Cloud processing, cloud evaporation and Angström exponent

With a cloud parcel model we investigated how cloud processing and cloud evaporation modify the size distribution and the Angström exponent of an aerosol population. Cloud processing causes a decrease in particle concentrations, relatively most efficiently in the coarse mode, and reduces the relative dispersion of the aerosol distribution. As a result the Angström exponent of the aerosol increases. The Angström exponent is subject to other influences. It is very sensitive for relative humidity, especially between 95% and 100%. In addition, kinetic limitations delay droplet evaporation during cloud dissipation, which hampers a direct relation between the Angström exponent and the relative humidity. Consequently, a direct interpretation of the Angström exponent in terms of aerosol properties that play a role in aerosol-cloud interactions, such as the fine mode fraction, is rather complex