Episode study of fine particle and ozone during the CAPUM-YRD over Yangtze River Delta of China: Characteristics and source attribution

Abstract

Fine particle (PM2.5) and ozone (O3) are major air pollutants in China, especially in developed city clusters. A comprehensive field campaign, the Campaign on Air Pollution and Urban Meteorology in Yangtze River Delta (CAPUM-YRD), eastern China is carried out in 2016 to enhance the understanding of regional PM2.5 and O3 pollution. Two super sites in Shanghai, one in Hangzhou, as well as nineteen national monitoring stations in Nanjing and Hefei jointly participate in the campaign. In this study, regional PM2.5 and O3 pollution episodes are characterized based on summer and winter experiments during the CAPUM-YRD. Further, trajectory simulation and source apportionment technology in Comprehensive Air Quality Model with Extensions (CAMx) are applied to explore the source of the pollution episodes. We find that regional synchronous PM2.5 pollution episodes (over 120 μg/m3) occur frequently in YRD. Secondary inorganic and carbonaceous components account for a major proportion (70–80%) in PM2.5. Nitrate concentration is significantly higher (lower) than sulfate in winter (summer). The secondary conversion of sulfur and nitrogen increases alongside elevated PM2.5 levels, with a maximum ratio of 0.28 and 0.18 in winter, respectively. Source apportionment results suggest that the contribution of emissions within YRD dominates in PM2.5 (over 60%) compared to other regions. The inter-regional transport from the north-northwest in eastern China can contribute over 20% to PM2.5 in YRD, with a maximum of 28.4% in Nanjing. Under the transport by enhanced northerly winds, PM2.5 concentrations may increase explosively, with observed peak values in Hefei (312.6 μg/m3) and Nanjing (255.7 μg/m3). Moreover, high levels and remarkable spatial heterogeneity of O3 are observed over YRD in summer, with daily maximum 8-h average O3 above 300 μg/m3 in Nanjing. The occurrence of O3 pollution episodes is significantly impacted by favorable weather conditions and transport at multiple spatial scales. Regional contribution results demonstrate that super-regional transport is prominent (29.4–50.7%) in O3 pollution, especially in coastal cities like Shanghai (30.3–63.0%). Our results suggest that enhanced regional collaboration and efforts are of extreme necessity in long-term and optimized control of severe PM2.5 and O3 pollution in YRD.