A modified semi-implicit method to obtain the evolution of an aerosol by coagulation

In the present work we improve the semi-implicit method previously developed by Jacobson et al. (1994. Atmospheric Environment 28, 1327–1338). In this the particle size distribution has to be discretized in several volume bins, and we must use partition coefficients that relate the particle formed in thecollision to the bin discretization. The original method has the problem of some numerical diffusion and we present two different methods for calculating the coefficients that provide better results. We compare the original and our methods with several analytical solutions for concrete cases of coagulation kernels. Finally, we apply our methods to a typical urban mono-component aerosol and to a multi-component case. More work has to be developed in the comparison of our methods with the original one when growth is present, because this phenomenon swamps out coagulation at large sizes.