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  

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

FIGURE 8.11 Schematic of steps involved in the transfer of S02 from the gas phase to the aqueous phase of an atmospheric water droplet and its oxidation in the liquid phase. S02(i) = S02 at the water-gas interface. 

Formation of aqueous-phase equilibria of the dissolved species, for example, (11)—(13) in the case of 

Schwartz and Freiberg (1981) have calculated the rates of these processes for S02 and expressed them in terms of characteristic times t, which for Step 5, chemical reaction, is the natural lifetime discussed in Section 5.A.I.C. For Steps 1-4, the characteristic time is the time to establish the appropriate steady state or equilibrium for the process involved for example, for Step 1, it is the time to establish a steady-state concentration of the gas in the air surrounding the droplet. Seinfeld (1986) discusses in detail calculation procedures for these characteristic times. A brief summary of the results of Schwartz and Freiberg (1981) for Steps 

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