Technical note: Influence of different averaging metrics and temporal resolutions on the aerosol pH calculated by thermodynamic modeling

Abstract

Aerosol pH is commonly used to characterize the acidity of aqueous aerosols and is of significant scientific interest due to its close relationship with atmospheric processes. The estimation of ambient aerosol pH usually relies on the thermodynamic modeling approach. In existing chemical transport model and field observation studies, the temporal resolution of the chemical and meteorological data given as input to thermodynamic models varies substantially, ranging from less than an hour to a year, because of the inconsistency in the resolution of the original data and the aggregation of time-series data in some studies. Furthermore, the average value of the aerosol pH are represented by diverse metrics of central tendency in existing studies. This study attempts to evaluate the potential discrepancies in the calculated average aerosol pH that arise from differences in both the averaging metric and the temporal resolution, based on the ISORROPIA-II thermodynamic model and example datasets prepared by the GEOS-Chem chemical transport model simulation. Overall, we find that the variation in the temporal resolution of input data may lead to a change of up to more than two units in the average pH, and the averaging metrics calculated based on the pH values of individual samples may be about two units higher than the averaging metrics calculated based on the activity of hydrogen ions. Accordingly, we recommend that the chosen averaging metrics and temporal resolutions should be stated clearly in future studies to ensure comparability of the average aerosol pH between models and/or observations.