(194c) Hybrid Water Activity – a Novel Framework for CCN Analysis of Sparingly Water Soluble Organic Aerosols

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 (Pth), isophthalic acid (IPth) and terephthalic acid (TPth) are some of the significant benzene polycarboxylic acids. The molecular structure of Pth, IPth and TPth contains two carboxylic acid groups attached on a benzene ring. Pth, IPth and TPth 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 Pth and IPth has been studied and explained using the traditional Köhler Theory (KT). The CCN activity and hygroscopicity of TPth 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 Pth due to its higher aqueous solubility compared to IPth and TPth. In this work, we studied the CCN activity and droplet growth for Pth, IPth and TPth. 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 for pure Pth, IPth, TPth and Pth-IPth internal mixtures. Furthermore, the analysis 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. Additionally, 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 IPth and TPth CCN activity as compared to the hygroscopicity parameter based on KT. Moreover, an improved agreement in experimental FHH-AT hygroscopicity was observed with respect to theoretical FHH-AT hygroscopicity for the AAC-CCN measurements. 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. These computations were found to be in line with either traditional KT or FHH-AT depending on the aqueous solubility of the sample.