Gaseous Air Pollutants with Atmospheric Aqueous Solutions

Interactions of Gaseous Air Pollutants with Atmospheric Aqueous Solutions 

Because of the gaseous nature of many of the important primary and secondary pollutants, the emphasis in kinetic studies of atmospheric reactions historically has been on gas-phase systems. However, it is now clear that reactions that occur in the liquid phase and on the surfaces of solids and liquids play important roles in such problems as stratospheric ozone depletion (Chapters 12 and 13), acid rain, and fogs (Chapters 7 and 8) and in the growth and properties of aerosol particles (Chapter 9). We therefore briefly discuss reaction kinetics in solution in this section and heterogeneous kinetics in Section E. 

The aqueous phase that serves as a reaction medium in the atmosphere is present in the form of clouds, fogs, rain, and particulate matter consisting of either an aqueous solution containing pollutants or a film of water surrounding an insoluble core (see Chapter 9). For example, at typical relative humidities, 30-50% of the aerosol mass is due to water (Graedel and Weschler, 1981). However, many of the species that are believed to react in such atmospheric solutions, for example, S02, 03, H202, and NO, are emitted or formed in the gas phase. Before reactions can occur in solution, then, several steps illustrated in Fig. 5.12 must first take place  

Diffusion of gases is fast relative to diffusion in the aqueous phase i.e., step 1 is fast relative to step 3. Thus diffusion coefficients for gases at 1 atm pressure are 0.1-1 cm2 s l, whereas in liquids they are 10 5 cm2 s 1 for small molecules. As discussed in detail by Schwartz and Freiberg (1981), gas-phase diffusion, in most (but not all) cases, will not be the slowest (i.e., rate-determining) step. 

Gases dissolve in aqueous solution to various extents, depending on the nature of the gas. At sufficiently long times, an equilibrium can be established between the gas- and liquid-phase concentrations, which is described by Henry s law  

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