Abstract. We applied a global 3-D chemical transport model (GEOS-Chem) to examine the variations in the frequency and intensity in severe winter haze days (SWHDs) in Beijing–Tianjin–Hebei (BTH) from 1985 to 2017 and quantified the roles of changes in anthropogenic emissions and/or meteorological parameters. Observed SWHDs were defined as the days with daily mean PM2.5 concentration exceeding 150 µg m−3, and simulated SWHDs were identified by using the same threshold but with adjustment on the basis of simulation biases. Comparisons between the simulated SWHDs and those obtained from the observed PM2.5 concentrations and atmospheric visibility showed that the model can capture the spatial and temporal variations in SWHDs in China; the correlation coefficient between the simulated and observed SWHDs is 0.98 at 161 grids in China. From 1985 to 2017, with changes in both anthropogenic emissions and meteorological parameters, the simulated frequency (total severe haze days in winter) and intensity (PM2.5 concentration averaged over severe haze days in winter) of SWHDs in BTH showed increasing trends of 4.5 d per decade and 13.5 µg m−3 per decade, respectively. The simulated frequency exhibited fluctuations from 1985 to 2017, with a sudden decrease from 1992 to 2001 (29 to 10 d) and a rapid growth from 2003 to 2012 (16 to 47 d). The sensitivity simulations indicated that variations in meteorological parameters played a dominant role during 1992–2001, while variations in both emissions and meteorological parameters were important for the simulated frequency trend during 2003–2012 (simulated trends were 27.3 and 12.5 d per decade owing to changes in emissions alone and changes in meteorology alone, respectively). The simulated intensity showed a steady increase from 1985 to 2017, which was driven by changes in both emissions and meteorology. Process analysis on all SWHDs during 1985–2017 indicated that transport was the most important process for the formation of SWHDs in BTH with a relative contribution of 65.3 %, followed by chemistry (17.6 %), cloud processes (−7.5 %), dry deposition (−6.4 %), and planetary boundary layer (PBL) mixing (3.2 %). Further examination showed that SWHDs exhibited large interannual variations in frequency and intensity, which were mainly driven by changes in meteorology. The results of this study have important implications for the control of SWHDs in BTH.

We applied the chemical transport model GEOS‐Chem to examine the changes in aerosols and tropospheric O3 in China from 2012–2017 and the associated radiative forcing and health impact. Simulated surface layer concentrations and column burdens of aerosols and O3 were evaluated by comparing with ground‐based and satellite‐retrieved measurements. Between 2012 and 2017, simulated annual mean concentrations of PM2.5 decreased by 21.0%, while O3 increased by 11.9% over eastern China (20–45°N, 105–122.5°E). Changes in aerosols and O3 over 2012–2017 jointly exerted a positive radiative forcing of 1.26 W/m2 over eastern China, which was dominated by the less cooling from PM2.5 reductions (1.18 W/m2). The Global Exposure Mortality Model predicted 268.3 (247.3–291.6) thousand (9.6%) fewer deaths in eastern China in 2017 relative to 2012. These results suggest an appreciable health benefit and a potential warming as a consequence of clean air actions.