Integrated impacts of synoptic forcing and aerosol radiative effect on boundary layer and pollution in the Beijing-Tianjin-Hebei region, China

Abstract. Rapid urbanization and industrialization have led to deterioration of air quality in the Beijing-Tianjin-Hebei (BTH) region with high loadings of PM2.5. The heavy aerosol pollutions frequently occur in winter, closely in relation to the meteorological conditions. To unravel the complicated impacts of large-scale atmospheric forcing and the local-scale planetary boundary layer (PBL) characteristics on the pollution there, this study combined long-term observational data analyses, synoptic pattern classification, and meteorology-chemistry coupled simulations. During the winter of 2017 and 2018, Beijing, Langfang, Tianjin, and Tangshan often simultaneously experienced heavy PM2.5 pollution, accompanying with strong thermal inversion aloft. These concurrences of pollution in different cities were primarily regulated by the large-scale atmospheric processes. Using the principal component analysis with the geopotential height fields at the 850-hPa level during winter, the typical polluted synoptic pattern in BTH was identified. The pattern was featured by westerly winds from upstream mountainous regions. By inducing warm advections from the west, the thermal inversion aloft in the BTH could be enhanced, leading to shallow daytime PBLs and high near-surface PM2.5 concentrations. In addition, the aerosol may also modulate the PBL structure through its radiative effect, which was examined using numerical simulations. The aerosol radiative effect can significantly lower the boundary layer height in the afternoon through cooling the surface layer and heating the upper part of PBL. Thus, more aerosols could be accumulated in the lower portion of PBL, bringing about heavy pollution in the BTH. This study has revealed the important roles played by the meteorology-aerosol interaction on the air quality.