A Modeling Study of the Impact of Crop Residue Burning on PM2.5 Concentration in Beijing and Tianjin during a Severe Autumn Haze Event

Crop residue burning is one of the important types of biomass burning in China and has potentially important effect on air quality and climate. A coupled meteorology and aerosol/chemistry model (WRF-Chem) with ground and satellite observations and biomass burning emission inventory were applied to investigate the spatial/temporal distribution and transport pathways of air pollutants and to quantify the contribution of crop residue burning to aerosol concentration in the North China Plain, with focus on Beijing and Tianjin during a severe haze episode on 7-11 October 2014, when the daily mean surface PM2.5 concentration in Beijing reached 317 mu g m(-3). During this period, intensive crop fires were detected over wide areas of eastern Henan, southern Hebei and western Shandong, and the crop residue burning emission was much larger than anthropogenic emission in major fire areas. Model comparison with ground observations demonstrated the WRF-Chem was able to generally reproduce surface meteorological variables and PM2.5 concentration, although it tended to overpredict wind speed and aerosol concentration in some locations. Taking crop residue burning into account can apparently improve PM2.5 prediction during the haze episode. The stagnant weather condition favored haze formation and maintenance in this region, and crop residue burning intensified haze pollution in both fire source and downwind regions. The crop residue burning emission on average contributed 19% to surface PM2.5 concentration in Beijing during the haze episode, in which it contributed 40% and 29% to organic carbon aerosol and primary PM2.5, respectively, and less to black carbon aerosol (4.9%). The impact of crop residue burning in Tianjin was smaller than that in Beijing, with an average contribution of 7.4% due to different fire sources and transport pathways.