The Program supports research on the sources, sinks, transport, and transformation of gases and aerosols in the atmosphere through models, observations, and experiments, including homogeneous and heterogeneous chemical reactions, emissions, deposition, atmospheric oxidation and photochemistry, aqueous-phase chemistry and aerosol processes; the formation of new particles and secondary organic aerosols, the modeling of atmospheric chemical processes, the study of chemical mechanisms in the atmosphere, optical properties of gases and aerosols, and improved methods for measuring the concentrations of trace species and their fluxes into and out of the atmosphere. The Program encourages principal investigators from a wide variety of institutions and backgrounds to submit proposals.