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https://doi.org/10.5194/acp-2020-879
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-879
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  10 Sep 2020

10 Sep 2020

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This preprint is currently under review for the journal ACP.

Significant contrasts in aerosol acidity between China and the Unites States

Bingqing Zhang1, Huizhong Shen1, Pengfei Liu2, Hongyu Guo3, Yongtao Hu1, Yilin Chen1, Shaodong Xie4, Ziyan Xi4, and Armistead G. Russell1 Bingqing Zhang et al.
  • 1School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
  • 2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
  • 3Cooperative Institute for Research in Environmental Sciences and Department of Chemistry, University of ColoradoBoulder, Boulder, Colorado 80309, USA
  • 4College of Environmental Sciences and Engineering, State Key Joint Laboratory of Environmental Simulation and Pollution Control, Peking University, Beijing, 100871, China

Abstract. Aerosol acidity governs several key processes in aerosol physics and chemistry, thus affecting aerosol mass and composition, and ultimately the climate and human health. Previous studies have reported the aerosol pH separately in China and the United States, implying a different aerosol acidity between these two countries. However, underlying mechanisms responsible for the pH difference are not fully understood, limited by the scarcity of simultaneous measurements of aerosol composition and gas species, especially in China. Here we conduct a comprehensive assessment of the aerosol acidity in China and the United States, using extended ground-level measurements and regional chemical transport model simulations. We show aerosol in China is significantly less acidic than that in the United States, with pH values 1–2 units higher. Based on a multivariable Taylor Series method and a series of sensitivity tests, we identify several major factors leading to the pH difference. Compared to the United States, aerosols in China are generally in total ammonia (TNH3 = NH4+ + NH3) rich conditions where particle phase ammonium (NH4+) concentrations are adequate enough to nearly neutralize major acidic inorganic anions such as sulfate, nitrate, and chloride, leading to a higher aerosol pH. Higher relative availability of the stronger acidic component, sulfate, compared with the weaker acidic component, total nitrate (TNO3 = NO3 + HNO3), also contributes to the lower aerosol pH in the United States. As a response to higher aerosol pH, the higher nitrate to sulfate molar ratios in China indicates a nitrate-rich condition, further leading to higher aerosol water uptake which will continually promote nitrate aerosol formation. Considering the historical emissions trends, the difference in aerosol acidity between these two countries is expected to continue as SO2 and NOx emissions are further controlled. The differences in aerosol acidity highlight in the present study imply potential differences in formation mechanisms, physicochemical properties, and toxicity of aerosol particles between China and the United States.

Bingqing Zhang et al.

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Short summary
Large-scale ground-level measurements are coupled with model simulations to comprehensively compare the aerosol acidity in China and the United States. Aerosol in China is significantly less acidic than that in the United States, with pH values 1–2 units higher. The abundance of ammonia and ammonium in China is the leading cause of the pH difference, and the large difference indicates potential distinctions in aerosol formation, composition, and toxicity between these two countries.
Large-scale ground-level measurements are coupled with model simulations to comprehensively...
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