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. Moreover, KT can be effectively applied for CCN activity analysis of Pth due to its higher water solubility compared to IPth and TPth. In this work, we studied the CCN activity and droplet growth for Pth, IPth and TPth by incorporating adsorption-based activation. We used the Frenkel-Halsey-Hill (FHH)-Adsorption Theory (FHH-AT) to explain the CCN activity of Pth, IPth and TPth. We performed two sets of CCN activity measurements using two different instrumentation setups. The first setup used the Differential Mobility Analyzer (DMA), whereas the second setup used the Aerodynamic Aerosol Classifier (AAC) for aerosol size-selection. The two setups provided measurements for CCN activity analysis with respect to two different representations of aerosol sizes – the electrical mobility diameter (obtained from the DMA-CCN measurements), and the aerodynamic diameter (obtained from the AAC-CCN measurements). The AAC-CCN measurements provided underpredicted CCN activity estimates compared to DMA-CCN measurements. Additionally, we also parameterized the single-hygroscopicity parameter using the FHH-AT. 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.

Effect of water adsorption on cloud condensation nuclei activity and hygroscopic properties of aromatic acid aerosols

ACS Spring 2022

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