Changes of Emission Sources to Nitrate Aerosols in Beijing After the Clean Air Actions: Evidence From Dual Isotope Compositions

Nitrate (NO3-) is a major contributing species to haze formation in Northern China. So far, formation processes and source apportionments of nitrate aerosols during haze pollution have not yet been well understood. In this study, the PM(2.5)samples were collected in Beijing from 13 November to 24 December 2018. In addition to water-soluble ions, oxygen (delta O-18-NO3-) and nitrogen (delta N-15-NO3-) isotopes in particulate NO(3)(-)were also determined, in order to investigate the formation pathways and potential sources of NO(3)(-)aerosols. The results showed that NO(3)(-)was a dominant species (43%) of secondary inorganic aerosols during the sampling period. The delta O-18-NO(3)(-)and delta N-15-NO(3)(-)values averaged at 83.8 +/- 13.4 and 11.5 +/- 5.0 parts per thousand, respectively. Combining isotope compositions and Bayesian isotope mixing model, we found that heterogeneous reaction and gas phase oxidation contributed equally to nitrate formation during the sampling period. However, the contribution of heterogeneous processes to nitrate increased from 39% during the clean period to 64% during the haze period. On average, coal combustion, biomass burning, vehicle emissions, and soil emission contributed 50%, 26%, 20%, and 4%, respectively, to nitrate aerosols during the sampling period. Compared to the result in 2013, the significant decrease (similar to 21%) of relative contribution of coal combustion to nitrate was due to strict reduction of coal consumption in Beijing. Finally, the relative contribution of traffic emissions to nitrate increased from 18% during the clean period to 30% during the haze period, suggesting that control of traffic emissions would be an important way to decrease nitrate concentrations and improve the air quality in Beijing.