TOPICAL SCIENCE
MAY 2018
M A Y
2 0 1 8
AUTHOR: M. FRANKLIN
Topical Science The Ozone Layer & The Montreal Protocol
In this issue Page 1: Introduction
Page 2: The ozone layer
Page 3 The Montreal Protocol
Page 4. Looking to the Future. Without the ozone layer, there would be a higher incidence of skin cancer in the population and more people would develop cataracts, leading to vision impairment.
The Ozone Layer, in the Stratosphere of Earth’s atmosphere, shields us from harmful UV radiation. But it is fragile. As summer approaches, many of us will head to the beach to enjoy the warm weather. No doubt we will apply sunscreen, to protect us from the harmful ultraviolet rays of the sun. But the ozone layer provides a natural protection from these rays. However, certain synthetic chlorine-containing chemicals can interact with ozone, causing depletion of the ozone layer.
Important Dates 1974 Molina & Rowland published a paper linking synthetic chlorofluorocarbons with ozone depletion.
1985 Computer images, using colour coding to show different levels of ozone, seem to show a ‘hole’ in the ozone layer over Antarctica.
1987 The Montreal Protocol was adopted, phasing out the use of ozone-depleting substances.
MAY 2018
TOPCAL SCIENCE
The Ozone Layer in Earth’s Atmosphere The atmosphere has four layers. The lowest layer, where our weather occurs and where all human activities take place, it called the troposphere. It extends from sea level to an altitude of about 10 -15 kilometres. Above this is the stratosphere, which extends for another 15 kilometres or so. There are two more layers above this, the mesosphere and the thermosphere. The chemical equation shown on the right represents a reversible chemical reaction, whereby 3 molecules of normal, diatomic oxygen (containing 2 oxygen atoms per molecule) are converted to two molecules of ozone. Ozone is a type of oxygen that has three oxygen atoms in each molecule, rather than two.
The Ozone Cycle
3O2 ! 2O3
The reaction requires ultra-violet (UV) light, which is absorbed by the ozone layer and therefore prevented from reaching the lower atmosphere. Ozone is inherently unstable and easily loses an oxygen atom, giving normal oxygen. When two oxygen atoms collide in the stratosphere, they re-combine to form normal diatomic oxygen. However, there is a constant input of energy in the form of UV radiation from the sun, which keeps the cycle going. The process acts as a filter for UV radiation.
Ozone-Depleting chemicals Scientists discovered that certain synthetic chlorine-containing chemicals, for example, chlorofluorocarbons (CFCs), could be broken down by UV radiation, releasing chlorine atoms which would react with ozone and destroy it, so that it would no longer provide an effective filter for UV rays. Computer images use colour coding to show ozone depletion. The images look as if a ‘hole’ has developed in the ozone layer over Antarctica. 2
MAY 2018
TOPCAL SCIENCE
The Montreal Protocol In the 1980s, when it was discovered that the ozone layer was becoming depleted, chlorofluorocarbons (CFCs) were in widespread use in refrigeration units and as propellants in aerosol cans. CFCs are non-flammable and they are non-toxic to humans and animals and so they were considered safe. When they were first introduced, it was not foreseen that they could interfere with the chemical processes in the ozone layer. But when scientists demonstrated that that CFCs and other chlorine containing compounds could cause ozone depletion, world leaders agreed to take action to phase out the use of ozone-depleting substances. Many chlorinated solvents could also break down to release free chlorine atoms, which are highly effective at destroying ozone molecules. On September 16th, 1987, twenty-four countries, under the United Nations Environment Programme, came together to sign the Montreal Protocol. Since then, a total of 197 countries, almost every country in the world, have signed up to it. This has been described as the most successful international treaty ever agreed. Under the agreement, further production of nearly 100 controlled substances has been discontinued and their use phased out. It has been necessary to find replacements for these substances and it could not be done overnight. In 1987, production of controlled ozonedepleting substances exceeded 1.8 million tonnes annually, but by the end of 2010, annual production of these substances had reduced to about 45,000 tonnes. This represents great progress. Apart from reducing the risks to health, the phasing out of CFCs has had another benefit. It has been found that CFCs strongly absorb infrared radiation and therefor contribute to global warming, by means of the so-called ‘greenhouse effect’. So, getting rid of them is good for the environment. Unfortunately, because CFCs are synthetic, nature has not evolved any natural enzymes to break them down. They are therefor non-biodegradable and so persist in the environment for many years. So, even though their use has been discontinued, so much has already been released into the atmosphere, that they continue to react with ozone and will do so for many years, until they are eventually broken down by UV radiation. But at last, there is evidence that the ozone layer is recovering and while there are seasonal fluctuations, the good news is that ozone levels are rising again.
A Scientist releases a balloon carrying an instrument to measure ozone levels in the stratosphere over Antarctica.
3
MAY 2018
TOPCAL SCIENCE
Looking to the Future The work of the Montreal Protocol is not yet finished. While scientists now predict that the ozone layer will have fully recovered later this century, those predictions rely on the assumption that the Montreal Protocol will be implemented in full by all countries. Among other things, this means completing the phase-out of hydrochloroflorocarbons (HCFCs), a class of chemicals that have been used as substitutes for CFCs. They are less harmful to the ozone layer, but they do cause some damage, so it is planned that they will be phased out by 2040.
The Parties to the Montreal Protocol are continuing to investigate opportunities for making other global agreements in an effort to protect the environment of our planet Earth.
About The Author Margaret Franklin, a retired chemistry lecturer, is the author of this ‘Topical Science newsletter. For over three years, her column appeared in ‘The Westmeath Independent’, a weekly local newspaper, based in the Midlands of Ireland. It is now published in electronic format, for a wider readership. Its aim is to explain the science behind various topical news items that are featured in the media. For more information, read this report from the United Nations Environment Programme: http://ozone.unep.org/Assessment_Panels/SynthesisReport2014.pdf
4