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

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 PM2.5. However, the mechanisms behind PM2.5 formation in river valleys and its impacts on surrounding regions remain poorly understood. This study investigates the formation mechanisms of PM2.5 in the Nanjing reach of the Yangtze River, considering its complex terrain and anthropogenic emissions, through numerical simulation. Simulated results revealed higher PM2.5 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 PM2.5 concentrations along the river. Our study demonstrates significant impact of local thermal circulations on PM2.5 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 PM2.5 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 PM2.5 formation, highlighting the need for comprehensive strategies to tackle air pollution in similar regions. Plain Language Summary: Throughout history, civilizations have thrived along major rivers across the globe. However, these densely populated areas often suffer from poor air quality due to high anthropogenic emissions. Notably, fine particulate matter (PM2.5) tends to be higher along rivers than in surrounding urban areas, raising questions about the mechanisms behind PM2.5 formation in river valleys and its impacts on neighboring regions. Using numerical experiments, this study focuses on the Nanjing reach of the Yangtze River as an example to model PM2.5 air quality. We observed a noticeable diurnal pattern of source regions for PM2.5 along the Yangtze River. The findings uncover that the topography of river valleys, coupled with urban terrain, can induce a unique small‐scale thermal circulation embedded within the conventional urban heat island circulation. This local thermal circulation acts as a physical barrier, trapping air pollutants within the valley during the daytime, and aggravating PM2.5 air quality along the river. Understanding the role of local thermal circulation is essential for developing comprehensive strategies for combatting air pollution in urban areas near major rivers. Key Points: The PM2.5 concentration along the Yangtze River is higher than in the neighboring Nanjing urban areasThe contribution from regions outside the Yangtze River to PM2.5 along the river shows a pronounced diurnal patternLocal thermal circulations, induced by the Yangtze River's water body and the urban terrains, interfere with PM2.5 formation [ABSTRACT FROM AUTHOR]