Chemically-speciated on-road PM_2.5 motor vehicle emission factors in Hong Kong

PM_2.5 (particle with an aerodynamic diameter less than 2.5 μm) was measured in different microenvironments of Hong Kong (including one urban tunnel, one Hong Kong/Mainland boundary roadside site, two urban roadside sites, and one urban ambient site) in 2003. The concentrations of organic carbon (OC), elemental carbon (EC), water-soluble ions, and up to 40 elements (Na to U) were determined. The average PM_2.5 mass concentrations were 229±90, 129±95, 69±12, 49±18 μg m^-3 in the urban tunnel, cross boundary roadside, urban roadside, and urban ambient environments, respectively. Carbonaceous particles (sum of organic material [OM] and EC) were the dominant constituents, on average, accounting for ~82% of PM_2.5 emissions in the tunnel, ~70% at the three roadside sites, and ~48% at the ambient site, respectively. The OC/EC ratios were 0.6±0.2 and 0.8±0.1 at the tunnel and roadside sites, respectively, suggesting carbonaceous aerosols were mainly from vehicle exhausts. Higher OC/EC ratio (1.9±0.7) occurred at the ambient site, indicating contributions from secondary organic aerosols. The PM_2.5 emission factor for on-road diesel-fueled vehicles in the urban area of Hong Kong was 257±31 mg veh^-1 km^-1, with a composition of ~51% EC, ~26% OC, and ~9% SO₄⁼. The other inorganic ions and elements made up ~11% of the total PM_2.5 emissions. OC composed the largest fraction (~51%) in gasoline and liquid petroleum gas (LPG) emissions, followed by EC (~19%). Diesel engines showed higher emission rates than did gasoline and LPG engines for most pollutants, except for V, Br, Sb, and Ba.