AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Measuring and Modeling the Water-Solubility Distribution of Organic Aerosol
SHUNSUKE NAKAO, Vikram Pratap, Aditya Kiran Srikakulapu, Clarkson University
Abstract Number: 612 Working Group: Aerosols, Clouds and Climate
Abstract The water-solubility of organics is a key parameter that may determine the hygroscopic growth of aerosol, cloud droplet activation, and liquid-liquid phase separation (LLPS). However, direct measurements of water-solubility are limited. Previous approaches to characterization of the solubility of OA includes extraction with multiple solvents with different polarity as well as activation of cloud condensation nuclei (CCN) with varying inorganic seed fraction. However, such approaches are labor intensive and only allow classification of water solubility into a few levels. This study developed a new experimental approach for characterizing the continuum of solubility distribution in OA based on the combination of high-performance liquid chromatography (HPLC) and charged aerosol detection (CAD). Unlike the conventional detection methods used in HPLC, such as UV-visible absorption or mass spectrometry, aerosol measurements allow quantification of analytes irrespective of chemical properties. The method was validated using a series of standard compounds showing a strong correlation between the HPLC retention time and estimated water-solubility. Semi-volatile compounds with saturation concentration above ~1μg m-3 were observed to significantly evaporate in the experimental setup; therefore, caution must be taken in interpreting the solubility distribution of semi-volatile OA. The method was applied to the characterization of secondary organic aerosol (SOA) produced from the ozonolysis of α-pinene. The results indicate that solubility of α-pinene SOA components varies by six orders of magnitude. The implication of the wide solubility distribution to cloud droplet activation was evaluated using the Köhler theory. It was estimated that approximately 10-40% of α-pinene SOA components remain undissolved at the point of droplet activation when the volume fraction of inorganic components in aerosol is varied from 0 to 50%. Molecular identities of the low-solubility compounds remain unknown. Implications of the polarity distribution on LLPS and the gap in sub-saturated and super-saturated aerosol hygroscopicity will be discussed.