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Chapter 6: Ocean storage 283 Box 6.1. Chemical properties of CO2 The oceans absorb large quantities of CO2 from the atmosphere principally because CO2 is a weakly acidic gas, and the minerals dissolved in sea water have created a mildly alkaline ocean. The exchange of atmospheric CO2 with ocean surface waters is determined by the chemical equilibrium between CO2 and carbonic acid H2CO3 in sea water, the partial pressure of CO2 (pCO2) in the atmosphere and the rate of air/ sea exchange. Carbonic acid dissociates into bicarbonate ion HCO3–, carbonate ion CO32–, and hydronium ion H+ by the reactions (see Annex AI.3): CO2 (g)+H2O↔H2CO3 (aq)↔HCO3– +H+ ↔ CO32– + 2H+ (1) which releases 2 mole-equivalents of Total Alkalinity and 1 mol of Dissolved Inorganic Carbon for each mole of CaCO3 dissolved. Increasing TAlk more than DIC leads to a decrease in the partial pressure of CO2 as seen in Figure 6.5. Because most Dissolved Inorganic Carbon is in the form of HCO3–, the main effect of dissolving CaCO3 in surface waters is (see Kheshgi, 1995) CaCO3 (s) + CO2 (g) + H2O ↔ Ca2+ + 2HCO3– (5) Total dissolved inorganic carbon (DIC) is the sum of carbon contained in H2CO3, HCO3–, and CO32–. The atmospheric concentration of CO2 in equilibrium with surface water can be calculated from well-known chemical equilibria that depend on ocean total dissolved inorganic carbon, alkalinity, temperature and salinity (Zeebe and Wolf-Gladrow, 2001). The partial pressure of CO2 in the ocean mixed layer equilibrates with the atmosphere on a time scale of about one year. Ocean surface waters are super-saturated with respect to CaCO3, allowing the growth of corals and other organisms that produce shells or skeletons of carbonate minerals. In contrast, the deepest ocean waters have lower pH and lower CO32– concentrations, and are thus undersaturated with respect to CaCO3. Marine organisms produce calcium carbonate particles in the surface ocean that settle and dissolve in undersaturated regions of the deep oceans. The ocean is a highly buffered system, that is the concentration of the chemical species whose equilibrium controls pH is significantly higher than the concentrations of H+ or OH–. The pH of sea water is the base–10 log of activity of H+. Total Alkalinity (TAlk) is the excess of alkaline components, and is defined as the amount of strong acid required to bring sea water to the ‘equivalence point’ at which the HCO3– and H2CO3 contributions are equal (Dickson, 1981). thereby shifting CO2 from the atmosphere to the oceans in equilibrium, neutralizing the effect of CO2 on pH. The principal effect of adding CO2 to sea water is to form bicarbonate ion, for example, CO2 + H2O + CO32– → 2HCO3–. (2) In addition, some CO2 undergoes simple reaction with water, for example, CO2 + H2O ↔ H+ + HCO3–. (3) In either case, Total Alkalinity does not change. The combined reactions lower both ocean pH, and carbonate ion concentration. For current ocean composition, CO2 that is added to sea water is partitioned primarily into HCO3– with the net reaction resulting in the generation of H+ and thus decreasing pH and making sea water more acidic; adding CO thereby decreases the concentration of CO 2–. 2 Total Alkalinity is increased when, for 3example, alkaline minerals such as CaCO are dissolved in sea water through the reaction, 3 CaCO3 (s) ↔ Ca2+ + CO32– (4) Figure 6.5 Composition diagram for ocean surface waters at 15°C (adapted from Baes, 1982). The white lines denote compositions with the same value of pCO2 (in ppm); the black lines denote compositions with the same pH. The tan shaded region is undersaturated and the green shaded region is supersaturated with respect to calcite at atmospheric pressure (calcite solubility increases with depth). Surface water and average ocean compositions are also indicated. Adding CO2 increases Dissolved Inorganic Carbon (DIC) without changing Total Alkalinity (TAlk); dissolving CaCO3 increases both DIC and TAlk, with 2 moles of TAlk added for each mole of DIC added.PDF Image | CARBON DIOXIDE CAPTURE AND STORAGE
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