Why would apparent <i>κ</i> linearly change with O/C? Assessing the role of volatility, solubility, and surface activity of organic aerosols

Correlations between O/C and 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 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 <i>κ</i>. The framework estimates intrinsic <i>κ</i> based on molecular formulas of organics composed of carbon, oxygen, and hydrogen that partition to condensed phase, and then it estimates apparent <i>κ</i> considering solubility distribution parameterized by O/C. This article applied the new framework to the two-dimensional volatility basis set (2D-VBS) and found that distribution of O/C and molecular size play key roles in determining apparent <i>κ</i>. For highly soluble organics, <i>κ</i> is dictated by gas-particle partitioning and it is unlikely for <i>κ</i> of organics to go beyond 0.3 in typical ambient organic material loadings. Sensitivity analysis showed that surface-active compounds are not likely to profoundly alter the overall trend within a reasonable range of surface activity. This framework provides a simple yet plausible explanation of why <i>κ</i> would and would not correlate with O/C; distributions of OA in the 2D-VBS as well as the presence of inorganic salts determine the trend. The framework opens up new opportunities to evaluate two-dimensional representations of organic aerosol aging using <i>κ</i>, a significant advancement from the current empirical linear fits to <i>κ</i> and O/C. Copyright © 2017 American Association for Aerosol Research