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

Abstract

Abstract. Aerosol pH is often calculated based on different standard states thus making it inappropriate to compare aerosol acidity parameters derived thereby. Such comparisons are however routinely performed in the 15 atmospheric science community. This study attempts to address this issue for the first time by providing a theoretical framework to compare and convert between aerosol pH values calculated based on molarity, molality and mole fractions. Using hourly ionic species measurements in Guangzhou, China, it is observed that pHx (mole fraction based) is always 1.74 pH unit higher than pHm (molality based) and follow the same trend, regardless of aerosol property. The difference between pHx and pHc (molarity based), on the other hand, ranges from 1.74 to 20 1.89 depending on the density of hygroscopic aerosol. It is observed that application of this pH standardization protocol can significantly influence conclusions on aerosol acidity reported by past studies and is thus highly recommended.

Aerosol pH Dynamics During Haze Periods in an Urban Environment in China: Use of Detailed, Hourly, Speciated Observations to Study the Role of Ammonia Availability and Secondary Aerosol Formation and Urban Environment

“…In addition to ISORROPIA‐II, we used E‐AIM to calculate the pH and the H + activity coefficients for some samples to compare the pH estimated by different models and to evaluate the effect of the H + activity coefficient on pH. For the same samples, we found the mean pH from E‐AIM was 4.1, a little bit lower than from ISORROPIA‐II (mean pH = 4.7), and the influence of H + activity coefficients on pH was weaker than that of H + molality (log 10 γ(H + ) = 0.32 ± 0.22, γ(H + ) is the activity coefficient of H + ), which agrees well with other recent assessments (Jia et al, ; Song et al, ).…”