关键词: 激光雷达, 高原, 西宁市, 消光系数, 退偏振比

Abstract: Objective　Sand and dust weather is one of the main meteorological disasters in Qinghai, China. With the increasing frequency, intensity, and scope of sand and dust weather, the degree of harm is also constantly increasing, seriously affecting human health, production, life, and ecological environment. Effective monitoring and forecasting can reduce the loss of people's lives and property to a certain extent. Aerosol lidar is an active remote sensing technology that can detect vertical distribution information of aerosols. By acquiring data such as extinction coefficient, backscattering coefficient, and depolarization ratio, aerosol lidar can detect the optical properties, spatial vertical distribution, and evolution process of atmospheric aerosols in the troposphere, and plays an important role in analyzing the mechanism of sandstorm weather, the sources and distribution of pollutants, and the transmission and deposition mechanism of dust aerosols. There are many cases of using lidar to monitor dust aerosols in China. In this work, we use the data detected by aerosol lidar to study the changes in ground particulate matter, in order to test the effectiveness of Qinghai's first aerosol lidar in pollution monitoring. Methods　Based on the observation data of lidar and ground particle stations, a dust transport process in Xining, Qinghai Province, China, in March 2021 is comprehensively analyzed. The study investigates a pollution weather event utilizing MICAPS meteorological observation data, upper air sounding data, and hourly PM2.5 and PM10 mass concentration average data from two national monitoring stations, including the Xining Environmental Monitoring Station and No.4 hospital. The lidar observation siteis located at the Ershilipu Urban Meteorological Station (36.72°N, 101.76°E) in Xining, which is away from the urban area, with low traffic volume and no obvious pollution sources. The station is equipped with atmospheric particulate matter monitoring lidar and automatic atmospheric particulate matter monitoring instruments. The atmospheric particulate matter monitoring lidar is AGHJ-I-LIDAR, which operates based on the Mie scattering principle. The lidar can emit lasers with two wavelengths of 532 nm and 355 nm, and the receiving telescope collects backscattered signals from aerosols, clouds, and other particles. By analyzing the backscattered 355 nm echo signals as well as the vertical and horizontal polarization signals at 532 nm, the lidar can determine the extinction characteristics and polarization properties of particles, which enables the identification of spatiotemporal distributions of particulate matter and facilitates the recognition of dust, clouds, and local pollutants. Results and Discussion　By analyzing the weather situation and the observation changes of ground particles, it was shown that the lidar space observation results were consistent with the near ground monitoring results. The results showed that the dust aerosol transport led to the severe pollution weather process in Xining area in March 2021. Before the pollution incident, the cold air from the north poured into the Qaidam Basin and failed to cross the Qilian Mountains, while the cold air from the east was pulled by 700–500 hPa low vortex shear and near ground thermal and low pressure, and poured back into Hehuang Valley under the influence of terrain, resulting in the pollution in Xining. Lidar had good image recognition significance in plateau polluted weather, especially in the vertical direction of space. During the process of the pollution, Xining city had obvious areas with high extinction coefficient and depolarization ratio, indicating the passage of sand and dust. The depolarization ratio reached about 0.3, indicating that sand and dust began to pass through Xining at this time and affected the air quality of Xining City, and the daily pollution index of Xining city reached severe pollution. From the combination of extinction coefficient and depolarization ratio, it could be seen that the vertical height of pollutants was relatively low, basically below 1 km, which was closely related to the high altitude of Qinghai Province. Conclusions　The weather pollution in Xining in March 2021 was mainly caused by the transport of sand and dust aerosols, which flowed back into the Hehuang Valley under the influence of terrain, resulting in pollution in Xining. Lidar can intuitively reflect changes in pollution in the vertical direction. From the extinction coefficient and depolarization diagram, it can be seen that the altitude of pollutants is relatively low, mostly below 1 km, which may be related to the higher altitude in Qinghai Province. As this study is a preliminary attempt of using lidar equipment in the Qinghai Plateau, many conclusions still need further validation.

Key words: lidar, plateau, Xining City, extinction coefficient, depolarization ratio