The vertical pollution structure reflected by aerosol optical properties in Beijing and its relation with meteorological conditions in the autumn and winter of 2017–2020

Abstract

The high temporal aerosol optical extinction vertical profiles observed by LiDAR in Beijing and its vicinity areas (BIV) in autumn and winter from October 2017 to February 2020 are investigated in this study. Through the clustering analysis using the machine learning, we separate and distinguish the clean conditions and aerosol pollution convergence situations at different altitudes, obtain six vertical aerosol pollution distribution patterns, and discuss the differences in the diurnal variation. Patterns with the entire clean layer are the typical category, accounting for 81.4% of the overall percentage. The patterns with pollution convergence at different heights at the top and above the boundary layer and in the lower and middle parts of the boundary layer account for 11.9% and 6.7%, respectively. The relationship among vertical pollution structure, the meteorological elements, and the near-surface PM2.5 mass concentration are discussed. Pollution convergences are mainly affected by northward aerosol pollution transport. Significant humidification and temperature inversion show aerosol pollution's influence on regional meteorological conditions. The heavy near-surface PM2.5 mass concentration is only related to the category with the aerosol pollution convergence in the lower and middle boundary layer when the high values of the PLAM index (parameters linking air quality to meteorological elements), which quantifies the regional air mass stability, and the boundary layer height (BLH) appear. Other aerosol pollution convergences above the boundary layer cannot reflect in the near-surface PM2.5 mass concentration. The pollution and associated meteorological conditions near the surface are more severe in the evening than in the morning. Regional unfavorable meteorological conditions contributed 47% and 62% to surface pollution in the morning and evening, respectively. Pollution transport and the two-way feedback jointly control the near-surface aerosol pollution in the morning; The unfavorable meteorological conditions may play a dominant role in the evening.