CHARACTERIZATION OF THE PARTICULATE PHASE IN THE EXHAUST FROM A DIESEL CAR

Diesel vehicles are a major source of fine, atmospheric particulate ma tter in urban environments. The influences diesel particles exert on s olar radiation, on atmospheric chemistry, and on humans depend crucial ly on the size and chemical character of these particles. In this work , size-resolved diesel particle chemistry has been examined by collect ing particles directly from a diesel car exhaust with a low-pressure i mpactor. The impactor samples have been weighed and analyzed chemicall y to construct continuous size distributions for selected compounds pr esent in the particulate phase. Submicron diesel-particle mass size di stributions displayed three log-normal modes that were centered at 0.0 9, 0.2, and 0.7-1 mu m of particle aerodynamic diameter (EAD) and that had average geometric standard deviations of 1.34, 1.61, and 1.34, re spectively. The lowest two modes had approximately the same particulat e mass, whereas over 80% of the number of particles were estimated to be found in the mode around 0.1 mu m. The third mode contained about 1 0% of the total particulate mass but less than 0.1% of the particles. The size distributions of elemental (EC) and organic carbon (OC) were quite different: EC peaked at 0.1 mu m, and OC peaked somewhere betwee n 0.1 and 0.3 mu m of EAD. The mass ratios of OC to EC were between 0. 3 and 0.5 in the bulk of the samples but were considerably lower for m ost of the particles. The presence of a catalytic converter reduced pa rticulate mass by 10-30%, with the removal being more efficient for OC than EC. The principal mechanism producing the mode around 0.1 mu m w as shown to be Brownian coagulation between small primary particles fo rmed during the combustion. The two larger size modes in the submicron particle range were hypothesized to be formed by activation and subse quent uptake of condensable organic compounds by some of the mode 1 pa rticles.