(263b) Why Would Cloud Condensation Nucleus Activity Linearly Change with O/C?  Assessing the Role of Volatility, Solubility, and Surface Activity

Aging of atmospheric organic aerosol (OA) has been studied in terms of average molecular properties such as elemental ratios (O/C, H/C), volatility, and hygroscopicity. Correlations between O/C and hygroscopicity (or cloud condensation nucleus activity), represented by κ, are a computationally efficient approach to estimate the impact of aerosol aging on cloud formation and climate; however, previously reported correlations between these two variables are largely empirical and vary widely in their slopes and extrapolations to high O/C values. This study proposes a theoretical framework that bridges elemental ratios, volatility, solubility, and κ of OA. The framework estimates inherent κ based on molecular formulas of organics composed of carbon, oxygen, and hydrogen that partition to condensed phase, and then it estimates apparent κ considering solubility distribution parameterized by O/C. This paper applied the new framework to the two-dimensional volatility basis set (2D-VBS) and found that distribution of O/C and molecular size plays a key role in determining apparent κ. Another finding is that for highly soluble organics, κ is dictated by gas-particle partitioning and it is unlikely for κ of organics to go beyond 0.3 in typical ambient organic material loading. Sensitivity analysis showed that presence of surface-active compounds may slightly enhance apparent κ when solutes are not entirely dissolved, but it would not change the major conclusions. Although inherent uncertainties in various parameters prevent exact underpinning of activation mechanisms, this framework provides plausible explanations of why κ would and would not correlate with O/C depending on product distribution in the 2D-VBS space. The framework opens up new opportunities to evaluate two-dimensional representations of organic aerosol aging using κ, a significant advancement from the current empirical linear fits to κ and O/C.