On-road and laboratory evaluation of combustion aerosols—Part 2:: Summary of spark ignition engine results

The primary objective was to characterize exhaust aerosols from a small group of in-use, light-duty, spark ignition (SI) vehicles operated on-road, and on a chassis dynamometer. A significant particle signature above background was not measured at highway cruise condition. Number emissions were much higher during acceleration, at high-speed cruise, and during cold–cold starts. Group average fuel-specific number emissions range from 3.9×1014 to 1.0×1017 particles/kg of fuel. Cold–cold start temperatures, driving cycles and vehicular condition influence SI emissions. Elemental carbon was a major contributor to mass emissions measured in chassis dynamometer Unified Driving Cycle (UDC) tests averaging 64% and 34% of the mass emissions for cold–cold and hot start cycles, respectively. Average ratios of cold–cold to hot start emissions were 3.3, 7.6, and 22 for CPC number, filter mass and SMPS volume, respectively. Apportionment results showed that on a weekly weighted basis and on weekdays, the majority of observed particle number was attributed to heavy-duty diesel traffic. Weekend production of particles was attributable to light-duty automobiles. On a per vehicle basis, heavy-duty vehicles produced substantially greater number concentrations. On a fuel-specific basis, heavy-duty vehicles produce slightly higher concentrations of particles than light-duty vehicles. The relative contribution of light-duty vehicles to particle number emissions increased as particle size decreased, for the smallest particles apportioned number emissions were 1.3×1016 and 7.1×1015 particles/kg of fuel for heavy-duty and light-duty vehicles, respectively. Comparison of on-road chase and apportionment results with chassis dynamometer tests in a certification type facility suggests that the latter may underestimate real-world number emissions.