Why would apparent κ 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 κ. The framework estimates intrinsic 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 play key roles in determining apparent κ. 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 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 κ 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 κ, a significant advancement from the current empirical linear fits to κ and O/C.