Study on Atmospheric Pollution Characteristics before a Snowfall Event in Autumn in the Beijing Urban Area Using Lidar

Lidar measurements were performed at a heavily polluted site between 5 Nov 2009 and 8 Nov 2009. Aerosol extinction coefficients, AOT (Aerosol Optical Thickness), and depolarization ratio were measured, temperature and relative humidity profiles were acquired from a microwave radiometer. Lidar is an efficient tool for detecting the ABL (Atmospheric Boundary Layer) and atmospheric aerosols. In this study, the aerosol extinction coefficient was retrieved from the measured signal of an ALS300 lidar system using the Fernald method. The ABL height was determined according to the maximum inflexion point of the inversed aerosol extinction coefficient (the height of the maximum decline rate). Atmospheric pollutant characteristics were analyzed using the depolarization ratio from the lidar. The temperature and relative humidity structure of air were obtained from microwave radiometer data. The results show that under the air pollution conditions, the atmosphere had a strong temperature inversion layer, with an inversion intensity up to 1 K (100 m)^-1, and a very low atmospheric relative humidity above 500 m; the atmosphere was highly stable. After the pollution events and before the start of snowfall, the inversion structure disappeared and the relative humidity increased significantly to reach saturation. Lidar depolarization ratio analysis indicated the pollution type to be a typical soot particulate matter pollution with regional characteristics. There was a significant linear relationship between PM2.5 and AOT, with a correlation coefficient of 0.72. Our results show that lidar systems can detect air pollution characteristics and ABL height in polluted weather before snowfall in autumn in urban Beijing.