SCAVENGING OF A COAGULATING FINE AEROSOL BY A COARSE PARTICLE MODE

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-mu-m, 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, t1/2, of the fine mode depends strongly on its initial mass concentration, m0, and on the first moment of the coarse mode size distribution, M1-c: t1/2 approximately m(0)4/M1-c5. There is a fairly sharp separation between the regime where a stable fine mode aerosol develops (t1/2 >> t(residence)) and the regime of almost complete scavenging of the fine mode by the coarse mode (t1/2 << t(residence)). Through m0, the half-life depends on gas temperature and chemical composition in a complex way.