Long-term planetary boundary layer features and associated PM2.5 pollution anomalies in Beijing during the past 40 years

Primary work on the planetary boundary layer (PBL) climatic features and associated air pollution in Beijing was carried out based on a 40-year dataset. The main results are identified as follows. The annual mean PBL height was 906 m (1455 m in daytime and 357 m in nighttime). Within the PBL, the average potential temperature, specific humidity, and wind speed were 287.3 K, 5.1 g kg(-1), and 6.5 m s(-1). Significant seasonal variations were observed in the PBL variables, leading to distinct pollutant diffusion conditions. The PM2.5 concentrations varied seasonally, with a high value in winter (97 mu g m(-3)) and a low value in summer (74 mu g m(-3)). The summertime PM2.5 was closely related to the local emission intensity (R = 0.79) and declined with a reduction in the emission intensity, while the winter PM2.5 showed large interannual variabilities and trends, relying more on the PBL conditions. A comparison of polluted and clean winters was performed, revealing that polluted winters were associated with a warmer layer above the PBL that inhibited the development of the PBL (933 m vs. 1283 m, daily maximum height for polluted vs. clean winters) and led to a humid (1.4 g kg(-1) vs. 0.8 g kg(-1)) and weak wind (5.8 m s(-1) vs. 8.2 m s(-1)) layer within the PBL. In terms of synoptic impacts, the warm advection above the PBL (0.07 K h(-1)) and subsidence heating (0.21 K h(-1)) near the top of the PBL contributed to the occurrence of a stable and polluted PBL.