Gas-to-particle conversion in the atmosphere: II. Analytical models of nucleation bursts

Simple models are developed to describe the formation of particles from condensable vapours in different atmospheric circumstances. The models are designed for use in large scale global transport models, where sub-grid descriptions are required for such phenomena. We solve the evolution equation for the density of a condensable vapour. When the concentration of existing aerosol is low, nucleation can occur, but only in intermittent, isolated bursts. In the absence of an initial aerosol, two analytical expressions are obtained for the number of particles produced in such bursts, valid for high and low rates of vapour production, respectively. These results compare favourably with calculations made using a detailed numerical code, using the homogeneous nucleation of sulphuric acid/water droplets as an illustration. Then we consider barrierless nucleation, where clusters are always stable against evaporation, which is relevant to the production of ammonium sulphate particles in the atmosphere. We go on to consider conditions where existing aerosol can affect the production of particles, and also consider slower bursts where the time dependence of the vapour production rate, and not condensation on the nucleated aerosol, cuts off nucleation. (C) 1998 Elsevier Science Ltd. All rights reserved.