Investigating the chemical species in submicron particles emitted by city buses

Detailed chemical characterization of exhaust particles from 23 individual city buses was performed in Helsinki, Finland. Investigated buses represented different technologies in terms of engines, exhaust after-treatment systems (e.g. diesel particulate filter, selective catalytic reduction and three-way catalyst) and fuels (diesel, diesel-electric (hybrid), ethanol and compressed natural gas). Regarding emission standards, the buses operated at EURO III, EURO IV and EEV (Enhanced Environmentally Friendly Vehicle) emission levels. The chemical composition of exhaust particles was determined by using a Soot Particle Aerosol Mass Spectrometer (SP-AMS). Based on the SP-AMS results the bus emission particles were dominated by organics and refractory black carbon (rBC). The mass spectra of organics consisted mostly of hydrocarbon fragments (54?86% of total organics), the pattern of hydrocarbon fragments being rather similar regardless of the bus type. Regarding oxygenated organic fragments, ethanol-fueled buses had unique mass-to-charge ratios (m/z) of 45, 73, 87 and 89 (mass fragments of C2H5O+, C3H5O2+, C4H7O2+ and C4H9O2+, respectively) that were not detected for the other bus types at the same level. For rBC there was a small difference in the ratio of C4+ and C5+ to C3+ for different bus types but also for the individual buses of the same type. In addition to organics and rBC, the presence of trace metals in the bus emission particles was investigated.