Time scales to achieve atmospheric gas-aerosol equilibrium for volatile species

Abstract

Time scales to achieve gas-aerosol equilibrium for volatile atmospheric species are systematically studied. It is found that equilibration between submicron aerosol and the gas phase is attainable on a time scale comparable to that of typical ambient gas and aerosol dynamics. However, the time required for the coarse aerosol to reach equilibrium is predicted to be sufficiently long that volatile species in atmospheric coarse aerosol particles may generally exist in nonequilibrium transition states. Variation of the gas-phase concentrations of volatile compounds is generally controlled by equilibration with the fine-mode particles. Volatile compounds in the gas phase can approach the equilibrium state on a much shorter time scale than that of the gas-aerosol system. Sensitivity analysis is performed to study how the equilibration times depend on relative humidity, initial gas-phase concentrations, aerosol mass concentration, aerosol size, temperature, accommodation coefficient, and different chemical compositions. The equilibration time increases with increasing particle size, or decreasing accommodation coefficient or temperature. Transport calculations performed for typical ambient aerosols confirm the results obtained from hypothetical cases. Equilibrium for ammonia between the gas phase and submicron marine aerosol is also examined.