Evaluating the impacts of cloud processing on resuspended aerosol particles after cloud evaporation using a particle-resolved model

Aerosol particles undergo physical and chemical changes during cloud processes. In this work, we quantified the changes in aerosol mixing state using a particle-resolved model. To this end, we coupled the particle-resolved aerosol model PartMC-MOSAIC with the aqueous chemistry module CAPRAM 2.4 and designed cloud parcel simulations that mimicked several cloud cycles that a particle population may be exposed to in polluted urban environments. With ammonium nitrate and ammonium sulfate added to the activated particles, after the cloud evaporated, the activation potential of the resuspended aerosol particles increased for supersaturation thresholds lower than the maximum supersaturation attained in the cloud. Formation of sulfate and nitrate increased the internally mixed state of all particle populations, quantified by the mixing state index chi. The change of aerosol mixing state due to aqueous-phase chemistry was related to the fraction of activated particles. For a case with low aerosol number concentration, where the activated fraction was up to 60%, chi increased by up to 50 percentage points after cloud processing, reaching an almost completely internal mixture. In contrast, for a case with high aerosol emissions and activated fraction of less than 20%, the increase in chi was less than 20 percentage points, and chi remained below 80% after cloud processing. The change in aerosol mixing state caused by coagulation within the cloud parcel was negligible. These findings highlight the complex influence of cloud processing on particle properties.