Stochastic approach to chemical kinetics in ultrafine aerosols

We consider chemical reactions taking place in aerosol droplets when the mean number of reactant molecules present is less than unity. Situations of this kind are possible for ultrafine droplets in an atmosphere containing trace amounts of gas phase reactants. We describe an analytic description of such ‘stochastic’ chemistry, and compare the results with the predictions of traditional chemical kinetics. We establish criteria to indicate when the traditional approach can overpredict a reaction rate considerably. We determine that it is rare for such conditions to occur in the atmosphere.