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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. NH _3 and HNO _3 ), SO _2 and major water-soluble inorganic ions of PM _2.5 simultaneously in a two-year (from November to April) study. We found that NH _3 and NO _3 ^− have the highest concentrations among the gaseous precursors and inorganic components of PM _2.5 , respectively. The total NH _x (gaseous NH _3 and particle NH _4 ^+ ) 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 HNO _3 , NH _3 and SO _2 to their corresponding particle phases. Under the heavy haze period, most HNO _3 (79%) has entered into NO _3 ^− , about 41% NH _3 remaining as free NH _3 , while only about 51% of SO _2 has been oxidized to SO _4 ^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 (SO _4 ^2− ), nitrate (NO _3 ^− ), ammonium (NH _4 ^+ )) formation and particulate NO _3 ^− formed at a relatively low RH (20%–60%) and SO _4 ^2− at a high RH (40%–80%). Thus, synchronized abatement of multi-pollutants emissions especially for NH _3 emission reduction at a regional scale is necessary for mitigating megacities ambient PM _2.5 pollution and achieving the UN sustainable development goal through improving N use efficiency in agriculture.