Effects of nocturnal boundary layer subsidence and long-distance transports on PM2.5 vertical profiles in the Yangtze River Delta of China measured by PM sensor on unmanned aerial vehicle and PM Lidar

Abstract

Atmospheric boundary layer structures and long-distance transports significantly affect fine particulate matter (PM_2.5) vertical profiles. In this study, the PM_2.5 vertical profiles in the Yangtze River Delta (YRD) of China were measured by PM sensor on unmanned aerial vehicle (UAV) and PM Lidar in 2022 (April, June, October) and 2023 (February). The results showed that the PM_2.5 vertical profiles appeared obvious stratification on the top of nocturnal boundary layer (NBL). The NBL subsidence stabilized the boundary layer structure and inhibited the vertical diffusion of PM_2.5, increasing the ground PM_2.5 concentrations. However, when there was an active turbulent motion during the NBL subsidence, the PM_2.5 from the surface might be transported upward. The PM_2.5 vertical mean concentrations (0-500 m) in the YRD decreased by 58-95% within 3 hours, which might be caused by the rapid shift of long-distance transport sources from the North China Plain to the Yellow Sea with higher wind speeds according to backward trajectory. When the ambient PM_2.5 concentrations were high (>20 μg·m^-3) and the weather was clear, the PM Lidar could also observe the diurnal variations of PM_2.5 vertical profiles (200-500 m) like the PM sensor on UAV. However, there were differences in the PM_2.5 vertical concentrations, and the differences of PM_2.5 vertical mean concentrations (200-500 m) measured by the two methods in different seasons were 2.2-13.6 μg·m^-3. When the PM_2.5 concentrations were lower than 17 μg·m^-3, the measurement performance of PM Lidar was significantly lower than those of the PM sensor on UAV.