A chemical kinetic model for reactive transformations of aerosol particles

Previous models of heterogeneous interactions have focused on reactive trace gas depletion in cloud or aerosol particles and droplets. 1,2,3 However, recent laboratory studies have demonstrated that reactive trace gas uptake can significantly transform the chemical composition of condensed phase species, as is evident from recent work on organic aerosols. 4,5 This model focuses on transformation rates within the particle itself. In limiting cases, the model leads to simple analytical expressions for the condensed phase species depletion as a function of aerosol/gas interaction time. The model takes into account gas phase diffusion, mass accommodation, bulk phase chemical reactions, surface reactions and particle phase reactant diffusion from the aerosol interior toward the surface. In this way, five limiting regimes of aerosol chemical kinetic processing are identified, quantified, and united to form a single comprehensive mathematical model of particle phase species kinetics. The presentation stresses the information to be gained from observing the size dependence of the particle phase decay rate. Indeed, decay rates exhibiting size dependence spanning particle radius, r 0 through r-2 are theoretically feasible, and rates going as r 0 and r-1 have been experimentally observed 1,4. In addition, an appendix is provided in which existing characteristic times are summarized or new ones are derived for each of the five relevant regimes of chemical processing. Characteristic times can be quickly compared to both timescales of observed changes in laboratory or field studies and apparent size dependence of reaction rates. Such time