How Does the Atmospheric Carbon Dioxide Affect Ocean Acidification?
December 3, 2022
Aleksandr Zhitomirskiy
The acidity (alkalinity) of sea water is determined by many factors: chemical composition, biochemical and photochemical processes, natural and manmade emissions. Against this background, it is unrealistic to single out the effect caused by changes in the concentration of carbon dioxide in the atmosphere. The magnitude of this effect can be calculated on the basis of data on the solubility of carbon dioxide in water depending on its partial pressure (Henry law) and the dissociation constant of the formed carbonic acid. Taking into account the interaction of hydrogen ions from carbonic acid with basic anions in sea water (carbonate, borate, phosphate, silicate), the calculated change in pH due to an increase in the concentration of CO2 in the atmosphere is less than the error in measuring pH.
Introduction.
At present, climatology has established the idea that the increase in the concentration of carbon dioxide in the atmosphere, due to human activity, has led to an increase in the acidity of the oceans. This idea is formulated in the report of the UN International Panel on Climate Change (IPCC) as follows: “Increasing atmospheric CO2 concentration leads directly to increasing acidification of the surface ocean. Projections based on SRES (School for Resource and Enviromental Studies) scenarios give reductions in pH of between 0.14 and 0.35 units in the 21st century (depending on scenario), extending the present decrease of 0.1 units from preindustrial times” [1].
It is known that the concentration of hydrogen ions in sea water, which determines the value of acidity, depends on many factors. Along with watersoluble atmospheric gases, acidity is affected by the chemical composition of sea water, which is not the same in different places of the ocean, biochemical and photochemical processes that occur differently in various places and at different times of the day. In this regard, it is hardly possible to identify reliably the role of one factor, in this case, the concentration of carbon dioxide in the atmosphere. To analyze the reasons for the change in the acidity of sea water, it is necessary first of all to consider the known experimental facts.
Results of measurements of pH of sea water in various places
The results of pH measurements in four areas located in the tropics and subtropics (Bermuda, Canary Islands, Cariaco Basin, Hawaii) are published and presented graphically [2]. The pH interval values for each region are as follows:
Region Time interval pH range
Bermuda 1983 – 2018
8.00 (2011) - 8.18 (1988)
Canary Islands 1996 – 2010 8.05 (2010) - 8.13 (1998)
Cariaco Basin 1995 – 2018 7.94 (2016) - 8.12 (1998)
Hawaii 1988 – 2018 8.04 (2018) - 8.15 (1994)
This data is a collection of peaks of varying heights showing the change in pH during each year. (The peaks themselves are attributed to seasonal fluctuations, without specifying which factors change from season to season). These peaks differ in height both in different years in the same place and for different places. The largest peak height was found in Bermuda (0.12-0.15 pH units), in other places it is about 0.05-0.08. The lack of initial numerical data does not allow us to assess the statistical significance of the trend in pH changes over time, but taking into account fluctuations in this value, we can assume that the correlation coefficient for such a dependence will be very low.
The cited source does not indicate how the pH measurements were made and how they vary within the same region (the geographical concept of Hawaii, Bermuda, etc. refers to a very large area). Therefore, along with the seasonal pH changes to which the peaks in the graphs are attributed, there are obvious differences depending on the sampling site, and it is not clear how to determine the "average" pH value for a given region, let alone on a planetary scale.
In fact, the range of pH values in sea water is significantly larger than noted above. The values from 7.91 to 8.46 have been reported at various locations in the ocean [3]. An even greater range of pH values in various places (15 locations) is recorded in the work by G.E.Hoffmann a.o. [4]: “These observations a continuum of month-long pH variability with standard deviations from 0.004 to 0.277 and ranges spanning 0.024 to 1.430 pH units.” (By the way, the change in pH of 1.43 corresponds to a difference in the concentration of hydrogen ions by about 27 times). Thus, variations in pH values at different places and at different times are many times greater than the change attributed to the influence of atmospheric CO2 concentration (0.1 units from preindustrial times).
K.M.G. Mostofa a.o. measured pH in seawater of the Bengal Bay and found diurnal variation from 8.12 to 8.37 [5].
