A numerical model of the cloud-topped planetary boundary-layer: chemistry in marine stratus and the effects on aerosol particles

Abstract

In a numerical study the effect of stratiform clouds on aerosol particles is investigated. This is done with the one-dimensional chemical microphysical stratus model CHEMISTRA. In the microphysical part of the model special emphasis is layed on 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 unactivated aerosols, small and large cloud droplets. Aqueous-phase chemical reactions are separately treated in the two droplet size classes. Numerical results show that within the boundary-layer, apart from chemical reactions, the time evolution of chemical reactants is strongly controlled by dynamic and microphysical processes. Due to the turbulent mixing the uptake of highly water soluble gas-phase compounds by cloud droplets yields a strong concentration decrease not only in cloudy regions but also below the cloud. The highest liquid-phase concentrations of these species are found in small cloud droplets while reactants with low water solubility yield higher concentrations in big than in small droplets. In the liquid phase the dominant sulfate source is the nucleation scavenging of aerosols. Sulfate production by chemical reactions takes place during the day when in the gas phase SO 2 is photochemically produced. Hereby the oxidation of sulfur by ozone is most important and is mainly observed in small droplets. The uptake of trace gases by cloud droplets and the sulfate production yield a distinct modification of the physico-chemical microstructure of the cloud condensation nuclei. 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.