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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 15 | Copyright
Atmos. Chem. Phys., 18, 11125-11133, 2018
https://doi.org/10.5194/acp-18-11125-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Technical note 09 Aug 2018

Technical note | 09 Aug 2018

Technical note: Comparison and interconversion of pH based on different standard states for aerosol acidity characterization

Shiguo Jia1,2, Xuemei Wang3, Qi Zhang1, Sayantan Sarkar4, Luolin Wu1, Minjuan Huang1,2, Jinpu Zhang5, and Liming Yang6 Shiguo Jia et al.
  • 1School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou 510275, P. R. China
  • 2Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, Sun Yat-sen University, Guangzhou 510275, P. R. China
  • 3Institute for Environmental and Climate Research, Jinan University, Guangzhou 510632, P. R. China
  • 4Department of Earth Sciences, and Centre for Climate and Environmental Studies, Indian Institute of Science Education and Research – Kolkata, Nadia 741246, West Bengal, India
  • 5Guangzhou Environmental Monitoring Center, Guangzhou 510030, P. R. China
  • 6Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576, Republic of Singapore

Abstract. Aerosol pH is often calculated based on different standard states thus making it inappropriate to compare aerosol acidity parameters derived thereby. However, such comparisons are routinely performed in the atmospheric science community. This study attempts to address this issue by comparing PM2.5 aerosol pH based on different scales (molarity, molality and mole fraction) on the basis of theoretical considerations followed with a set of field data from Guangzhou, China as an example. The three most widely used thermodynamic models (E-AIM-IV, ISORROPIA-II, and AIOMFAC) are employed for the comparison. Established theory dictates that the difference between pHx (mole fraction based) and pHm (molality based) is always a constant (1.74, when the solvent is water) within a thermodynamic model regardless of aerosol property. In contrast, pHm and pHc (molarity based) are almost identical with a minor effect from temperature and pressure. However, when the activity coefficient is simplified as unity by thermodynamic models, the difference between pHm and pHc ranges from 0.11 to 0.25pH units, depending on the chemical composition and the density of hygroscopic aerosol. Therefore, while evaluating aerosol acidity (especially, trend analysis) when the activity coefficient is simplified as 1, considering the pH scale is important. The application of this pH standardization protocol might influence some conclusions on aerosol acidity reported by past studies, and thus a clear definition of pH and a precise statement of thermodynamic model parameters are recommended to avoid bias when pH comparisons are made across studies.

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Aerosol pH are often directly compared across studies while ignoring the inconsistency in standard states. This study attempts to address this issue by comparing aerosol pH with different standard states on the basis of theoretical considerations followed with a set of field data as an example. Application of a pH standardization protocol including a precise statement of thermodynamic model parameters is recommended to avoid biases in cross-comparison.
Aerosol pH are often directly compared across studies while ignoring the inconsistency in...
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