In this regard, it is necessary to discuss various factors affecting the
concentration of hydrogen ions in sea water and evaluate the likely contribution due to changes in the concentration of carbon dioxide in the atmosphere.
Effect of temperature
In pure water, there are equal amounts of hydrogen ions and hydroxyl ions, which are formed as a result of self-ionization of water:
H 2 O H ↔ + + OH-
The equilibrium constant of this process K = [H+] [OH- ] /[H2 O], where symbol [ ] refers to molar concentration of corresponding species. Since [H2 O] is many million times greater than [H+] and [OH- ], it is customary to use the ionic product K w = [H+] [OH- ] instead of the equilibrium constant, This value increases with temperature and, accordingly, the concentrations of hydrogen and hydroxyl ions increase, equal to the square root of the ionic product. Therefore, the pH value for pure water (pH = - log [H+]), corresponding to an equal concentration of both ions, decreases with increasing temperature: 7.47 at 0 °C and 7.00 at 25 °C.
In real conditions, temperature affects the solubility of solids and gases in seawater, which changes the pH of water, the rate of biological processes, and the rate of diffusion of ions. Therefore, any change in pH is in one way or another partly related to a change in temperature.
Chemical composition of sea water
The total average concentration of salts dissolved in sea water is estimated at 35 g/kg, while the observed concentration range is from 10.8 to 37.6 g/kg [3, p.54].
Sea water contains a significant amount of salts, which are completely dissociated, so it is convenient to represent the composition of this solution in the form of ions. These are mainly chlorides and sulfates of sodium, magnesium, calcium and potassium, completely dissociated in water. The corresponding ions practically do not affect the acidity of the solution, with the exception of magnesium. Magnesium hydroxide is a weaker base than sodium, potassium and calcium hydroxides, respectively, the magnesium ion can be considered as a weak Lewis acid (electron acceptor). The acidic properties of cations, as well as ordinary hydrogen-containing acids, are characterized by the value of the acid dissociation constant K a or the value of pKa = - log Ka . The vaues of pKa for cations Na+ , Ca2+ and Mg2+ are 14.8, 12.8 and 11.4 [6], therefore, when evaluating the effect of sea water composition on acidity, the ratio of magnesium to calcium can also be important, especially when it comes to small changes in pH.
Anions of weak acids, which have the basic properties, have a much greater effect on pH. The concentrations of these anions in sea water (µmol/ kg) are in
the range: dissolved inorganic carbon (sum of dissolved CO2, HCO3 - and CO 3 2- ) 1837 to 2204, CO3 2- 80 to 303, PO4 3- 0.02 to 2.11, silicate ion in terms of SiO 2 0.37 to 101 [3]. Data on the change in the concentration of borate ions were not found, but for a conditional “reference” sample of sea water, it was taken equal to 101 µmol/ kg with a total concentration of boron 415 µmol/ kg [3, p.23, Table 1.2]. List [3] does not include hydrophosphate (HPO4 -) and dihydrophosphate (H2 PO 4 2-) ions, as well as hydrosulfide (HS-), the last one is present in significant quantities in the Black Sea where observed values of pH are in the range 8.2 –8.6 [7].
A measure of the basicity of an anion is the equilibrium constant of the reaction of its interaction with water (Kb) with the formation of a hydroxyl ion, usually for comparison, the negative logarithm of this value pKb is used, or the index of the acid dissociation constant pKa = 14 – pKb. For polyprotic acids (H2 CO 3, H3 PO 4, etc.), the constants of each dissociation step at 25 °C are considered [6].
According to this data, the basic strength of corresponding anions inceases in the row: HPO 4 2 - < HS- < HCO 3 - < H 2 BO 3 - < H 3 SiO 4 - < PO 4 3- , CO3 2- . Boric acid and silicic acid, for which pK1 > 7, can themselves be regarded as bases B(OH)3 and Si(OH)4 . All of the listed acids in combination with the corresponding anions form buffer systems.
