Hybrid Water Activity – A Novel Framework for CCN Analysis of Sparingly Water Soluble Organic Aerosols

KANISHK GOHIL, Chun-Ning Mao, Dewansh Rastogi, Chao Peng, Mingjin Tang, Akua Asa-Awuku, University of Maryland

     Abstract Number: 215
     Working Group: Aerosol Chemistry

Abstract
Organic aerosols are comprised of hundreds of organic compounds either emitted directly into the atmosphere (Primary Organic Aerosols; POAs) or formed due to chemical transformations in the atmosphere (Secondary Organic Aerosols; SOAs). One such class of compounds are aromatic acids that can act as both POAs and SOAs. Phthalic acid (PTA), isophthalic acid (IPTA) and terephthalic acid (TPTA) are some of the significant benzene polycarboxylic acids. The molecular structure of PTA, IPTA and TPTA contains two carboxylic acid groups attached on a benzene ring. PTA, IPTA and TPTA are structural isomers and differ significantly from each other in terms of water solubility which affects their cloud condensation nuclei (CCN) activity. So far, the CCN activity of PTA and IPTA has been studied and explained using the traditional Köhler Theory (KT). The CCN activity and hygroscopicity of TPTA has not been characterized in the past in the context of cloud activity and droplet formation. Moreover, KT can be effectively applied for CCN activity analysis of PTA due to its higher aqueous solubility compared to IPTA and TPTA. In this work, we studied the CCN activity and subsaturated droplet growth for PTA, IPTA and TPTA. As with other hygroscopicity studies, the supersaturated and subsaturated hygroscopicity derived from KT principles do not agree. In response, we developed the Hybrid Activity Model (HAM) by incorporating the aqueous solubility of a solute within an adsorption-based activation framework. Frenkel-Halsey-Hill (FHH)-Adsorption Theory (FHH-AT) was combined with the compound solubility to develop HAM. Analysis from HAM was validated using laboratory measurements of pure PTA, IPTA, TPTA, and PTA-IPTA internal mixtures. Furthermore, the results generated using HAM were tested against traditional KT and FHH-AT to compare their water uptake predictive capabilities for the compounds selected for this work. A single-hygroscopicity parameter was also parameterized based on the FHH-AT and HAM frameworks. The hygroscopicity parameter developed using FHH-AT provided a better quantification of IPTA and TPTA CCN activity as compared to the hygroscopicity parameter based on KT. It was also found that the hygroscopicity parameter based on HAM could successfully simulate the water uptake behavior of the pure and internally mixed samples. Results suggest that the HAM framework can be applied to aerosols of varying aqueous solubility.