Ground-level ozone in four Chinese cities: precursors, regional transport and heterogeneous processes

Abstract. We analyzed the measurements of ozone (O₃) and its precursors made at rural/suburban sites downwind of four large Chinese cities – Beijing, Shanghai, Guangzhou and Lanzhou, to elucidate their pollution characteristics, regional transport, in situ production, and impacts of heterogeneous processes. The same measurement techniques and observation-based model were used to minimize uncertainties in comparison of the results due to difference in methodologies. All four cities suffered from serious O₃ pollution but showed different precursor distributions. The model-calculated in situ O₃ production rates were compared with the observed change rates to infer the relative contributions of on-site photochemistry and transport. At the rural site downwind of Beijing, export of the well-processed urban plumes contributed to the extremely high O₃ levels (up to an hourly value of 286 ppbv), while the O₃ pollution observed at suburban sites of Shanghai, Guangzhou and Lanzhou was dominated by intense in situ production. The O₃ production was in a volatile organic compound (VOC)-limited regime in both Shanghai and Guangzhou, and a NO_x-limited regime in Lanzhou. The key VOC precursors are aromatics and alkenes in Shanghai, and aromatics in Guangzhou. The potential impacts on O₃ production of several heterogeneous processes, namely, hydrolysis of dinitrogen pentoxide (N₂O₅), uptake of hydro peroxy radical (HO₂) on particles and surface reactions of NO₂ forming nitrous acid (HONO), were assessed. The analyses indicate the varying and considerable impacts of these processes in different areas of China depending on the atmospheric abundances of aerosol and NO_x, and suggest the urgent need to better understand these processes and represent them in photochemical models.