Impact of the coal banning zone on visibility in the Beijing-Tianjin-Hebei region.

The Beijing-Tianjin-Hebei (BTH) region, which has the most severe air pollution in China, built a 10,000 km2 coal banning zone for pollution control in 2017. In this study, to evaluate the impact of banning coal zone on visibility (VIS), a chemical composition analysis, a chemical mass closure and the revised IMPROVE algorithm were applied to estimate the chemical components and lighting extinction coefficients (bext) of the fine particulate matter (PM2.5) collected at three urban sites (Beijing (BJ), Tianjin (TJ) and Shijiazhuang (SJZ)) and a regional background site (Xinglong (XL)) during autumn and winter of 2016-2017. Compared to measurements from 2016, the average PM2.5 from 2017 decreased by 44 μg m-3 (BJ), 37 μg m-3 (TJ), 69 μg m-3 (SJZ) and 10 μg m-3 (XL), respectively, accompanied by an improved VIS (3.2-4.6 km). The degradation of VIS caused by atmospheric aerosol is due to the light extinction. The bext clearly decreased by 58%, 51%, 56% and 54% at BJ, TJ, SJZ and XL, respectively. However, the reductions/improvements were more significant in winter than those in autumn, especially at BJ and TJ located in the coal banning zone. The decline (improvement) in PM2.5 (VIS) was 16%-37% (15%-27%) in autumn but 29%-60% (21%-83%) in winter. The reductions in SO42- (Cl-) in winter were 2.8 (3.2) and 7.4 (16.4) times larger than those in autumn at BJ and TJ, respectively. Reductions in ammonium sulfate, one of the main species of PM2.5 caused by coal burning, were particularly pronounced at three urban sites in winter (59%-68%). In addition, the reductions in bext in winter were 2.3 (BJ), 339.4 (TJ), 1.9 (SJZ) and 0.4 (XL) times larger than those in autumn. The results reveal that banning coal zone has a marked effect on controlling pollution in the BTH, especially in winter (scattering aerosol sulfate).