Atmospheric Chemistry and Physics

Abstract. With a cloud parcel model we investigate how cloud processing and cloud evaporation modify the size dis-tribution and the Angström exponent of an aerosol popula-tion. Our study provides a new explanation for the observed variability of the aerosol optical thickness and Angström ex-ponent in the vicinity of clouds. Cloud processing causes a decrease of aerosol particle concentrations, relatively most efficiently in the coarse mode, and reduces the relative dis-persion of the aerosol distribution. As a result the Angström exponent of the aerosol increases. The Angström exponent is very sensitive for changes in relative humidity during cloud evaporation, especially between 90 % and 100%. In addition, kinetic limitations delay evaporation of relatively large cloud drops, especially in clean and mildly polluted environments where the coarse mode fraction is relatively large. This ham-pers a direct relation between the aerosol optical thickness, the Angström exponent and the ambient relative humidity, which may severely complicate interpretation of these pa-rameters in terms of aerosol properties, such as the fine mode fraction.