Scavenging of a coagulating fine aerosol by a coarse particle mode

Abstract

Aerosol emissions from coal combustion and waste incineration often have a bimodal mass distribution. The coarse mode consists mainly of entrained ash. The fine (submicron) mode may form by volatilization of a small fraction of mineral matter followed by reoxidation and recondensation. Fine mode particles may grow by coagulation into a size range near 0.1 μm, where they are stable over the process residence time. Alternatively, the fine mode may be scavenged by diffusion to the coarse mode. In this paper, we derive a simple analytical criterion for determining whether diffusion to the coarse mode will suppress growth of the fine mode. The analysis is based on two different analytical methods: (1) the self-preserving analysis, and (2) the monodisperse aerosol model and a (numerical) moment method. The half-life, t 1 2 , of the fine mode depends strongly on its initial mass concentration, m 0, and on the first moment of the coarse mode size distribution, M 1 c : t 1 2 ∼ m 0 4/M 1 c5 . There is a fairly sharp separation between the regime where a stable fine mode aerosol develops ( t 1 2 ⪢ t residence ) and the regime of almost complete scavenging of the fine mode by the coarse mode ( t 1 2 ⪡ t residence ). Through m 0, the half-life depends on gas temperature and chemical composition in a complex way.