Impact of Biomass Burning and Vertical Mixing of Residual‐Layer Aged Plumes on Ozone in the Yangtze River Delta, China: A Tethered‐Balloon Measurement and Modeling Study of a Multiday Ozone Episode

Ozone (O3) is one of the most important trace gases in the troposphere because of its impact on human health, crop yield, climate change, and atmospheric oxidizing capacity. In recent years, China has suffered from deleterious O3 pollution in many regions, especially in eastern China during summer. However, understanding the causes of high O3 pollution episodes is still limited because existing measurements were mainly conducted at the surface level. In this study, we conducted an intensive tethered‐balloon measurement at the Station for Observing Regional Processes of the Earth System in the Yangtze River Delta during a multiday O3 episode in mid‐June 2014, with the highest hourly surface O3 record (142 ppbv) at the station in 2014. By integrating available ground‐based, remote sensing and aircraft measurement data together with chemical transport modeling and Lagrangian dispersion modeling, we carried out a comprehensive analysis of the main source and formation mechanism for two typical days during the multiday O3 episode. On 11 June, agricultural straw burning in the north significantly enhanced O3 concentration (40–50 ppbv) above the planetary boundary layer (PBL) over Nanjing and further influenced the downwind surface O3 in the following days. On 14 June, regional photochemically aged air masses from city clusters in the Yangtze River Delta substantially influenced early morning ground O3 concentrations by vertical mixing from the residual layer and contributed to PBL daytime O3 buildup. This study provides a clear picture of the transport and mixing of O3 and relevant pollutants in the PBL from different sources and highlights the significance of regional emission control in the mitigation of photochemical pollution in eastern China.