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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 15 Jul 2019

Submitted as: research article | 15 Jul 2019

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

Observation of nitrate dominant PM2.5 and particle pH elevation in urban Beijing during the winter of 2017

Yuning Xie1, Gehui Wang1,2, Xinpei Wang1, Jianmin Chen2,3, Yubao Chen1, Guiqian Tang4, Lili Wang4, Shuangshuang Ge1, Guoyan Xue1, Yuesi Wang4, and Jian Gao5 Yuning Xie et al.
  • 1Key Laboratory of Geographic Information Science of the Ministry of Education, School of Geographic Sciences, East China Normal University, Shanghai 200241, China
  • 2Institute of Eco-Chongming, 3663 N. Zhongshan Rd., Shanghai 200062, China
  • 3Department of Environmental Science and Technology, Fudan University, Shanghai, China
  • 4State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100080, China
  • 5Chinese Research Academy of Environmental Sciences, Beijing 100000, China

Abstract. Particle acidity is crucial to understand secondary formation processes in pollution events because of its substantial impacts on the physiochemical properties of PM2.5. Recently, particle pH predicted by thermodynamic modeling were used to elucidate the sulfate formation mechanism in China, but the results were biased and controversial. In this article, particle pH was found to be increase as a result of effective sulfur emission control. Benefit from strict pollution control actions, average PM2.5 concentration reduced to a low level (39.7 μg/m3) in urban Beijing during winter of 2017. Compare to history record (2014–2017), SO2 gradually decreased to a low level (3.2 ppbv in 2017 winter) while NO2 kept increasing (21.4 ppbv in 2017 winter). As a response, nitrate's contribution (23.0 μg/m3) to PM2.5 become dominant over sulfate (13.1 μg/m3) during the PM2.5 pollution. The nitrate to sulfate molar ratio significantly increased from 1 to 2.7 (value of 1999 and 2017). As particulate nitrate fraction significantly elevated, particle pH was also found to increase in winter Beijing given sufficient ammonia (average concentration 7.1 μg/m3, 12.9 μg/m3 during pollution). During PM2.5 pollution episodes, the particle pH predicted increased from 4.4 (moderate acidic) to 5.4 (near neutral) as nitrate to sulfate molar ratio increased from 1 to 5. It is found that the major H+ contributor S(VI) was mostly in the form of sulfate, showing anions were more neutralized as nitrate content enriched. In the final part, future prediction of particle acidity change was discussed via sensitivity tests: On one hand, nitrate rich particles would absorb more water compared to the sulfate rich particles. This absorption contrast doubles with low to moderate RH (20 % ~ 50 %). On the other hand, increased level of nitrate and ammonia would have synergetic effects leading to rapid elevation of particle pH to merely neutral (above 5.6). As moderate haze might occur more frequently with high ammonia and particulate nitrate concentration, the major chemical processes during haze events and the control target shall be re-evaluated to obtain the most effective control strategy.

Yuning Xie et al.
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Yuning Xie et al.
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Short summary
As a result of strict emission control, nitrate dominant PM2.5 in pollution episodes was observed in urban Beijing during the winter of 2017–2018. With the help of sufficient ammonia, simulated Particle pH could increase to near neutral (5.4) as nitrate mass fraction increases. Further tests imply that airborne particle hygroscopicity would be enhanced at moderate RH in nitrate dominant particles, and pH elevation might be accelerated when ammonia and particulate nitrate both increases.
As a result of strict emission control, nitrate dominant PM2.5 in pollution episodes was...