Aerosol effective radius governs the relationship between cloud condensation nuclei (CCN) concentration and aerosol backscatter

Understanding the vertical distribution of cloud condensation nuclei (CCN) concentrations is crucial for reducing uncertainty associated with aerosol-cloud interactions (ACIs) and their effective radiative forcing. Many studies take advantage of widely available remote sensing observations to develop proxies, parameterizations, and relationships between CCN concentration and aerosol optical properties (AOPs). Such methods generally provide a good constraint for CCN concentration, but many uncertainties and limitations exist, generally related to high relative humidity (RH), environments with internal or external mixtures of several different aerosol types, and differences in parts of the aerosol size distribution relevant to both CCN and AOPs. In this study, we use in situ observations of the aerosol size distribution and chemical composition in a recent airborne field campaign to inform theoretical calculations of CCN concentration (CCNtheory) and aerosol backscatter at 532 nm (BSCtheory) with the purpose of understanding the dominant governing factors of the CCNtheory-BSCtheory relationship. Estimates from random forest models indicate that, for smoke, marine, and urban aerosols, the aerosol size distribution, as parameterized by the effective radius (Reff), is the most important predictor of the CCNtheory-BSCtheory relationship. We further investigate how Reff impacts CCNtheory : BSCtheory and find an exponential relationship between the parameters. We find that modeling CCNtheory : BSCtheory using this exponential Reff relationship can explain about 68 %-79 % of the variance in the CCNtheory-BSCtheory relationship. These findings suggest that including information about aerosol size is critical for future studies in constraining CCN concentration from AOPs.