NO promotion of SO conversion to sulfate: An important mechanism for the occurrence of heavy haze during winter in Beijing.

In this study, concentrations of NO, SO, O and fine particles (PM) 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 NO promoted the conversion of SO 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 SO, NH, NO and organic aerosol were positively related to the concentration of added NO. These results provide definitive evidence that NO can promote the conversion of SO to sulfate. At the same time, we found that NO can promote the formation of NH 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.