Two Trans-Boundary Aerosol Transport Episodes In The Western Yangtze River Delta, China: A Perspective From Ground-Based Lidar Observation

Aerosol pollution is a significant threat to public health, and ground-based lidar can effectively monitor air pollution and regional air quality, including providing accurate observations of the vertical distribution of aerosols. Two long-distance trans-boundary aerosol episodes (named P2 and P5; P2 was the heavy pollution period (from 19:00 on January 19 to 15:00 on January 20) and P5 was the moderate pollution period (from 03:00 on January 25 to 21:00 on January 25)) occurred in the western Yangtze River Delta region, China during 17-26 January 2019, causing significant PM2.5 pollution. The ground-based lidar average value of the aerosol extinction coefficient below 0.5 km was highly correlated with surface PM2.5 concentration, and the correlation coefficient was 0.89. The regression relationship between the planetary boundary layer (PBL) height and PM2.5 concentration value was significantly exponential with a correlation coefficient of -0.59. The PBL height during P2 and P5 was below 0.5 km, relative to other periods with PBL height ranging from 0.5 to 2.0 km. Based on the backward trajectory and weather pattern analysis and surface PM(2.5 )observations, aerosols during P2 and P5 were mainly contributed by transported PM2.5 blown from northern China during the winter monsoon. The depolarization ratios derived from the lidar data show that the persistent pollution episode was mainly caused by strong mixing of transported and local aerosols. Relative to the P2 period, the wind speed in the P5 period was relatively strong, so transmission and diffusion took place relatively quickly and the duration of the pollution process was shorter. Our findings provide scientific evidence that transboundary air pollution were closely associated with different weather conditions and mixed aerosol sources, particularly for various PBL heights.