AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Acid Dissociation in Organic-Solvent/Water Mixtures and Its Relevance to Gas/Particle Partitioning to Atmospheric OPM
JULIA DEGAGNE, James F. Pankow, Portland State University
Abstract Number: 331 Working Group: Aerosol Chemistry
Abstract The gas/particle partitioning behavior of organic compounds to atmospheric particulate matter (PM) is affected by atmospheric conditions, the volatilities of the partitioning compounds, and the composition of the particular PM. The acidity characteristics of the PM affect the dissociation equilibria of organic acids as well as ammonium and protonated organic bases. Each conjugate acid/base pair is distributed between a neutral and ionic form. Organic salts can precipitate under certain conditions. The gas/particle partitioning behavior of organic acids and bases, which depends on volatility, is highly dependent on this speciation. Descriptions of acid dissociation behavior and volatility in atmospheric PM have, to date, focused primarily on mostly aqueous phases. However, atmospheric PM is a generally very complex, and includes water, inorganics, and up to 90% organic matter. Non-aqueous acid/base chemistry is not adequately represented in current atmospheric aerosol formation models.
We present data describing the acid dissociation behavior of organic acids and protonated amines in mixtures composed of a single organic solution (chosen to approximate the characteristics of one type of organic PM) with varying levels of water content. In such mixtures, the preferential solvation of ions and neutral molecules (by the aqueous portion or the organic portion, respectively) affects the acid-base equilibria of the solutes. We show how the pK$_a values differ from those of an aqueous solution and how they can vary with water content. As water content decreases, in general pK$_a values of organic acids, including acetic, maleic, and oxalic acids, will increase. In contrast, pK$_a values of certain protonated amines can first decrease to a local minimum and then increase, but more gradually than for organic acids. This situation can result in a drastically different speciation than would be expected in an aqueous solution, a consideration with important implications for the gas/particle partitioning of atmospheric organic PM.