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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-270
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 May 2018

Research article | 14 May 2018

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

Aerosol pH and its influencing factors in Beijing

Jing Ding2,1, Pusheng Zhao1,3, Jie Su1, Qun Dong1, and Xiang Du1 Jing Ding et al.
  • 1Institute of Urban Meteorology, China Meteorological Administration, Beijing 100089, China
  • 2State Environmental Protection Key Laboratory of Urban Ambient Air Particulate Matter Pollution Prevention and Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China
  • 3Max Planck Institute for Chemistry, Mainz 55128, Germany

Abstract. Acidity (pH) plays a key role in the physical and chemical behavior of aerosol and cannot be measured directly. In this work, aerosol liquid water content (ALWC) and size-resolved pH are predicted by thermodynamic model (ISORROPIA-II) in 2017 of Beijing. The mean aerosol pH over four seasons is 4.3±1.6 (spring), 4.5±1.1 (winter), 3.9±1.3 (summer), 4.1±1.0 (autumn), respectively, showing the moderate aerosol acidity. The aerosol pH in fine mode is in the range of 1.8 ~ 3.9, 2.4 ~ 6.3 and 3.5 ~ 6.5 for summer, autumn and winter, respectively. And coarse particles are generally neutral or alkaline. Diurnal variation of aerosol pH follows both aerosol components (especially the sulfate) and ALWC. For spring, summer and autumn, the averaged nighttime pH is 0.3~0.4 unit higher than that on daytime. Whereas in winter, the aerosol pH is relatively low at night and higher at sunset. SO42− and RH are two crucial factors affecting aerosol pH. For spring, winter and autumn, the effect of SO42− on aerosol pH is greater than RH, and it is comparable with RH in summer. The aerosol pH decreases with elevated SO42− concentration. As the NO3 concentration increases, the aerosol pH firstly increases and then decreases. Sulfate-dominant aerosols are more acidic with pH lower than 4, whereas nitrate-dominated aerosols are weak in acidity with pH ranges 3~5. In recent years, the dominance of NO3 in inorganic ions may be another reason responsible for the moderately acidic aerosol. ALWC has a different effect on aerosol pH in different seasons. In winter, the increasing RH could reduce the aerosol pH whereas it shows a totally reverse tendency in summer, and the elevated RH has little effect on aerosol pH in spring and autumn when the RH is between 30% and 80%. The dilution effect of ALWC on Hair+ is only obvious in summer. The elevated NH3 and NH4+ could reduce aerosol acidity by decreasing Hair+ concentration exponentially.

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Latest update: 15 Jul 2018
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Short summary
The pH values of aerosol had been calculated in some studies for the North China Plain in recent years. However, the range of aerosol pH value is controversial at present, mainly due to the differences in calculation methods or observation periods. To provide a more comprehensive and systematic reference for the aerosol pH, this study has carried out a detailed investigation of the relevant methods and used the latest data covering four seasons and different particle sizes in the Beijing area.
The pH values of aerosol had been calculated in some studies for the North China Plain in recent...
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