Vertical Structure and Driving Mechanism of PM_2.5 and PM₁₀ Aerosols in Hefei Based on LiDAR Observations (2021–2023)

Abstract. Aerosol pollution remains a significant environmental concern in China. However, the vertical structure and evolution of particulate matter are poorly understood due to the lack of long-term, high-resolution observations. In Hefei, the aerosols during the study period were dominated by a mixture of fine particulate matter (PM_2.5) and coarse particulate matter (PM₁₀), mainly originating from urban traffic emissions, industrial activities, and regional transport, with significant contributions from secondary inorganic aerosols and occasional dust events. To address the knowledge gap in aerosol vertical distribution during different pollution episodes, this study employed an aerosol LiDAR system with 532 nm band to investigate the vertical profile characteristics of aerosols, with a focus on comparing the stratification differences of optical properties between PM_2.5 and PM₁₀ pollution events over Hefei across different periods and altitudes. The seasonal and diurnal variations of aerosol profiles were investigated, and vertical structures were compared on polluted and clean days. The relationship between near-surface particulate matter concentrations and aerosol stratification was analyzed, alongside the dynamic evolution of aerosol layers during typical pollution events. Our results demonstrated that the extinction coefficient (532 nm) of PM_2.5-polluted days below 0.6 km was approximately three times that of PM₁₀-polluted days. In contrast, the depolarization ratio of PM₁₀-polluted episodes remains consistently higher than that of PM_2.5-polluted cases throughout the entire observed altitude range. The differences in extinction between polluted and clean days for PM_2.5 were most pronounced below 0.9 km and subsequently decreased as altitude increased, whereas the differences in PM₁₀ remained significant below 1.2 km. For PM_2.5, the strongest enhancement appeared between 7:00 and 14:00 (Beijing time, BJT). A subtle lifting with height was observed around midday. PM₁₀-polluted days were characterized by a greater vertical extension of high aerosol extinction (reaching up to ~1.2–1.4 km) but a shorter duration of strong extinction, in contrast to PM_2.5-polluted days, which exhibited a more persistent but vertically confined aerosol layer. PM₁₀ pollutant tended to accumulate within the altitude range of 0.4–1.2 km on polluted days. The vertical wind shear (VWS) was weaker on PM_2.5-polluted days compared to clean days. On PM₁₀-polluted days, the VWS in the near-surface layer (1000–900 hPa) was significantly stronger than that on clean days, especially during the early morning and evening periods. The PM_2.5 pollution in Hefei was mostly contributed by temperature inversion and high relative humidity, while PM₁₀ pollution was driven by long-range transport of aerosol particles under the cold front system and dry conditions. These findings highlight the complex interactions between aerosol optical properties, boundary-layer dynamics, and synoptic-scale meteorology, providing new insights into the vertical processes governing air quality in eastern China.