A fast and efficient modified sectional method for simulating multicomponent collisional kinetics

A fast and efficient method for simulating the evolution of internally mixed multicomponent particle size distributions for aerosol coagulation and droplet coalescence is developed. The technique is based upon a bin-wise sectionalization of the particle mass domain and by imposing the condition of mass conservation for each component. The distribution of each species as a function of the total particle mass is represented in each mass bin as a two-parameter exponential function. Particles of a given mass are assumed to be internally homogeneously mixed. The method is shown to be numerically stable for a wide range of time steps. The numerical solution is compared with both analytical results and results from other well-accepted numerical schemes. This comparison reveals that the proposed technique offers the advantage of being fast and accurate, even for coarse spectral resolution. The method is computationally attractive and easily allows the treatment of ten or more different chemical species in a collisionally evolving particle size distribution. The applicability of the method is demonstrated with several examples: Coalescence growth of multicomponent cloud droplet spectra, coagulation of measured multi-species aerosol particle size distributions, and the simulation of the accumulation mode due to a source of small aerosol particles. The technique is ideally suited for modelling the interaction of microphysics and chemistry in a size-bin resolving aerosol or cloud model.