On the severe haze in Beijing during January 2013: Unraveling the effects of meteorological anomalies with WRF-Chem

Despite the stringent emission reduction measures implemented in Beijing over the past decade, a series of unprecedentedly severe haze events hit this megacity in January 2013. It is of great interest to find out the cause so as to provide a scientific basis for refining emission control measures. In the present study, we examine long-term (2000-2014) surface meteorological observations and simulate four recent winter haze episodes in 2010-2014 using a coupled meteorology-chemistry model (WRF-Chem). In addition to confirming the large-scale meteorological anomalies in northern China, the analysis of local meteorological parameters revealed that January 2013 had more frequent sustained weak southerly winds and high relative humidity in Beijing. Comparison of WRF-Chem simulations of the four episodes unambiguously shows that the combination of anomalously strong contribution of local and regional sources resulted into the extreme event in 2013: meteorological anomalies caused thicker temperature inversion, lower boundary layer, and hence stronger local accumulation of PM2.5in urban Beijing (212μgm-3in 2013 case vs. 112-114μgm-3in historical cases); longer duration of southerly winds transported more pollutants to urban area (107μgm-3vs. 38-82μgm-3) from eastern China. Our study also suggests that, although the emissions in Beijing have been decreased, they were still the major contributor (61-77%) to surface-layer PM2.5over the urban area in recent winter episodes. Since adverse weather conditions such as those in January 2013 are uncontrollable, to alleviate severe haze pollution, Beijing must further strengthen its emission reduction measures and similar control should be extended to the entire eastern China.