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To investigate the formation and evolution mechanism of haze pollution in Beijing cold days, we measured reactive nitrogen gases (e.g. NH3 and HNO3), SO2 and major water-soluble inorganic ions of PM2.5 simultaneously in a two-year (from November to April) study. We found that NH3 and NO3 − have the highest concentrations among the gaseous precursors and inorganic components of PM2.5, respectively. The total NH x (gaseous NH3 and particle NH4 +) was mostly in excess the need to neutralize acid compounds. During the whole study period, the aerosol pH with an average value of 4.05. From normal period into haze episodes, the aerosol pH tends to decrease and the concentration of all species (gases and particles) increases. Meanwhile, declined gas fractions exhibited that enhanced partitioning from HNO3, NH3 and SO2 to their corresponding particle phases. Under the heavy haze period, most HNO3 (79%) has entered into NO3 −, about 41% NH3 remaining as free NH3, while only about 51% of SO2 has been oxidized to SO4 2−, implying the severe Nr pollution in atmosphere of Beijing in winter. Further analysis shows relative humidity (RH) plays an important driving role on the SNA (sulfate (SO4 2−), nitrate (NO3 −), ammonium (NH4 +)) formation and particulate NO3 − formed at a relatively low RH (20%–60%) and SO4 2− at a high RH (40%–80%). Thus, synchronized abatement of multi-pollutants emissions especially for NH3 emission reduction at a regional scale is necessary for mitigating megacities ambient PM2.5 pollution and achieving the UN sustainable development goal through improving N use efficiency in agriculture.