Significant decline in aerosols in the mixing layer in Beijing from 2015 to 2020: Effects of regional coordinated air pollution control

Beijing's air quality has improved significantly since the implementation of the Air Pollution Prevention and Control Action Plan in 2013, but the local and regional contributions to this improvement have rarely been studied. Here, the vertical profile of the atmospheric backscattering coefficient (ABC) was measured by a ceilometer in Beijing from 2015 to 2020. The results show that the ABC in Beijing decreased the most at ground level from 2015 to 2020, decreasing 51.4%. Interannual variability decreased with height, and no noticeable change was found in the height range above 600 m. The most apparent declines occurred in autumn and winter, with decreases greater than 55.0%, and the minimum decrease occurred in summer, with a reduction of only 20.0%. To analyze the reasons for the autumn and winter declines, we divided the whole day into four periods according to the evolution characteristics of the atmospheric boundary layer. The significant decrease in the backscattering coefficient near the ground during the daytime confirms the effect of local emission reductions. In contrast, the considerable decreases in the backscattering coefficient measured at different heights in the midday mixing layer demonstrate the contribution of regional transport reduction. The above research results confirm the importance of regional coordinated air pollution control.