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. Aerosol pH is often calculated based on different standard statesthus making it inappropriate to compare aerosol acidity parameters derivedthereby. However, such comparisons are routinely performed in the atmosphericscience community. This study attempts to address this issue by comparingPM 2.5 aerosol pH based on different scales (molarity, molality and molefraction) on the basis of theoretical considerations followed with a set offield data from Guangzhou, China as an example. The three most widely usedthermodynamic models (E-AIM-IV, ISORROPIA-II, and AIOMFAC) are employed forthe comparison. Established theory dictates that the difference betweenpH x (mole fraction based) and pH m (molality based) is always aconstant (1.74, when the solvent is water) within a thermodynamic modelregardless of aerosol property. In contrast, pH m and pH c (molarity based) are almost identical with a minor effect from temperatureand pressure. However, when the activity coefficient is simplified as unityby thermodynamic models, the difference between pH m and pH c rangesfrom 0.11 to 0.25 pH units, depending on the chemical composition and thedensity of hygroscopic aerosol. Therefore, while evaluating aerosol acidity(especially, trend analysis) when the activity coefficient is simplified as1, considering the pH scale is important. The application of this pHstandardization protocol might influence some conclusions on aerosol acidityreported by past studies, and thus a clear definition of pH and a precisestatement of thermodynamic model parameters are recommended to avoid biaswhen pH comparisons are made across studies.

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