Effect of Ambient Dilution on Coagulation of Particulate Matter in a Turbulent Dispersing Plume

In recent years, there has been an increasing need for accurately predicting the nucleation, coagulation, and dynamics of particulate matter (PM) emissions from diesel engines. The proposed United Sates Environmental Protection Agency (USEPA) standard on fine particles, is focused on allowing levels of 50 μg/m³ annual average concentration of PM10 (particles smaller than 10 μm aerodynamic diameter) and an additional annual average standard of 15 μg/m³ of fine particles smaller than 2.5 μm in the atmosphere. Existing legislation for particulates is however, based on measurement by mass but not on the particle number density. The current system does not properly account for the small particulates, mostly of the nucleation type, which have an insignificant mass despite being present in very high numbers. These small particulates in high numbers can contribute extremely large surface areas for biological interaction, and they can pose a serious health threat. To this end, the public health community, automotive industry and aerosol physicists are focusing their attention on developing sampling and measurement techniques that will enable a better understanding of the origin and fate of combustion generated particulate matter (soot). This paper discusses a numerical model that includes the effect of ambient dilution on coagulation of particulate matter emissions in the exhaust plume of a heavy-duty diesel-fueled truck. The integro-differential formulation of polydisperse coagulation equation is solved for the time dependent PM concentration using a semi-implicit finite difference scheme. In these simulations, measured PM data at 20″ (0.51 m) near the plume source is used as an initial condition. The PM size distribution and concentration variations at any spatial location are predicted by the numerical algorithm. This computational model is also validated with the experimental data collected by the West Virginia University researchers.