NO_x promotion of SO_2 conversion to sulfate: An important mechanism for the occurrence of heavy haze during winter in Beijing

摘要

AbstractIn this study, concentrations of NOx, SO2, O3and fine particles (PM2.5) were measured at three monitoring stations in Beijing during 2015. For extreme haze episodes during 25 Nov. - 3 Dec. 2015, observation data confirmed that high concentrations of NOxpromoted the conversion of SO2to sulfate. Annual data confirmed that this is an important mechanism for the occurrence of heavy haze during winter in Beijing. Furthermore, in situ perturbation experiments in a potential aerosol mass (PAM) reactor were carried out at Shengtaizhongxin (STZX) station during both clean and polluted days. The concentrations of SO42−, NH4+, NO3−and organic aerosol were positively related to the concentration of added NO2. These results provide definitive evidence that NO2can promote the conversion of SO2to sulfate. At the same time, we found that NO2can promote the formation of NH4+and organic compounds in the aerosols. Our results illustrate that strengthened controls of nitrogen oxides is a key step in reducing the fine particles level in China.Graphical abstractDisplay OmittedHighlights•NOxpromoted the conversion of SO2to sulfate.•An important mechanism in the heavy haze occurrence in winter in Beijing.•NOxpromoted the formation of NH4+and organic compounds in the aerosols.•Controls NOxis the key steps to reduce the urban PM level in China.