Impacts of lake on diurnal evolution of surface PM_2.5 concentrations around a typical megacity of China

Abstract. Lake-land thermal contrasts significantly modulate regional air quality, yet the coupling mechanisms which inland lakes regulate the diurnal evolution of PM_2.5 and its components remain poorly understood. This study conducts high-resolution (1 km) WRF-Chem simulations over Lake Chaohu and the adjacent megacity of Hefei, China, to elucidate these interactions. Results reveal a distinct diurnal reversal effect. During daytime, the lake presence facilitates a PM_2.5 increase exceeding 10 ug/m³ both over the lake and in surrounding urban areas by suppressed planetary boundary layer height, weakened vertical mixing, and reduced dry deposition velocities, which collectively transform the lake into a "storage zone" that prolongs PM_2.5 lifetimes. This accumulation is dominated by secondary PM_2.5, as the cooler and more humid lake air thermodynamically favors the ammonium nitrate formation. Furthermore, convergence zones where lake breezes meet background winds create localized stagnation traps that intensify shoreline pollution. At night, while the lake surface maintains higher PM_2.5 concentrations than surrounding land, its impact on the city reverses, exerting a purification effect with urban PM_2.5 decreasing by over 10 μg/m³ as land-breeze circulation enhances vertical mixing and facilitates primary pollutant dispersion. Sensitivity experiments reveal that failing to distinguish lake surfaces in emission inventories can significantly amplify daytime pollution. These findings emphasize that lakes act as complex dual regulators of urban air quality, with the identified mechanisms likely applicable to other urban-lake systems globally. This study highlights the necessity of high-resolution meteorological modeling and precise surface characterization for improved air quality forecasting in lake-adjacent megacities regions.