Factors Affecting Thermodynamic Modeling of Aerosol pH

Abstract

Aerosol acidity (pH) is a fundamental property of aqueous particles that affects many chemical and physical processes in the atmosphere. Overall, aerosol pH is a vitally important but ultimately poorly represented parameter in the aerosol literature. Though aerosol pH is currently reported in an increasing number of works, 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. These models rely on the input of detailed speciated aerosol- and gas-phase composition and meteorological data to accurately predict the equilibrium concentrations of components in both the gas and particle phases. While several models exist, not all treat the thermodynamic solution paradigm equally, with emphasis on computational speed or wider species applicability, many are restricted to inorganic species only, with very limited inclusion of water-soluble organic species such as carboxylic acids and sugars. This work seeks to explore how individual variables can affect thermodynamic equilibrium model predictions of aerosol pH in a controlled manner using a combination of ambient and hypothetical aerosol compositions and concentrations. Three specific factors are examined: the meteorological effect; the water-soluble organic composition and concentration effect; and the source influence effect. The meteorological effect is examined by investigating aerosol pH under changing urban-rural meteorological conditions (the urban heat island effect) with temporally- and spatially-invariant aerosol compositions with an inorganics-only aerosol thermodynamic equilibrium model, revealing that urban aerosols are systematically more acidic than rural aerosols for identical aerosol composition due to differences in relative humidity and aerosol liquid water content between the two regions. The water-soluble organic composition effect is examined by extending invariant ambient inorganic sampled aerosols from two distinct regions with hypothetical organic acid and non-acid organics, and utilizing a functional group activity coefficient model. Organic species up to 60% organic dry mass fraction are shown to influence the aerosol pH regularly by as much as 0.5 pH units, with non-acid aerosol species having a larger effect due to their influence on the H+ activity coefficient in solution. The source influence effect is examined in the course of a month-long ambient sample campaign where varied local source influences (agricultural and industrial) were determined to affect measured aerosol composition. It was discovered that during periods of local intense agricultural influence (high gas-phase NH3 contribution), ambient aerosols were not at thermodynamic equilibrium, and that aerosol nitrate was significantly higher than other nearby regions on the east coast. These findings imply that aerosol pH will continue to grow more acidic, as the world becomes increasingly urbanized, while simultaneously working to achieve reductions in regulated emissions known to contribute to aerosol acidity (SO2 gas). The tool most widely used by researchers, reliant on aerosols achieving thermodynamic equilibrium when sampled to generate accurate predictions, may be less useful as more local point sources develop. Development of instruments to directly measure aerosol pH for particles independent of size and composition may become a necessity as these conditions progress in the future.