Growth laws for atmospheric aerosol particles: An examination of the bimodality of the accumulation mode

The growth of atmospheric aerosol particles was examined by first deriving growth laws and their corresponding time scales for all the important aerosol processes: condensation, coagulation, aqueous-phase reactions in droplets or aerosol particles, and reactions taking place at the interface of two phases. These: tools were then used to explore and explain the frequently observed splitting of the accumulation mode into two individual modes. Two environments—the remote marine boundary layer and the South Coast Air Basin of Los Angeles (SoCAB)—were considered. Our analysis points out that the observed mode separation cannot result from condensational or coagulational growth. In both the marine boundary layer and the SoCAB, H2O2/O3 sulfate formation in droplets during clouds or fogs provides all the features required for explaining the bimodality. In the SoCAB, this process may be assisted or even dominated by the trace metal catalyzed droplet-phase SO2 oxidation.