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Discussion papers
https://doi.org/10.5194/acp-2019-994
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-994
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 13 Nov 2019

Submitted as: research article | 13 Nov 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Formation mechanisms of atmospheric nitrate and sulfate during the winter haze pollution periods in Beijing: gas-phase, heterogeneous and aqueous-phase chemistry

Pengfei Liu1,2,3,5, Can Ye1,3, Chaoyang Xue1,3, Chenglong Zhang1,2,3, Yujing Mu1,2,3,4, and Xu Sun1,6 Pengfei Liu et al.
  • 1Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China
  • 2Center for Excellence in Urban Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, 361021, China
  • 3University of Chinese Academy of Sciences, Beijing, 100049, China
  • 4National Engineering Laboratory for VOCs Pollution Control Material & Technology, University of Chinese Academy of Sciences, Beijing, 100049, China
  • 5Key Laboratory of Atmospheric Chemistry, China Meteorological Administration, Beijing, 100081, China
  • 6Beijing Urban Ecosystem Research Station, Beijing, 100085, China

Abstract. A vast area in China is currently going through severe haze episodes with drastically elevated concentrations of PM2.5 in winter. Nitrate and sulfate are main constituents of PM2.5 but their formations via NO2 and SO2 oxidation are still not comprehensively understood, especially under different pollution or atmospheric relative humidity (RH) conditions. To elucidate formation pathways of nitrate and sulfate in different polluted cases, hourly samples of PM2.5 were collected continuously in Beijing during the wintertime of 2016. Three serious pollution cases were identified reasonably during the sampling period and the secondary formations of nitrate and sulfate were found to make a dominant contribution to atmospheric PM2.5 under the relatively high RH condition. The significant correlation between NOR and NO2 × O3 during the nighttime under the RH ≥ 60 % condition indicated that the heterogeneous hydrolysis of N2O5 involving aerosol liquid water was responsible for the nocturnal formation of nitrate at the extremely high RH levels. The more coincident trend of NOR and HONO × DR (direct radiation) × NO2 than Dust × NO2 during the daytime under the 30 % < RH < 60 % condition provided convincing evidence that the gas-phase reaction of NO2 with OH played a pivotal role in the diurnal formation of nitrate at moderate RH levels. The extremely high mean values of SOR during the whole day under the RH ≥ 60 % condition could be ascribed to the evident contribution of SO2 aqueous-phase oxidation to the formation of sulfate during the severe pollution episodes. Based on the parameters measured in this study and the known sulfate production rate calculation method, the oxidation pathway of H2O2 rather than NO2 was found to contribute greatly to the aqueous-phase formation of sulfate.

Pengfei Liu et al.
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Short summary
A vast area in China is currently going through severe haze episodes with drastically elevated concentrations of PM2.5 in winter. Nitrate and sulfate are main constituents of PM2.5 but their formations via NO2 and SO2 oxidation are still not comprehensively understood. Our results found that the gas-phase reaction of NO2 with OH and the heterogeneous hydrolysis of N2O5 play key roles in nitrate formation, and SO2 aqueous-phase oxidation with H2O2 rather than NO2 contributed greatly to sulfate.
A vast area in China is currently going through severe haze episodes with drastically elevated...
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