Aerosol vertical distribution and optical properties of different pollution events in Beijing in autumn 2017

As one of the largest metropolitan areas in the world, Beijing has a high aerosol loading. Although the government has taken various measures to reduce emissions, the large-scale and intermittent occurrence of pollution is still a major issue. To understand the mechanisms and characteristics of pollution, Raman-Mie lidar and CE-318 sun photometer were used to study the aerosol vertical distribution and optical properties during dust, haze and clean periods in Beijing that occurred in September to October 2017. Combined with meteorological data, the sources and transmission paths of the pollution events were analyzed. The results are as follows. For the dust event, irregular particles with a large linear particle depolarization ratio played a leading role. The aerosol boundary layer height reached 3.5 km. The volume concentration of coarse particles was obviously larger in the volume size distribution. Strong northwest winds at high altitudes cause high-altitude long-distance transmission of pollutants. And this process can exacerbate air quality of Beijing. For the haze event, high-spherical water-soluble particles were dominant. The extinction coefficient exceeded 1 km−1 while the linear particle depolarization ratio was relatively small. The aerosol boundary layer height was <1 km. The range of aerosol optical depth at 440 nm varied between 0.8 and 2.4. Compared with the dust process, meteorological conditions contribute differently to the haze pollution. Aerosols from the south-west is one of the major resources of haze in Beijing. Combined with static weather such as low wind speed, the pollution level can be continuously increased. During the clean period, favorable weather conditions, such as high wind speed, provided good conditions for the diffusion of local particles. The extinction coefficient and the linear particle depolarization ratio were significantly lower. The single-scattering albedo fluctuated between 0.75 and 0.85, the scattering of light was weak and the visibility was generally high.