Reconstruction of the chemical composition and pH of surface sea water over the previous 100 million years [8] indicates an increase in the ratio [Mg2+]/[Ca2+] with an increase in the concentration of the CO3 2- ion by about 4 times. At the same time, the pH value increased from about 7.5 to 8.2. These facts show that the effect of the CO 3 2- ion as a relatively strong base prevails over the effect of the very weak acid Mg2+. During the same period, the concentration of atmospheric CO2 significantly exceeded the current level [9].
factors affecting the acidity of sea water
In addition to the dissolution of various minerals mentioned above, which determines the chemical composition of sea water, natural biochemical processes, photochemical processes and volcanic activity influence the acidity value. Biochemical and photochemical processes in most cases need to be considered together, since biochemical processes are associated with exposure to sunlight.
According to various researchers, from one third to one half of the dissolved carbon dioxide is bound annually by marine phytoplankton [5]. Partial conversion of dissolved carbon dioxide to a non-electrolyte organic compound reduces the overall acidity of the solution.
An important role in the change in acidity during daylight hours can also be played by the photochemical generation of hydrogen peroxide associated with the absorption of H+ ions. A linear positive correlation has been established between the pH value and the H2 O 2 concentration, as well as between pH and the intensity of solar UV radiation [5, Fig.3]. It is significant that the diurnal range of pH changes is not the same in different places: an interval of 8.16-8.25 and 7.82-8.28 was observed.
The acidity of sea water can also be influenced by the mineralization of dissolved organic matter, the anaerobic oxidation of methane, sulfides, and possibly other biochemical and photochemical processes that have not yet been studied.
Volcanic eruptions release mainly acid gases (SO2 , CO2,, sometimes in small amounts HCl) into the atmosphere. Sulfur dioxide forms sulfurous acid (H2 SO 3 ) with water, which is much stronger than carbonic acid (H2 CO 3 ), and in an aqueous solution it is easily oxidized into strong sulfuric acid H2 SO 4. Assuming that the gases emitted by underwater volcanoes have similar chemical composition. as gaseous emissions from terrestrial volcanoes, this explains the decrease in pH with depth found, for example, by Y.M.Astor et al. [10]. The difference in the concentration of hydrogen ions with depth is the driving force of concentration diffusion, the rate of which also depends on temperature and also affects the pH value in the surface layer.
Thus, various natural factors can have an opposite effect on the acidity of water, and this effect differs depending on place and time and cannot be accurately accounted for.
The role of human factors
As a result of the removal by rivers, household waste, rainwater from agricultural land and industrial wastewater enter the ocean. These contaminants affect the acidity of seawater both indirectly through biological processes and directly. Mineral fertilizers washed off by rains affect the acidity of water in
different ways. Nitrogen fertilizers such as urea and calcium cyanamide decompose with the formation of ammonia, which, when dissolved in water, alkalizes it, while ammonium nitrate solution creates a slightly acidic environment.
Wastewater from industrial plants contains various ratios of acids and alkalis, as well as metal ions that are predominantly acidic, in particular aluminum and iron ions. The effect of wastewater on the acidity of seawater usually affects estuaries and adjacent areas, although it is not possible to estimate the magnitude of this effect and its distribution.
Unlike industrial effluents, gas emissions from thermal power plants, internal combustion engines, chemical and metallurgical enterprises are carried in the atmosphere over long distances and, getting into the ocean, can change the acidity (alkalinity) of sea water. These gases include carbon dioxide СО2, nitrogen oxides NO x and sulfur dioxide SO 2 . Along with industrial emissions, a significant amount of ammonia NH3 enters the atmosphere as a result of the decomposition of animal waste and partially nitrogen fertilizers. Annual emissions NO x and SO 2 are estimated at 2 Tmol (teramole), NH3 at 4 Tmol [11].
