A simple formalism is presented to model chemical interactions between aerosols and reactive trace gases over a wide range of conditions. 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. While previous models have focused on the heterogeneous uptake of trace gases by atmospheric droplets and particles, this model focuses on the reactive transformation of condensed phase species. In limiting cases, the model leads to simple analytical expressions for the condensed phase species depletion as a function of aerosol/gas interaction time.