10th International Aerosol Conference
September 2 - September 7, 2018
America's Center Convention Complex
St. Louis, Missouri, USA

Abstract View


Factors Affecting Thermodynamic Modeling of Aerosol pH

MICHAEL BATTAGLIA JR., Rodney J. Weber, Athanasios Nenes, Christopher Hennigan, University of Maryland, Baltimore County

     Abstract Number: 931
     Working Group: Aerosol Chemistry

Abstract
Aerosol pH is a fundamental property of aqueous particles that affects many chemical and physical processes in the atmosphere. Presently, aerosol scientists lack the ability to directly measure aerosol pH across all ranges of composition and particle sizes. Because of this, the use of aerosol thermodynamic equilibrium models run with gas + aerosol composition inputs has become the most accurate way to predict aerosol pH. Many aerosol thermodynamic models have been developed, but few systematic comparisons of aerosol pH between models have been carried out. This is especially important for aerosol pH since each model possesses unique constraints and limitations on the composition inputs and solution methodology. In this work, we carry out a detailed analysis of aerosol pH computed with three widely used thermodynamic equilibrium models: the Extended Aerosol Inorganics Model (E-AIM), ISORROPIA-II, and AIOMFAC. We focus on pH differences that arise due to factors previously identified as important: the inclusion of aerosol liquid water associated with inorganic and organic constituents, and treatment of non-volatile cations (Ca2+, Mg2+, K+, Na+). We add to this analysis the effects of the hydrogen ion (H+) activity coefficient calculation, and the activity basis (molality, mol fraction, or molar concentration) used to compute pH. The SOAS campaign is used as the basis for comparison since this dataset has been analysed extensively, including analyses of aerosol pH. Overall, we find the activity basis is the most critical factor when comparing different model results, as pH values computed on different bases can lead to inter-model variations of up to several pH units. The other factors are shown to have a cumulative effect on the model prediction of aerosol pH, but at a lesser magnitude than that of inter-model basis consistency.