The Effects of Clouds on Aerosol and Chemical Species Production and Distribution. Part III: Aerosol Model Description and Sensitivity Analysis

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, SO is convectively transported to the mid- to upper troposphere, where it is oxidized to gas-phase H 2 SO 4 . 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 H 2 SO 4 mass mixing ratio, assumed OH. profile, and nucleation rate factor are discussed. The small particie 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 H 2 SO 4 (g) concentration is lowered, and the nucleation rare 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.