Examining the major contributors of ozone pollution in a rural area of the Yangtze River Delta region during harvest season

Abstract. Open biomass burning (OBB) emits significant amounts of non-methane hydrocarbons (NMHCs), and the mixing of OBB with urban plumes could exacerbate regional ozone (O₃) pollution. In the present study, an observational field campaign was performed in a rural area at the northern edge of the Yangtze River Delta region (YRDR) from 15 May to 24 June 2010, during intensive open burning of wheat residues. The net photochemical production rate of oxidant (O_x = O₃ + NO₂) at the site was evaluated based on a box model (Regional Atmospheric Chemical Mechanism, Version 2) constrained by real-time ambient measurements (e.g., O₃, volatile organic compounds (VOCs), NO_x (NO₂ + NO), J values). Our results showed that both in situ photochemistry and direct transport from urban areas in the YRDR were responsible for the high O_x concentration at the site. During an OBB-impact case, net photochemical production of O_x in the daytime was pronounced, with a 6 h averaged O_x production rate of 13 ± 4 ppbv h⁻¹ (maximum value of 21 ppbv h⁻¹ at 12:00 CST). Photochemical O_xproduction changed from VOC-limited in the morning to NO_x-limited in the afternoon due to the rapid photochemical consumption of NO_x during the day. A combined analysis with positive matrix factorization demonstrated that O₃ pollution in the rural area of the YRDR was largely affected by urban emission, and OBB-related emissions also contributed to in situ photochemical production, particularly in the afternoon. Our study suggested that a joint effort in reducing both NMHCs (e.g., aromatics) and NO_x emissions in the urban area, as well as local OBB activities, may be effective in eliminating high-O₃ pollution risk in the rural areas of the YRDR.