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2.2 X 104

" From Olson and Hoffmann (1989).

" From Olson and Hoffmann (1989).

14 {[RCHO]aq + [RCHO(OH)2]aq}[HSOj]aq for various large aldehydes as well as formaldehyde (Olson and Hoffmann, 1989). As discussed in detail by Hoffmann and co-workers (e.g., see Olson and Hoffmann, f988a, 1988b, 1989; Olson et al., 1988; and Betterton and Hoffmann, 1988), formaldehyde, glyoxal, hydroxyacetaldehyde, and, to a smaller extent, methyl-glyoxal all meet these criteria and hence can be important reservoirs for S(IV) in droplets, whereas acetalde-hyde and benzaldehyde do not. Other multifunctional compounds such as glyoxylic acid [CH0C(0)0H] may also contribute in weakly acidic solutions (Olson and Hoffmann, 1988a, 1988b, 1989).

Although the S(IV)-aldehyde adducts are stable toward oxidation, one or more of the oxidation processes for HSO^ or SO2- described below are likely to be much faster than adduct formation under typical fog and cloud conditions. For example, Fig. 8.10 shows the calculated times for complexing S(IV) with HCHO compared to the time for oxidation by H202 at different concentrations typical of various clouds and fogs as a function of pH (Rao and Collett, 1995). Even at the lowest H202 concentrations and highest HCHO concentrations, complexation only competes with oxidation at pH values above about 4.5. Thus the two processes,

FIGURE 8.10 Calculated characteristic times for formation of the HCHO-S(IV) complex or oxidation by H202 under various conditions (adapted from Rao and Collett, 1995).

FIGURE 8.10 Calculated characteristic times for formation of the HCHO-S(IV) complex or oxidation by H202 under various conditions (adapted from Rao and Collett, 1995).

complexation and oxidation, are expected to occur in parallel, with oxidation being more important as the droplets become acidified.

The formation of such adducts, however, can play a significant role in determining the composition of clouds and fogs. Thus much larger aqueous-phase concentrations of aldehydes and S(IV) than predicted from simple Henry's law equilibria are possible if the adducts are formed. For example, Munger et al. (1986) measured the concentration of hydroxymethanesulfonate (HMSA), the HCHO-S(IV) adduct, in fog water as well as the aqueous-phase S(IV) and HCHO and found that in some cases, the fogs were supersaturated in S(IV) by as much as a factor of fO compared to that expected on the basis of Henry's law. Similarly, Rao and Collett (1995) showed that most of the S(IV) in cloudwater samples collected in five different locations across the United States was in the form of HMSA, while there was excess HCHO, and interestingly, H202. Over the range of values of pH typically found in fogs and clouds, ~2-5, the HCHO-S(IV) adduct is primarily in the monovalent form shown in Eq. (14) because the CH2(0H)S03" is a relatively weak acid, pK.d = 11.7 (S0rensen and Andersen, 1970; Jacob and Hoffmann, 1983; Deister et al., 1986). HMSA has also been measured in aerosol particles, presumably due to formation in cloudwater droplets followed by evaporation of water (e.g., Dixon and Aasen, 1999).

In short, complex formation involving S(IV) and aldehydes is now known to be important in a number of cases and must be considered in the chemistry of fogs and clouds.

With these caveats in mind concerning possible complex formation, we examine potential oxidants for S(IV) in solution. These include 02, 03, H202, free radicals such as OH and H02, and oxides of nitrogen (e.g., NO, N02, HONO, and HN03). Metal catalysis may play a role in some of these reactions.

There are two major factors to be considered in assessing the contribution of potential oxidants for S(IV) to the net aqueous-phase oxidation. The first is the aqueous-phase concentration of the species, and the second is the reaction kinetics, that is, the rate constant and its pH and temperature dependencies. As a first approximation to the aqueous-phase concentrations, Henry's law constants (Table 8.1) can be applied. It must be noted, however, that as discussed earlier for S(IV) this approach may lead to low estimates if complex formation occurs in solution. On the other hand, high estimates may result if equilibrium between the gas and liquid phases is not established, for example, if an organic film inhibits the gas-to-liquid transfer (see Section 9.C.2).

Note that the Henry's law constants given in Table 8.1 are generally for 25°C. At lower temperatures, the values will, of course, be larger due to the increased solubility. While the rate constants for most reactions, especially those in the liquid phase, decrease with decreasing temperature, the increased reactant concentrations tend to counterbalance this effect. Thus rates of reactant loss and product formation are not as sensitive to temperature as the rate constant alone.

b. Physical and Chemical Steps in Aqueous-Phase Oxidation

In the discussion that follows, we focus on the kinetic studies of S(IV) oxidations in aqueous solutions. However, it must be recognized that the oxidation itself is only one portion of a sequence of processes that leads from gas-phase S02 to aqueous-phase sulfate. The sequence of steps, depicted in Fig. 8.11, is as follows:

1. Transport of the gas to the surface of the droplet

2. Transfer of the gas across the air-liquid interface (note that the formation of unique surface species may occur; see below)

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