Articles | Volume 18, issue 10
Atmos. Chem. Phys., 18, 7423–7438, 2018
Atmos. Chem. Phys., 18, 7423–7438, 2018

Research article 28 May 2018

Research article | 28 May 2018

Fine-particle pH for Beijing winter haze as inferred from different thermodynamic equilibrium models

Shaojie Song1,*, Meng Gao1,*, Weiqi Xu2, Jingyuan Shao3,4, Guoliang Shi5, Shuxiao Wang6, Yuxuan Wang7,8, Yele Sun2,9, and Michael B. McElroy1,10 Shaojie Song et al.
  • 1School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
  • 2State Key Laboratory of Atmospheric Boundary Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
  • 3Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, China
  • 4Department of Atmospheric Sciences, University of Washington, Seattle, Washington 98195, USA
  • 5State 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
  • 6State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China
  • 7Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77004, USA
  • 8Department of Earth System Science, Tsinghua University, Beijing 100084, China
  • 9University of Chinese Academy of Sciences, Beijing 100049, China
  • 10Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
  • *These authors contributed equally to this work.

Abstract. pH is an important property of aerosol particles but is difficult to measure directly. Several studies have estimated the pH values for fine particles in northern China winter haze using thermodynamic models (i.e., E-AIM and ISORROPIA) and ambient measurements. The reported pH values differ widely, ranging from close to 0 (highly acidic) to as high as 7 (neutral). In order to understand the reason for this discrepancy, we calculated pH values using these models with different assumptions with regard to model inputs and particle phase states. We find that the large discrepancy is due primarily to differences in the model assumptions adopted in previous studies. Calculations using only aerosol-phase composition as inputs (i.e., reverse mode) are sensitive to the measurement errors of ionic species, and inferred pH values exhibit a bimodal distribution, with peaks between −2 and 2 and between 7 and 10, depending on whether anions or cations are in excess. Calculations using total (gas plus aerosol phase) measurements as inputs (i.e., forward mode) are affected much less by these measurement errors. In future studies, the reverse mode should be avoided whereas the forward mode should be used. Forward-mode calculations in this and previous studies collectively indicate a moderately acidic condition (pH from about 4 to about 5) for fine particles in northern China winter haze, indicating further that ammonia plays an important role in determining this property. The assumed particle phase state, either stable (solid plus liquid) or metastable (only liquid), does not significantly impact pH predictions. The unrealistic pH values of about 7 in a few previous studies (using the standard ISORROPIA model and stable state assumption) resulted from coding errors in the model, which have been identified and fixed in this study.

Short summary
Severe haze events occur frequently over northern China, especially in winter. Acidity plays a critical role in the formation of secondary PM2.5 and its toxicity. Using field measurements of gases and particles to critically evaluate two thermodynamic models routinely employed to determine particle acidity, we found that China's winter haze particles are generally within a moderately acidic range (pH 4–5) and not highly acidic (0) or neutral (7) as has been previously reported in the literature.
Final-revised paper