Mass Transport Limitations in Aqueous-Phase Chemistry

The transition regime formulas can then be extended to account for imperfect accommodation by multiplying the LHS of (12.27) by a. The Fuchs expression in (12.31) becomes 

The formula of Loyalka is applicable only for a = 1, but the theory of Sitarski and Nowakowski (1979) can be used for any accommodation coefficient setting 

2 MASS TRANSPORT LIMITATIONS IN AQUEOUS-PHASE CHEMISTRY 

Dissolution of atmospheric species into cloud droplets followed by aqueous-phase reactions involves the following series of steps  

Diffusion of the reactants from the gas phase to the air-water interface 

FIGURE 11.4 Mass transfer rates as a function of particle diameter for accommodation coefficient values 1.0, 0.1, and 0.01 for the approaches of Sitarski and Nowakowski (1979), Fuchs and Sutugin (1970), and Dahneke (1983). 

Measurements of accommodation coefficients have recently been made by a series of investigators. These will be presented later in this chapter in Example 11.1. 

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MASS TRANSPORT LIMITATIONS IN AQUEOUS-PHASE CHEMISTRY 611 a point close to the particle surface (r — Rp) < and... 

The equations above have been the basis of most atmospheric aqueous-phase chemistry models that include mass transport limitations [e.g., Pandis and Seinfeld (1989)]. These equations simply state that the partial pressure of a species in the cloud interstitial air changes due to mass transport to and from the cloud droplets (incorporating both gas and interfacial mass transport limitations). The aqueous-phase concentrations are changing also due to aqueous-phase reactions that may be limited by aqueous-phase diffusion included in the factor Q. 

Gas-Phase limitation The problem of coupled gas-phase mass transport and aqueous-phase chemistry was solved in Section 12.3.1 resulting in (12.82). Solving for the aqueous-phase reaction term / aq and noting that in this case Henry s law will be satisfied at the interface (pA(Rp) = Caq/// ) ... 

The large S02 mass accommodation coefficient (7 - 0.11) indicates that interfacial mass transport will not limit the rate of S02 uptake into clean aqueous cloud and fog droplets. Either gas phase diffusion, Henry s law solubility, or aqueous reactivity will control the overall rate of aqueous S(IV) chemistry. This conclusion is demonstrated by modeling studies of S02 oxidation in clouds by Chamedies (3) showing that the conversion time of S(FV) to S(IV) is independent of the mass accommodation coefficient for 1 7 > 10 2 Schwartz (1 ) has also shown that, with 7 as large as our measured value, the interfacial mass transport is unlikely to inhibit the oxidation of SC by or Ho02 in cloud droplets for gas concentrations typical of non-urban industrialized regions. 

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