Spatial variability of air pollutants in a megacity characterized by mobile measurements: Chemical homogeneity under haze conditions

Abstract. Characterization of the spatial distributions of air pollutants on an intracity scale is important for understanding localized sources, secondary formation, and human exposure. In this study, we conducted in situ mobile measurements for the chemical composition of fine particles, volatile organic compounds (VOCs), oxygenated VOCs (OVOCs), and common gas pollutants in winter in the megacity of Beijing. The spatial variations of these gaseous and particulate pollutants under different pollution conditions are investigated. During the less-polluted periods, a large spatial variability exists in the inorganic composition of fine particles, suggesting a wide range of particle neutralization in Beijing. Significant spatial variations are also observed in the composition of organic aerosol (OA), which is mainly driven by local emissions of primary OA from vehicle and cooking exhaust. The spatial variations of VOCs and OVOCs vary by species. In general, hydrocarbon compounds show a large spatial variability driven by traffic emissions, while secondary OVOCs are more spatially homogeneous in concentration. Other gas pollutants show relatively low spatial variabilities, although hot spots of concentration frequently appear which are plausibly caused by high-emitting plumes as well as fast on-road ozone titration. During the haze periods, the spatial variabilities of air pollutants are largely reduced because of the contribution of regional transport. Hydrocarbons and less-oxygenated OVOCs show good positive spatial-temporal correlations in concentration. More-oxygenated OVOCs show good positive correlations among themselves and moderate negative correlations with hydrocarbons, less-oxygenated OVOCs, and particulate components. The results highlight the potential role of chemical homogeneity on the SOA production in the megacity under haze conditions. On the other hand, the spatial heterogeneity of air pollution calls a future need of using fine-resolution models to evaluate human exposure and pollution control strategies.