The amount of CO 2 in the air is many thousand times greater than the amount of oxides of nitrogen and sulfur. However, when comparing the relative roles of these gases in ocean acidification, there are important considerations to keep in mind. First, the solubility of CO2 in sea water is relatively low and it decreases with increasing temperature and salt concentration, while nitrogen and sulfur oxides are mixed with water indefinitely. Secondly, CO2 forms a weak carbonic acid (the dissociation constant for the equilibrium H2 CO 3 ↔ H+ + HCO 3 - is about 1*10-6 in the sea water), while nitrogen and sulfur oxides eventually form completely dissociated strong acids H2 SO 4 and HNO 3 in water. Thirdly, the most of ammonia is nitrified to nitrate (NO3 - ) in the upper ocean [11].
How to determine the change in the acidity of sea water due to changes in the concentration of CO2 in the atmosphere?
First of all, it is obvious that this value cannot be determined on the basis of changes in the pH value in natural conditions, since many factors that affect acidity are continuously changing depending on the time and place of sampling. As far as we know, a model experiment with measuring the pH of sea water of a certain composition at a constant temperature and illumination depending on the concentration of CO 2 in the air has not been carried out. However, since the solubility of CO2 in seawater and the process of dissociation of carbonic acid have been sufficiently studied, the effect of the concentration of CO2 in the
atmosphere on the acidity of sea water can be estimated by calculation. To calculate the effect of atmospheric CO2 absorbed by sea water on the change in its acidity, three questions must be considered: a) how much CO2 is additionally dissolved in water as a result of an increase in its concentration in the atmosphere? b) how does the concentration of hydrogen ions change as a result of the dissociation of the formed carbonic acid? c) how does this additional amount of hydrogen ions interact with other ions in sea water? The solubility of carbon dioxide in seawater depends on the partial pressure of the gas, water temperature and the concentration of dissolved salts. The solubility increases with the partial pressure of gas (according to Henry law) and decreases with increasing temperature and concentration of salts. Based on the available experimental data [12], it is possible to calculate the solubility of CO2 in pure water at a given partial pressure and temperature. The values of the Henry constant kH are given in the pressure range 5-100 kPa and temperatures 0-100 °C in units mole CO 2 /(mole H2O * kPa). For subsequent calculations, it is more convenient to express the Henry constant in mole CO2 /(kg H2O atm) using relationships 1 kg H2O = 55.5 mole and 1atm = 101.3 kPa. The values of kH in the range 0 - 30oC are:
As is known, the partial pressure of CO2 over the past century has changed from 0.0003 to 0.0004 atm. For a conditional temperature of 15 °C, kH = 0.0462, and the solubility of carbon dioxide at the above mentioned partial pressures will be 13.9 and 18.5 μmol/kg. According to F.J.Millero calculations for the reference sea water sample, the concentration of dissoled CO2 is 9.6 μmol/kg [3, p.24, Table 1.2]. The concentration of dissolved CO2 in sea water is significantly lower than in pure water due to the effect of dissolved salts and the biological consumption of CO2 during photosynthesis. Assuming that these factors acted in the same way at different times, it it is possible to estimate the increase in CO2 concentration in sea water over the industrial period as 18.5 - 13.9 = 4.6 μmol/kg.
The formation of hydrogen (hydroxonium) ions from dissolved CO2 is described by the equations:
HCO 3 - H ↔ + + CO 3 2- (3)
For the reaction (1), equilibrium ratio [H2 CO 3]/[CO2])aq) in 0.65 molal NaCl solution at 15oC is of 1/840 [13]. Ionization constant Ka (equation (2)) is determined as log Ka = - 0.994 – 610.5/T [13], so at T= 288 K value of Ka = 7.71*10-4. Combined constant for equations (1) and (2) is: K1 = [H+][HCO-
3]/[CO2] = 0.000771/840 = 9.18*10-7 .
So, pK1 = - log K1 = 6.037, and this value is comparable with data by F.J.Millero a.o. [14] who found pK1 = 5.947 at 15 oC and salinity about 35. The dissociation constant for hydrocarbonate ion K2 (equation (3) is more than 1000 times less than K 1 (pK2 ~ 9.1), then amount of H+ ions forming according to equation (3) is negligibly small.
