NOx promotion of SO2 conversion to sulfate: An important mechanism for the occurrence of heavy haze during winter in Beijing.

In this study, concentrations of NOx, SO2, O3 and 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 NOx promoted the conversion of SO2 to 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 NO2 can promote the conversion of SO2 to sulfate. At the same time, we found that NO2 can 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.