Other Forms of Water in the Atmosphere

Our discussion in the preceding sections has focused on warm nonraining tropospheric clouds. Water in the atmosphere can also exist as ice, rain, snow, and so on. We summarize here aspects of the formation and removal of these water forms that are most associated with atmospheric chemistry. The interested reader is referred for more information to Pruppacher and Klett (1997) and references therein. 

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A second pathway to the formation of sulfuric acid depends on the presence of hydrogen peroxide (H2O2) in clouds, fog, rain, and other forms of water in the atmosphere. Hydrogen peroxide is now known to form in such locations when hydroperoxyl radicals react with each other ... 

Since is in the form of CO2 in the atmosphere and enters into the surface ocean water as CO2 in a timescale of decades, its partition between the atmosphere and the oceans yields a reliable estimate for the mean CO2 gas transfer rate over the global oceans. This yields a CO2 gas exchange rate of 20 + 3 mol CO2 m y that corresponds to a sea-air CO2 transfer coefficient of 0.067mol CO2 m y uatm. Wanninkhof in 1992 presented an expression that satisfies the mean global CO2 transfer coefficient based on and takes other field and wind tunnel results into consideration. His equation for variable wind speed conditions is ... 

Although it is one of the smallest reservoirs in terms of water storage, the atmosphere is probably the second most important reservoir in the hydrosphere (after the oceans). The atmosphere has direct connections with all other reservoirs and the largest overall volume of fluxes. Water is present in the atmosphere in solid, liquid, and vapor forms, all of which are important components of the Earth s natural greenhouse effect. Cycling of water within the atmosphere, both physically (e.g. cloud formation) and chemically, is also integral to other biogeochemical cycles and climate. Consult Chapter 17 for more details. 

Little is known concerning the chemistry of nickel in the atmosphere. The probable species present in the atmosphere include soil minerals, nickel oxide, and nickel sulfate (Schmidt and Andren 1980). In aerobic waters at environmental pHs, the predominant form of nickel is the hexahydrate Ni(H20)g ion (Richter and Theis 1980). Complexes with naturally occurring anions, such as OH, SO/, and Cf, are formed to a small degree. Complexes with hydroxyl radicals are more stable than those with sulfate, which in turn are more stable than those with chloride. Ni(OH)2° becomes the dominant species above pH 9.5. In anaerobic systems, nickel sulfide forms if sulfur is present, and this limits the solubility of nickel. In soil, the most important sinks for nickel, other than soil minerals, are amorphous oxides of iron and manganese. The mobility of nickel in soil is site specific pH is the primary factor affecting leachability. Mobility increases at low pH. At one well-studied site, the sulfate concentration and the... 

At 298 K and atmospheric pressure with 50% relative humidity, about 0.2 HO" are produced per O( D) atom formed. Photolysis of 03 in the presence of water vapor is the major tropospheric source of HO", particularly in the lower troposphere where water vapor mixing ratios are high (for an explanation of the term mixing ratio see below). Other sources of HO" in the troposphere include the photolysis of nitrous acid (HONO), the photolysis of formaldehyde and other carbonyls in the presence of NO, and the dark reactions of 03 with alkanes. Note that all these processes involve quite complicated reaction schemes. For a discussion of these reaction schemes we refer to the literature (e.g., Atkinson, 2000). 

The formation of hydrogen peroxide by photolysis of natural waters is discussed in Chapter 6. It is also formed by illumination of some sands and semiconductor oxides (Kormann et al., 1988 see also Section 6.E.3). Other sources of H2O2 include formation in the gas phase of the troposphere by the self-termination (dismutation) reaction of OOH and the autooxidation of reduced transition metals such as iron (Equation 4.4). The formation and fate of H2O2 in the atmosphere has been reviewed (Gunz and Hoffmann, 1990 Sakugawa et al., 1990). 

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