It is possible to calculate the amount of hydrogen ions from above mentioned increase of dissolved CO 2 concentration 4.6 μmol/kg. Taking values of [H+] and [HCO3 -] as x and amount of undissociated H2 CO 3 as 4.6*10-6 -x, we get: x2 / (4.6*10-6 -x) = K1. Average value of pK1 is about 6 (between 6.037 and 5.947), so K 1 = 1.00*10-6 , and x = 1.88*10-6 mol/kg or 1.88 μmol/kg. This additional amount of hydrogen ions, due to the increase in the concentration of CO2 in the atmosphere, is about a hundred times less than the concentrations of carbonate ions and borate ions mentioned above.
Discussion
In order to find out how justified the idea of the effect of changes in the concentration of CO 2 in the atmosphere on the acidity of the ocean, it is necessary to consider at least three questions.
First, what facts give grounds to believe that the acidity (alkalinity) of the ocean can be characterized by some average value? - No, such facts have not been found. On the contrary, studies show that in different parts of the ocean, the difference in the pH values of sea water can reach 1.43 [4] , which corresponds to a 27-fold change in the concentration of hydrogen ions. When averaging pH changes over a month, the standard deviation from the mean reaches 0.277 pH units [4] (almost a 2-fold change in hydrogen ion concentration), while the change in pH from pre-industrial times, attributed to the effect of atmospheric CO 2, is 0.1 [1]. Therefore, the concept of the average acidity of the ocean not only has no physical meaning, but is not statistically reliable.
Secondly, how can one separate the effect caused by changes in the concentration of atmospheric CO2 under natural conditions from other factors
affecting the acidity of sea water? - These factors include differences in the concentration of ions that affect the acidity (alkalinity) of sea water [3] , biological and photochemical processes [4], emissions of volcanic gases from terrestrial and underwater volcanoes, the removal of wastewater from industrial enterprises into the ocean, washout of mineral fertilizers from fields, etc. . These factors obviously cannot remain unchanged in time and they act differently in different places in the ocean, therefore, it is impossible to isolate the effect of atmospheric CO2 on pH based on simple observations of pH change over time. Thirdly, how to quantify the change in the acidity of sea water when the concentration of CO 2 in the atmosphere changes by a certain amount? - This question can be answered experimentally or by calculation. In the experiment, it is necessary to maintain a constant temperature and exclude the effect of solar radiation. The chemical composition of sea water must be precisely known, especially the concentration of ions that affect the pH value (carbonate, bicarbonate, borate, silicate, phosphate). A certain amount of sea water in the experiment would be in contact with a certain volume of air, the carbon dioxide content of which must be obviously higher than in the atmosphere with which the water sample was in contact before the start of the experiment. During the experiment, changes in the pH of water, the concentration of CO2 in the air, and the concentration of the mentioned ions would be recorded. Such an experiment is quite complicated, but the change in acidity due to the dissolution of CO 2 can be approximately estimated, knowing the dependence of CO 2 solubility on partial pressure and temperature and the dissociation constant of carbonic acid in water. The inaccuracy of the calculation is due to the influence of salt concentration on the solubility of CO2 in sea water and the dissociation constant of the acid. Nevertheless, from the calculation given in the previous section, it follows that a change in the concentration of CO2 in the atmosphere from 0.03% to 0.04% leads to an increase in the concentration of H+ ions by approximately 2 (1.88) μmol/kg. These additional hydrogen ions in solution react with buffering anions, for example, convert carbonate to hydrogen carbonate: H+ + CO 3 2- HCO → 3 - . The change in the pH value in this case can be determined by the HendersonHasselbach equation: pH = pKa + log ([A- ]/ [HA])
To calculate the pH change in the HCO3 - - CO 3 2- buffer system, it is necessary to know the concentrations of the corresponding ions before and after the addition of the acid (H+ ions). According to reference composition of sea water [3, Table 1.2] these values are taken as 239 μmol/kg for CO3 2- (A- ) and 1718 μmol/kg for
HCO 3 - (HA). Then change of pH after addition to this system 2 μmol/kg H+ will be log (239/1718) – log (237/ 1720) = 0.004 which is less than the pH measurement error.
Thus, the effect of changes in the concentration of carbon dioxide in the atmosphere on the acidity of sea water is negligible.
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