Abstract
Abstract. Given significant challenges with available measurements of aerosol acidity, proxy methods are frequently used to estimate the acidity of atmospheric particles. In this study, four of the most common aerosol acidity proxies are evaluated and compared: (1) the ion balance method, (2) the molar ratio method, (3) thermodynamic equilibrium models, and (4) the phase partitioning of ammonia. All methods are evaluated against predictions of thermodynamic models and against direct observations of aerosol–gas equilibrium partitioning acquired in Mexico City during the Megacity Initiative: Local and Global Research Objectives (MILAGRO) study. The ion balance and molar ratio methods assume that any deficit in inorganic cations relative to anions is due to the presence of H+ and that a higher H+ loading and lower cation / anion ratio both correspond to increasingly acidic particles (i.e., lower pH). Based on the MILAGRO measurements, no correlation is observed between H+ levels inferred with the ion balance and aerosol pH predicted by the thermodynamic models and NH3–NH4+ partitioning. Similarly, no relationship is observed between the cation / anion molar ratio and predicted aerosol pH. Using only measured aerosol chemical composition as inputs without any constraint for the gas phase, the E-AIM (Extended Aerosol Inorganics Model) and ISORROPIA-II thermodynamic equilibrium models tend to predict aerosol pH levels that are inconsistent with the observed NH3–NH4+ partitioning. The modeled pH values from both E-AIM and ISORROPIA-II run with gas + aerosol inputs agreed well with the aerosol pH predicted by the phase partitioning of ammonia. It appears that (1) thermodynamic models constrained by gas + aerosol measurements and (2) the phase partitioning of ammonia provide the best available predictions of aerosol pH. Furthermore, neither the ion balance nor the molar ratio can be used as surrogates for aerosol pH, and previously published studies with conclusions based on such acidity proxies may need to be reevaluated. Given the significance of acidity for chemical processes in the atmosphere, the implications of this study are important and far reaching.
Highlights
The acidity of atmospheric particles is a critical parameter that affects air quality and the health of aquatic and terrestrial ecosystems
Prior studies have observed and discussed large differences in aerosol acidity predicted by different models (Ansari and Pandis, 1999; Yao et al, 2006); we do not revisit this analysis but instead seek to understand some of the limitations and uncertainties of using thermodynamic equilibrium models to predict aerosol pH
Fountoukis et al (2009) found that the equilibration timescales for NH3 / NH+4 were on par with the measurement integration timescales during Megacity Initiative: Local and Global Research Objectives (MILAGRO), strongly suggesting that the assumption of equilibrium is valid. This further supports the conclusion above that the reverse-mode models vastly overstate the acidity of the Mexico City aerosol. These results suggest that the two best proxy methods for estimating aerosol pH are (1) thermodynamic equilibrium models run using gas + aerosol inputs, and (2) the phase partitioning of ammonia
Summary
The acidity of atmospheric particles is a critical parameter that affects air quality and the health of aquatic and terrestrial ecosystems. While the trends in emissions are promising in the US and western Europe, ecosystem recovery from the effects of acid deposition is a slow process that can take decades (Likens et al, 1996; Stoddard et al, 1999). Particle acidity affects global biogeochemical cycles by controlling the solubility – and bioavailability – of limiting nutrients that are delivered through atmospheric deposition in many marine environments (Meskhidze et al, 2003, 2005; Nenes et al, 2011). This has important implications for marine primary productivity, the carbon cycle, and even climate (Mahowald, 2011)
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