The largest uncertainty in the global aerosol burden lies in the removal rate, currently dominated by scavenging during rain and fog events. As such, understanding the scavenging process is critical to understanding both aerosol radiative forcing and the interactions between aerosols, clouds, and rain in the hydrologic cycle. This presentation will investigate the interactions between aerosol chemistry and rain and fog through recent field measurements and process modeling.

Ambient aerosol size distributions and chemical composition were measured at several East Coast sites during summer 2003. Precipitation events during the sampling period kept aerosol concentrations low, with an average gravimetric PM1 of 8.2 µg m^-3 and an average Fourier transform infrared (FTIR) spectroscopy-measured PM1 of 8.6 µg m^-3. The average submicron scavenging coefficient for the rain events was 7x10^-5, with variability apparently caused by heterogeneity in chemical composition. The water-rinsed fraction of the aerosol organic mass decreased during rain events, with averages of 55% at the start and 30% at the end of the observed rain events.

Modeling of fog events in polluted regions also shows an important role for chemical composition in aerosol removal by wet deposition. Using detailed chemical thermodynamics and microphysics, a fog model in a boundary layer with low entrainment was used to show the role of organic compounds in providing fog nuclei. In turn, wet scavenging and deposition play important roles in controlling liquid water content.