Effects of reactive nitrogen gases on the aerosol formation in Beijing from late autumn to early spring

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 NHx (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 SO42-, 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 (SO42-), nitrate (NO3-), ammonium (NH4+)) formation and particulate NO3- formed at a relatively low RH (20%-60%) and SO42- 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.

Automated summary

The study in Beijing during cold days found that NH3 and NO3- were the predominant components in PM2.5, with total NHx exceeding the neutralization capacity for acid compounds. Aerosol pH decreased and concentrations of all species increased during haze episodes, with gas fractions shifting towards their corresponding particle phases. High levels of aerosol pollution were associated with elevated NH3 and HNO3 concentrations in the atmosphere.