SIZE DISTRIBUTIONS OF POLYCYCLIC AROMATIC-HYDROCARBONS AND ELEMENTAL CARBON .2. AMBIENT MEASUREMENTS AND EFFECTS OF ATMOSPHERIC PROCESSES

A comprehensive data set of polycyclic aromatic hydrocarbon (PAH) and elemental carbon (EC) size distributions has been-measured in ambient aerosol and in vehicular emissions, during the same time frame; using identical sampling instruments and analysis methods. Ambient PAH and E C size distributions are bimodal with peaks' in the 0.05-0.12-mu m (mo de I) and 0.5-1.0-mu m (mode II) size ranges. Mode I was attributed to primary emissions from combustion sources while mode II was attribute d to the accumulation of secondary reaction products on primary aeroso l particles. Size distributions of the 4-ring PAHs (FLT, PYR, CHR, BAA ) show larger secondary peaks than the 5-ring and larger PAHs (BBF, BK F, BAP, DBA, BGP, INP). This can be explained by differences in their sources or in their gas-particle partitioning behavior. The more react ive PAHs are delighted td greater extents in mode II aerosol, which is evidence for aerosol aging. The shift of mass to the secondary mode b y atmospheric processes lowers the average deposition velocity by fact ors of 1.5-3.5, indicating longer atmospheric residence times for thes e species.