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Given significant challenges with available measurements of aerosol acidity,proxy methods are frequently used to estimate the acidity of atmosphericparticles. In this study, four of the most common aerosol acidity proxies areevaluated and compared: (1) the ion balance method, (2) the molar ratiomethod, (3) thermodynamic equilibrium models, and (4) the phase partitioningof ammonia. All methods are evaluated against predictions of thermodynamicmodels and against direct observations of aerosol–gas equilibriumpartitioning acquired in Mexico City during the Megacity Initiative: Local and Global Research Objectives (MILAGRO) study. The ion balance and molar ratiomethods assume that any deficit in inorganic cations relative to anions isdue to the presence of H+ and that a higher H+ loading and lowercation / anion ratio both correspond to increasingly acidic particles(i.e., lower pH). Based on the MILAGRO measurements, no correlation isobserved between H+ levels inferred with the ion balance and aerosol pHpredicted by the thermodynamic models and NH3–NH4+partitioning. Similarly, no relationship is observed between thecation / anion molar ratio and predicted aerosol pH. Using only measuredaerosol chemical composition as inputs without any constraint for the gasphase, the E-AIM (Extended Aerosol Inorganics Model) and ISORROPIA-IIthermodynamic equilibrium models tend to predict aerosol pH levels that areinconsistent with the observed NH3–NH4+ partitioning. Themodeled pH values from both E-AIM and ISORROPIA-II run with gas + aerosolinputs agreed well with the aerosol pH predicted by the phase partitioning ofammonia. It appears that (1) thermodynamic models constrained bygas + aerosol measurements and (2) the phase partitioning of ammoniaprovide the best available predictions of aerosol pH. Furthermore, neitherthe ion balance nor the molar ratio can be used as surrogates for aerosol pH,and previously published studies with conclusions based on such acidityproxies may need to be reevaluated. Given the significance of acidity forchemical processes in the atmosphere, the implications of this study areimportant and far reaching

A critical evaluation of proxy methods used to estimate the acidity of atmospheric particles