Evolution of nanoparticle size and mixing state near the point of emission

Abstract

This study examines the evolution of the size distribution and mixing state of soot and background particles near a point and line source of emission. This evolution occurs invariably at a spatial scale smaller than that of the grid scale of urban through global atmospheric models, and the evolved distribution is that which is properly the source distribution “emitted” into such models. A recent set of field data showed that, within minutes of emission, the soot particle size distribution evolved substantially, and it was hypothesized that Brownian coagulation was the main cause of the evolution. Here, it is found that Brownian coagulation, alone, may be insufficient to account for the observed rapid evolution of the size distribution. Enhancement of Brownian coagulation due to van der Waals forces offset by viscous forces and fractal geometry may account for a greater share of the evolution. These coagulation processes are represented together with aerosol emissions, nucleation, condensation, dissolution, hydration, and chemistry among 10 aerosol classes in a high-resolution three-dimensional numerical simulation. Dilution is found to be more important than coagulation at reducing the total number concentration of particles near the source of emission, but the relative importance of dilution versus coagulation varies with concentration. It is also found that heterocoagulation of emitted soot with background particles produces new mixtures in increasing concentration with increasing distance from the emission source. However, self-coagulation of emitted soot reduces particle number concentration by an order of magnitude more than does heterocoagulation of emitted soot with background particles in the first few minutes after emission. Heterocoagulation increases in relative importance as emitted particles age.