Source apportionment of diesel and spark ignition exhaust aerosol using on-road data from the Minneapolis metropolitan area

Air quality measurements were made on interstate highways in the Minneapolis metropolitan area. Gas and aerosol concentrations were measured on weekdays and weekends. By exploiting the difference in the relative volumes of heavy duty (HD) diesel and light duty (LD) spark ignition (SI) vehicles on weekdays and weekends, we were able to estimate apportioned fuel specific emissions. The on-road, apportioned, fuel specific particle number emissions factors, estimated from condensation particle counter (CPC) measurements were 1.34±0.2×10¹⁶ particles kg^-1 for diesels and 7.1±1.6×10¹⁵ particles kg^-1 for spark ignition vehicles. Estimates from the scanning mobility particle sizer (SMPS) measurements were 2.1±0.3×10¹⁵ particles kg^-1 for diesels and 3.9±0.6×10¹⁴ particles kg^-1 for SI vehicles. The difference between CPC and SMPS measurements is mainly due to different lower size detection limits of the instruments, ∼3 and ∼10 nm, respectively. On a weekly weighted basis and on weekdays, the majority of particle number was attributed to HD diesel traffic. Weekend production of particles can be primarily attributed to light duty SI automobiles. On a per vehicle basis, HD vehicles produced substantially greater numbers of particles. On a fuel specific basis, HD vehicles produce slightly higher concentrations of particles than light duty vehicles. The relative contribution of LD vehicles to particle number emissions increased as particle size decreased. The HD apportioned size distributions were similar to size distributions measured during other on-road and laboratory studies. The LD apportioned size distribution was bounded by laboratory and on-road size distributions. Our work is representative of summer, highway cruise conditions. It is likely that under cold start and high load operating conditions LD emissions will increase relative to HD emissions.