Synoptic Effect of a Heavy Haze Episode over North China

Version 3.6 of the online-coupled meteorology-chemistry model WRF-Chem (Weather Research Forecasting Model with Chemistry) was applied to simulate the feedback effect of a heavy haze episode (15-17 February 2013) on the synoptic conditions, based on optimization of the model scheme with observed data. We focus on the contribution of anthropogenic emissions, including primary particles, inorganic gas compositions, and volatile organic compounds (VOCs), to the formation of fine particulate matter (PM2.5), and the feedback effect on meteorological variables. The model results show that the contribution rate of anthropogenic emissions to the regional average PM2.5 concentration is 91.27% over North China and 96.9%, 95.9%, and 97.2% in Beijing, Qinhuangdao, and Cangzhou, respectively, leading to a 15.99% decrease in regional downward shortwave radiation, with regional radiative forcing at the surface estimated to be -26.51 W m^-2. Consequently, the 2-m temperature decreases by 0.14℃ (3.68%), and the inversion intensifies so that the vertical temperature gradient (∂T/∂z) increases by 0.026 K km^-1, which leads to a boundary layer height reduction of 18.92 m (8.77%). The 10-m wind speed decreases by 0.014 m s^-1 (0.35%), surface relative humidity increases by 0.51%, and surface pressure reduces by 0.86 Pa. For the haze episode during 15-17 February, the feedback effect of anthropogenic emissions on downward shortwave radiation plays a leading role in the weather process, which has a great influence-but not a leading role-on boundary layer height, and has little influence on temperature, wind speed, relative humidity, and surface pressure. As the precursors of secondary organic aerosol (SOA), volatile organic compounds (VOCs) contribute 99.6% to the concentration of SOA; moreover, VOCs transform inorganic gas into inorganic salt by adjusting the reactivity of the atmosphere, and they contribute greater than 50% of sulfates and nitrates. Even so, the contribution of VOCs to the PM2.5 concentration is less than 1/4 of the contribution of all anthropogenic emissions. The feedback effect of anthropogenic VOCs is about the same as anthropogenic emissions, but the effect of VOCs on surface pressure is led by thermal factors and all the anthropogenic emissions are led by dynamic factors. These results imply that the meteorological feedback effect induced by haze pollution was not conducive to the dispersal of air pollutants but contributed to local accumulation, which resulted in a more prolonged and serious pollution episode than might otherwise have been the case. Hence, the self-adjustment of air pollutants is an ineligible factor in analyzing the formation mechanism of regional haze.