Coagulation in a spatially inhomogeneous plume: Formation of bimodal size distribution

Aerosol systems are more often spatially heterogeneous than homogeneous due to various factors such as turbulence in the atmosphere, flow field in a pipe, and varying generation rates at the source. As a general result, we demonstrate that an initially monodisperse and spatially heterogeneous aerosol system evolves into a bimodal size distribution purely by coagulation. The spatial inhomogeneity in the particle number concentration initiates differential coagulation rates which lead to a distribution with larger size modes in regions with higher concentration. When averaged over space, this would appear as a bimodal size distribution. We show this effect through a free-molecular coagulation model for a spatially heterogeneous system combined with the scaling theory of self-preserving distributions. It is found that sharper the occurrence of spatial heterogeneity, more pronounced is the bimodal effect. The study clearly demonstrates spatial heterogeneity as an additional factor for the origin of bimodality in aerosols.