Roadside and rooftop measurements of polycyclic aromatic hydrocarbons in PM2.5 in urban Guangzhou: Evaluation of vehicular and regional combustion source contributions

Concurrent sampling of PM2.5 aerosol at a roadside of heavy traffic (1.2 m above ground) and on a nearby rooftop (50 m above ground) was conducted at a same location in urban Guangzhou in September, October 2006 and January 2007. The samples were analyzed for eighteen polycyclic aromatic hydrocarbons (PAHs), together with major aerosol constituents and certain organic tracers for vehicular emissions (hopanes) and biomass burning (levoglucosan). Elemental carbon (EC) and hopanes were observed to be lower by 21–38% and 28–84%, respectively, at the rooftop than the roadside, confirming vehicular emissions as a significant local PM source. On the other hand, sulfate showed little vertical gradient, consistent with its secondary origin and its regional characteristics. The roadside-rooftop sample pairs have provided an opportunity in evaluating relative contributions of vehicular emissions and regional sources to ambient PAHs in this urban location. Concentrations of the total PAHs were ∼43% lower at rooftop in the September 2006 samples while they were at similar levels between rooftop and roadside in the October 2006 and January 2007 samples. Sources of PAHs were investigated through comparing ambient data of PAH isomer pairs and PAH/EC ratios with relevant source profiles including those of Guangzhou roadway tunnel emissions, rice straw/sugarcane leave combustion, and industrial coal combustion. The 4-ring PAHs such as pyrene and fluoranthene had a shift in their dominating source from vehicular emissions in September and October to regional combustion source in January. A few major 5- and 6-ring PAHs such as benzo[ghi]perylene and indeno[1,2,3-cd]pyrene were likely heavily influenced by regional biomass burning emissions in all three sampling months. a)pyrene-equivalent carcinogenic potency (BaPeq) was calculated to evaluate the cancer risk of carcinogenic PAHs on the public. BaPeq levels in PM2.5 were significantly higher at the roadside than those at the rooftop in September; however, levels of BaPeq at the rooftop were drastically elevated and became comparable to those at the roadside in October and January due to regional sources dominating the carcinogenic PAHs. This suggests that it is important to control regional combustion sources to reduce air pollution-related health risk in urban Guangzhou.