American Association for Aerosol Research - Abstract Submission

AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA

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A Critical Evaluation of Proxy Methods used to Estimate the Acidity of Atmospheric Particles

CHRISTOPHER HENNIGAN, Jessica Izumi, University of Maryland, Baltimore County

Abstract Number: 84
Working Group: Aerosol Chemistry

Abstract
Aerosol acidity is an important parameter that impacts human health, ecosystems, and biogeochemical cycles. Aerosol acidity also affects many chemical and physical processes in the atmosphere, including aerosol water uptake and the gas/particle partitioning of many semi-volatile compounds. Despite its importance, the acidity of atmospheric particles is generally quite poorly constrained. This is due to both the difficulty in measuring non-conservative H$^+ and to the general disagreement among proxy methods commonly used to estimate aerosol acidity. In this study, five proxy methods of aerosol acidity are evaluated and compared using as inputs a month of ambient measurements collected in Mexico City during the MILAGRO study. The proxy methods evaluated include: 1) the ion balance method; 2) the NH$_4$^+/(Cl$^(-) + NO$_3$^(-) + 2*SO$_4$^(2-)) molar ratio; 3) the ISORROPIA and AIM thermodynamic equilibrium models run in reverse mode (aerosol composition only); 4) the ISORROPIA and AIM thermodynamic equilibrium models run in forward mode (gas + aerosol inputs); and 5) gas-aerosol phase partitioning of the semi-volatile NH$_3/NH$_4$^+ and HNO$_3/NO$_3$^(-) systems. It is shown that the two most common measures of aerosol acidity – the inferred H$^+ loading (in nmol/m$^3) and aerosol pH – have no correlation, a finding that has important implications for the interpretation of many ambient studies. We further highlight major differences and investigate the largest sources of uncertainty across the various proxy methods. Ultimately, the accuracy of each method is assessed and recommendations are made for their appropriate use based on the limitations observed under different atmospheric conditions.