The Importance of Local Thermal Circulations in PM2.5 Formation in a River Valley: A Case Study From the Lower Yangtze River, China

Abstract Poor air quality is often experienced in densely populated areas located alongside large rivers. The presence of a river valley can modify local meteorology, potentially interfering with the formation of air pollutants such as PM 2.5 . However, the mechanisms behind PM 2.5 formation in river valleys and its impacts on surrounding regions remain poorly understood. This study investigates the formation mechanisms of PM 2.5 in the Nanjing reach of the Yangtze River, considering its complex terrain and anthropogenic emissions, through numerical simulation. Simulated results revealed higher PM 2.5 concentrations along the Yangtze River compared to the average concentrations in Nanjing, primarily driven by elevated nitrate level. Factors such as higher humidity, wind speed, ageostrophic wind field, and shallow boundary layer were identified as key contributors to the increased PM 2.5 concentrations along the river. Our study demonstrates significant impact of local thermal circulations on PM 2.5 formation, resulting from the interaction between the large water body of the Yangtze River and the urban area. This phenomenon was confirmed through sensitivity experiments, which showed attenuated local thermal circulations in the absence of the Yangtze River. Source apportionment and process analysis results confirm the dominant roles of local thermal circulation related horizontal and vertical advections, with diurnal fluctuations, in shaping PM 2.5 variations along the river. Inorganic chemistry processes also played a non‐negligible role, particularly during polluted conditions. Local thermal circulations in river valleys are crucial for PM 2.5 formation, highlighting the need for comprehensive strategies to tackle air pollution in similar regions.