Modeling of coagulation and dispersion of aerosolsin the atmosphere

Abstract

A simple model to analyze the effects of coagulation and turbulent dispersion on the behaviour of atmospheric aerosols is proposed here. This model does not use the concept of monomeric structure for the aerosol particles as proposed by Smoluchowsky to describe the coagulation. Instead, a probabilistic estimate of the production rate of the aerosol particles that are created by coagulation is carried out in the model as a function of the diameter ranges of the particles that collide. Only collisions of two particles with preservation of mass and volume are taken into account, and it is assumed that coagulation always occurs. The dispersion process of the aerosol is implemented by means of a simple Monte Carlo approach, where the mass fluxes are estimated with the mean wind field and the distribution of the turbulent fluctuations of wind velocity. Coagulation and dispersion are coupled in the mathematical formulation and numerical solution of the mass balance equations of the model. With this model we studied the steady state behaviour of an aerosol released to the atmosphere by an elevated point source. In particular, we studied the way in which the processes of coagulation and dispersion affect the distribution of the particle number, and also their effects on the concentration of the number of particles as a function of the downwind distance from the emission source. Qualitatively, some results obtained, such as a very fast consumption of the smallest aerosol particles near the source, and the accumulation of larger particles at greater distances, are in agreement with documented experimental findings.