A simple model for the time evolution of the condensation sink in the atmosphere for intermediate Knudsen numbers

Transformation of the mass flux towards the particle from the kinetic regime to the continuum regime is often described by the Fuchs–Sutugin coefficient. Kinetic regime can be obtained as a limiting case when only one term of the expansion of the Fuchs–Sutugin coefficient at small 1 ∕ Kn is considered. Here we take the two first terms into account, and get a mass flux which agrees well with the full mass flux down to Kn ∼ 0.5. This procedure allows an analytical solution of the condensation equation valid for the range of intermediate Knudsen numbers to be obtained. The expansion is further applied to analytically calculate the condensation sink. The formula for the condensation sink is tested against field observations. The relative contribution of different aerosol modes to the condensation sink is discussed. Furthermore, we present a simple model describing the coupled dynamics of the condensing vapour and the condensation sink. The model gives reasonable predictions of condensation sink dynamics during the periods of the aerosol modes' growth by condensation in the atmosphere.