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

Shiguo Jia,Qi Zhang,Sayantan Sarkar,Liming Yang,Minjuan Huang,Jinpu Zhang,Xuemei Wang,Luolin Wu

doi:10.5194/acp-18-11125-2018

Abstract

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 0.25 pH 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.

Highlights

Aerosol acidity is of great scientific interest due to its effects on human health and atmospheric chemical processes (Amdur and Chen, 1989; Xue et al, 2011)
Taking pHm as an example, the averaged pHm calculated by ISORROPIA-II (2.77±0.36) is 0.25 pH unit higher than that calculated by E-AIM-IV (2.52 ± 0.28), which is consistent with the result reported by Song et al (2018) and Liu et al (2017)
It is worthwhile to note that the activity coefficient of H+ calculated by E-AIMIV (0.57 ± 0.19) is 2.7 times higher than that calculated by AIOMFAC (0.21±0.08) while the molality of H+ calculated using AIOMFAC ((1.98 ± 2.50) × 10−2) is 2.5 times higher than that calculated by E-AIM-IV ((7.80 ± 9.52) × 10−3) the resultant pHm is similar

Summary

Introduction

Aerosol acidity is of great scientific interest due to its effects on human health and atmospheric chemical processes (Amdur and Chen, 1989; Xue et al, 2011). Acidic aerosols are found to correlate with health effects including asthma, bronchitis, and others respiratory diseases along with reduced lung function (Amdur and Chen, 1989; Ricciardolo et al, 2004; Longo and Yang, 2008). Aerosol acidity plays a key role in the gas-particle partitioning of species such as HCl/Cl−, HNO3/NO−3 and NH3/NH+4 , and is vital for predicting lifetimes of gaseous compounds such as HCl, NH3. S. Jia et al.: Aerosol pH comparison for different standard states and HNO3 in the atmosphere (Nemitz et al, 2004; Oss et al, 1998). Aerosol acidity is known to affect the formation of secondary organic aerosols (SOA); e.g., experimental studies show that seed aerosols with acidic surfaces can enhance the formation of organosulfate SOA upon reaction with volatile organic compounds such as octanal, carbonyls, isoprene, limonene, and caryophyllene (Jang et al, 2002)

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Conclusion