Abstract
Abstract. Road traffic has significant impacts on air quality particularly in densely urbanized and populated areas where vehicle emissions are a major local source of ambient particulate matter. Engine type (i.e., fuel use) significantly impacts the chemical characteristics of tailpipe emission, and thus the distribution of engine types in traffic impacts measured ambient concentrations. This study provides an estimation of the contribution of vehicles powered by different fuels (gasoline, diesel, LPG) to carbonaceous submicron aerosol mass (PM1) based on ambient aerosol mass spectrometer (AMS) and elemental carbon (EC) measurements and vehicle count data in an urban inner city environment in Hong Kong with the aim to gauge the importance of different engine types to particulate matter burdens in a typical urban street canyon. On an average per-vehicle basis, gasoline vehicles emitted 75 and 93 % more organics than diesel and LPG vehicles, respectively, while EC emissions from diesel vehicles were 45 % higher than those from gasoline vehicles. LPG vehicles showed no appreciable contributions to EC and thus overall represented a small contributor to traffic-related primary ambient PM1 despite their high abundance (∼ 30 %) in the traffic mix. Total carbonaceous particle mass contributions to ambient PM1 from diesel engines were only marginally higher (∼ 4 %) than those from gasoline engines, which is likely an effect of recently introduced control strategies targeted at commercial vehicles and buses. Overall, gasoline vehicles contributed 1.2 µg m−3 of EC and 1.1 µ m−3 of organics, LPG vehicles 0.6 µg m−3 of organics and diesel vehicles 2.0 µg m−3 of EC and 0.7 µg m−3 of organics to ambient carbonaceous PM1.
Highlights
1.1 Particulate matter from motor vehicles in urban areasEmissions from on-road motor vehicles comprise gas-phase species (CO, CO2, NOx, SO2) and volatile organic compounds – among them important precursors of secondary organic aerosol (SOA) – as well as primary particulate matter predominantly in the fine (PM2.5) and ultrafine particle size range (PM0.1), mostly as carbonaceous species which encompass primary organic aerosol (POA) components and elemental carbon (Giechaskiel et al, 2014)
Hong Kong. Organic species concentrations (HOA) was resolved and identified as the traffic exhaust related factor, exhibiting chemical characteristics typically associated with freshly emitted primary organic aerosol from fossil fuel combustion (Zhang et al, 2005) and with appreciable correlations to other traffic-related species such as NOx or pentanes (Lee et al, 2015)
While this study has focused on primary emissions, gasphase emissions from the tailpipe can lead to subsequent condensation of organics or atmospheric oxidation to form secondary organic aerosol, significantly enhancing postemission particulate matter concentrations (Gentner et al, 2012; Platt et al, 2013) and should be the subject of further investigation
Summary
1.1 Particulate matter from motor vehicles in urban areasEmissions from on-road motor vehicles comprise gas-phase species (CO, CO2, NOx, SO2) and volatile organic compounds – among them important precursors of secondary organic aerosol (SOA) – as well as primary particulate matter predominantly in the fine (PM2.5) and ultrafine particle size range (PM0.1), mostly as carbonaceous species which encompass primary organic aerosol (POA) components and elemental carbon (Giechaskiel et al, 2014). In urban areas with high population and building densities, proximity to vehicle emissions poses a significant public health risk and renders paramount importance to the characterization and quantification of vehicle emissions (Kumar et al, 2014; Uherek et al, 2010) Their contribution to total ambient concentrations remains elusive, with considerable variability in measured emission rates and species composition between as well as within vehicle classes from exhaust measurements in laboratory settings, vehicle chase or portable emission measurement system (PEMS) studies (Franco et al, 2013; Kwak et al, 2014; Karjalainen et al, 2014; Giechaskiel et al, 2014; Alves et al, 2015). Among major primary and secondary aerosol sources of ambient particulate matter which were identified from speciated long-term filter samples (1998–2008) of PM2.5 by source apportionment analysis utilizing positive matrix factorization, vehicle emissions were shown to make substantial contributions: at general measurement sites (urban, rooftop), vehicle emissions accounted for 8–9 μg m−3 equivalent to 11–25 % of total PM2.5 with lower fractions in the fall and winter seasons, primarily due to the masking in-
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