Abstract
A modeling study of the effects of clouds on the evolution and redistribution of aerosol particles in the troposphere is presented. A two-mode, two-moment aerosol evolution model is coupled with a two-dimensional, mixed-phase, two-moment microphysics, Eulerian cloud model and a sulfate cloud chemistry model. The coupled model is used to simulate evolution of a convective cloud with different assumptions about the initial chemical and aerosol fields. In the simulations, SO2 is convectively transported to the mid- to upper troposphere, where it is oxidized to gas-phase H2SO4. After cloud processing, cloud condensation nuclei (CCN) particles are removed by precipitation and graupel to form a CCN-depleted region above cloud top and in the cold and humidified cloud outflow region. These conditions are favorable for binary homogeneous nucleation of ultrafine sulfuric acid particles to take place. The new particle formation in the mid- and upper troposphere interacts with cloud processing and transport of aerosol particles and produces a peak of small particle concentration in the outflow region. Sensitivity tests varying initial aerosol composition and mass mixing ratio, initial H2SO4 mass mixing ratio, assumed OH· profile, and nucleation rate factor are discussed. The small particle concentration in the upper troposphere is most sensitive to initial aerosol composition and assumed OH· profile. When the nucleation rate factor is increased, the critical H2SO4(g) concentration is lowered, and the nucleation rate adjusts to changes in environmental variables more quickly. The model results suggest that both aerosols and aerosol precursors can be transported into the mid- and upper troposphere by convective clouds, affecting vertical profiles of aerosol concentrations.
Corresponding author address: Dr. Sonia M. Kreidenweis, Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523.