A numerical model of the cloud-topped planetary boundary-layer: cloud processing of aerosol particles in marine stratus

In a numerical study with the one-dimensional chemical microphysical stratus model CHEMISTRA the effect of stratiform clouds on aerosol particles is investigated. The microphysical part of CHEMISTRA consists of a detailed description of cloud microphysical processes by means of a joint two-dimensional particle distribution for aerosols and cloud droplets. In the chemical part of the model the particle spectrum is subdivided into three categories referring to inactivated aerosols, small and large cloud droplets. Aqueous phase chemical reactions are separately treated in the two droplet size classes. Numerical results are presented demonstrating that the uptake of trace gases by cloud drops and the sulfate production yield a distinct modification of the physico-chemical microstructure of the activated aerosols. After cloud evaporation a local minimum evolves in the aerosol spectra separating cloud processed from interstitial aerosols. The residual aerosols have an increased mass forming a second maximum in the accumulation mode of the size distributions. (C) 1999 Elsevier Science Ltd. All rights reserved.