Reactive Uptake Of Gases By Cloudwater

Without further reaction the fractional uptake of S02 into cloudwater is low, even at fairly high pH. The same is true a fortiori for N02, which does not undergo acid dissociation reaction in aqueous solution. However, there is a strong thermodynamic driving force in clouds for the reactive uptake of these gases to form sulfuric and nitric acids, respectively, the principal species contributing to acid deposition. This situation has stimulated substantial research interest in the processes whereby these gases are transformed into the acids and incorporated into cloudwater. The present understanding of these rcactivc uptake processes is that in the case of S02, the process consisting of uptake of S02 followed by aqueous-phase oxidation contributes substantially to the uptake of sulfuric acid by cloudwater and to the deposition of this material in precipitation. In contrast, the uptake process for N02 to form nitric acid appears to be dominated by gas-phase oxidation followed by uptake of the oxidized species. This section presents the formalism by which the rate of aqueous-phase reaction in cloudwater may be evaluated treating these two gases as examples.

Consider the rate of aqueous-phase reaction of dissolved sulfur-IV to be given by

^[S(IV)]=^[S(VI)] = fc(1)[s(iv)] dt dt where k<l> denotes an effective first-order rate coefficient, which in general may be equal to a second-order rate coefficient times the concentration of a second reagent. The reaction need not be first-order in the reagent sulfur-IV; additional power(s) of [S(IV)] could be incorporated within the effective first-order rate coefficient kil>. It is useful to refer the reaction rate to the total S(IV) concentration because of equilibration of individual sulfur-IV species within solution, S02(aq), HSOJ or bisulfite, and S032- or sulfite, that is rapid relative to depletion by reaction. The aqueous-phase reaction rate can be related to the gas-phase mixing ratio of S02 by solubility equilibria between aqueous-phase concentration of S(IV) and gas-phase partial pressure of S02, under assumption that these equilibria apply. This phase equilibrium is expected to hold if mass transport rates coupling the two phases are sufficiently fast to replenish the aqueous-phase material that is depleted by reaction, a situation that is normally expected to obtain, as discussed below. Hence

Under assumption that the aqueous-phase rate is uniform within a given region of a cloud, then the rate of reaction, expressed as a rate of decrease in the mixing ratio of S02, is

The quantity in braces is an effective first-order rate coefficient of aqueous-phase reaction, referred to the gas-phase mixing ratio; this quantity may be directly employed in evaluating rates of reactions or in comparison to rate coefficients for loss by gas-phase reactions. Note that k^ scales linearly with liquid water content and with Henry's law solubility coefficient. The more water present to serve as volume of reactor, the faster the reaction. Likewise, the more soluble the reagent gas, the faster the reaction. Evaluation of the rate of a specific reaction requires knowledge of the effective first-order rate coefficient of aqueous-phase reaction, For this, one must identify the mechanism and rate of aqueous-phase reaction.

There is a strong thermodynamic driving force for oxidation of dissolved S02 by molecular oxygen, which, because of its abundance, might be thought to be the key oxidant of S02 in cloudwater. However, this reaction is quite slow unless catalyzed, for example, by transition metal ions. Although catalyzed oxidation of dissolved sulfur-IV by dissolved molecular oxygen may be of some importance in some circumstances, the species that have been identified as of principal importance in oxidation of sulfur-IV in cloudwater are the strong oxidants ozone (03) and hydrogen peroxide (H202). Ozone is commonly present in the atmosphere at a mixing ratio of 30 to 50 nmol/mol. Hydrogen peroxide is present at much lower abundance, ~1 nmol/mol. These mixing ratios compare with those for S02 of order 10 nmol/mol in regions influenced by industrial emissions, to much lower at locations well removed from sources.

Consider first the ozone reaction. The rate of aqueous-phase reaction is given as where ki2) is a second-order rate constant that must be determined by laboratory measurement and has been found to exhibit a strong pH dependence, increasing with increasing pH (Fig. 2a). The concentration of dissolved ozone is related to the gas-

Figure 2 Effective second-order rate coefficients for aqueous-phase reaction of S(IV) with 03 (a) and with H202 (b) as a function of pH. Modified from Schwartz (1988).

Figure 2 Effective second-order rate coefficients for aqueous-phase reaction of S(IV) with 03 (a) and with H202 (b) as a function of pH. Modified from Schwartz (1988).

phase mixing ratio of this species again under assumption of solubility equilibrium, as

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