Sav so2 h2o hso so32

The oxidized form of sulfur (i.e., sulfuric acid and sulfate) is in the +6 oxidation state and hence is commonly referred to as S(VI).

The individual reactions in the equilibria represented by (11)—(13) are relatively fast (Martin, 1984).

01 23456789 1011 12 pH

FIGURE 8.7 Mole fraction of sulfur species in solution at different acidities (adapted from Martin and Damschen, 1981).

01 23456789 1011 12 pH

FIGURE 8.7 Mole fraction of sulfur species in solution at different acidities (adapted from Martin and Damschen, 1981).

For example, the rate constant for dissociation of hydrated S02, kn, is 3.4 X 106 s"1 so that the half-life for dissociation of the hydrated S02 is only 0.2 /as. Similarly, the second ionization, reaction (13), occurs on time scales of less than a millisecond (Schwartz and Freiberg, 1981). Thus, regardless of which of the three species, S02 • H20, HSO^, or SOj~, is the actual reac-tant in any particular oxidation, the equilibria will be reestablished relatively rapidly under laboratory conditions, and likely under atmospheric conditions as well. The latter is complicated by such factors as the size of the droplet, the efficiency with which gaseous S02 striking a droplet surface is absorbed, the chemical nature of the aerosol surface, and so on; for example, the presence of an organic surface film on the droplet could hinder the absorption of S02 from the gas phase.

As expected from the equilibria (11)—(13) and Le Chatelier's principle, the more acidic the droplet, the more equilibria will shift to the left, that is, the less the dissolved S02. Figure 8.8 shows the range of dissolved S(IV) concentrations expected in aqueous solutions that are in equilibrium with S02 in the gas phase at concentrations of 0.2-200 ppb and over a pH range of 0-6. It is seen that a wide range of concentrations, from ~10"9 to 10"3 mol L"1, of S(IV) is anticipated, depending on the pH and on the concentration of S02 in the gas phase. As expected, the aqueous-phase S(IV) concentration falls as the pH falls.

This dependence of the S(IV) concentrations on the pH of the droplet plays a critical role in determining which oxidant dominates the S(IV) oxidation. As discussed in more detail later, the rates of the various aqueous-phase reactions show different dependencies on pH. Some have rate coefficients that increase with

FIGURE 8.8 Range of expected aqueous S(IV) concentrations as a function of acidity for gas-phase S02 concentrations of 0.2-200 ppb (adapted from Martin, 1984).

increasing pH (e.g., 03) whereas others (e.g., H202) show the opposite trend.

In the first case shown schematically in Fig. 8.9a, both the rate constant and solubility of S(IV) vary with pH in the same manner. As a result, the overall rate of production of S(IV), i.e., &[S(IV)], by such reactions k [S(IV)]

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