A severe fog-haze episode in Beijing-Tianjin-Hebei region: Characteristics, sources and impacts of boundary layer structure

Beijing-Tianjin-Hebei (BTH) region is recognized as one of the most polluted regions in China. In this study, characteristics, synoptic condition, boundary layer structure, and sources of a severe fog-haze episode during the December of 2016 and January of 2017 are investigated. The results show that BTH region is controlled by the uniform pressure field and southwest wind, and the average mass concentrations of PM2.5 in Beijing (BJ), Tianjin (TJ), and Shijiazhuang (SJZ) are 263.5, 192.7, and 296.1 mu g/m(3), respectively. Besides, the maximum hourly concentration and growth rate of PM2.5 are 578.8 mu g/m(3) and 13.1 mu g/(m(3).h), respectively for BJ. According to the wind profiler radar and L-band radar data, the maximum concentration of PM2.5 in two stages basically coincides with the minimum ventilation index (VI) value, and the minimum VI value in stage I and stage II are 5124 m(2)/s and 4524 m(2)/s. Moreover, the two stages are accompanied by significant inversion layer in the low levels. Although the maximum temperature inversion (10.8 degrees C) in stage I is higher that in stage II (8.3 degrees C) in the boundary layer, the maximum inversion strength in stage II (0.33 degrees C/hPa) is stronger than that in stage I (0.20 degrees C/hPa). Besides, weighted potential source contribution function (WPSCF) and conditional probability function analysis(CPF) are applied to investigate the potential source regions of PM2.5, the results show that southern BJ, as well as central and southern Hebei province (HB) are the main potential sources, and CPF values from the wind direction (180 degrees-270 degrees) are all over 0.3.