone in a vertical column of the atmosphere. A series of four instruments has been making daily global maps of the earth's ozone field. total ozone (TO) - Amount of ozone, measured from Earth's surface to the top of the atmosphere, over a given surface area. trace gas - A minor constituent of the atmosphere. The most important trace gases contributing to the greenhouse effect are water vapor, carbon dioxide, ozone, methane, ammonia, nitric acid, nitrous oxide, ethylene, sulfur dioxide, nitric oxide, CFC-11, CFC-12, methyl chloride, carbon monoxide, and carbon tetrachloride. Such trace gases are sometimes referred to as trace species. tropopause - Boundary between the troposphere and stratosphere, from 8 km in the polar regions to 15 km in the tropics. Marks the vertical limit of most weather phenomena. troposphere - Lowest region of the atmosphere, defined by a steady decrease in temperature with altitude. Extends to approximately 15 km above Earth's surface. The troposphere is characterized by decreasing temperature with height, appreciable vertical wind motion, appreciable water vapor content, and weather. tropospheric - Having to do with the lowest region of the atmosphere, which extends to approximately 15 km above Earth's surface.
U UARS (Upper Atmosphere Research Satellite) - NASA satellite launched in September, 1991 with instruments to measure temperature, wind, and composition of the upper atmosphere. Instrument made the first space-based measurements of clorine monoxide, a principal compound in ozone-depletion chemistry. ultraviolet (UV) - a portion of the electromagnetic spectrum with wavelengths shorter than visible light. The sun produces UV radiation, which is commonly split into three bands: UVA, UVB, and UVC.
UVA (wavelengths 320-400nm) is radiation not absorbed by ozone. UVB (280-320nm) is mostly absorbed by ozone, although some reaches the Earth, and has several harmful effects. UVC (wavelengths shorter than 280nm) is completely absorbed by ozone and normal oxygen.
V vibrational energy level - The energy associated with the vibrational motion of an atoms in molecule.