Aircraft measurements of single particle size and composition reveal aerosol size and mixing state dictate their activation into cloud droplets

Shallow convective clouds are common in many regions of the world. Currently, aerosol-cloud interactions parameterizations for convective clouds are a major source of uncertainty in global climate model predictions of radiative forcing. Size and composition of individual aerosol particles are the most important properties that determine aerosol activation into cloud droplets and the impacts of aerosol on aerosol-cloud-climate interactions. A challenge to accurately describe aerosol activation is often due to a lack of measurements of individual particle size and composition, making it necessary to rely on simplistic and, most often, unrealistic aerosol mixing state assumptions, which are known to lead to significant errors in predicted concentrations of cloud condensation nuclei (CCN). We present the aircraft-based single-particle measurements of the size and composition of individual below-cloud particles, interstitial aerosol particles, and cloud droplet residuals during two contrasting seasons. Measurements reveal enhanced contribution from larger and sulfate-rich particles in cloud droplet residuals and provide direct evidence for sulfate and isoprene-epoxydiol-derived secondary organic aerosol (IEPOX-SOA) formation in cloud droplets. We observe a strong dependence of the size and composition of below-cloud aerosol on their cloud droplet activation fraction during the spring campaign, when the observed dynamic range in aerosol properties was large. Furthermore, we report clear seasonal differences in the aerosol activation fraction (0.38 ± 0.21 for spring and 0.20 ± 0.08 for summer) over the Atmospheric Radiation Measurements (ARM), Southern Great Plains (SGP) atmospheric observatory in Oklahoma, consistent with high fluxes of biogenic volatile organic compounds that drive the formation and growth of less-hygroscopic organic components during summer but not during spring when the measured aerosol composition was more variable. A closure between measured cloud droplet number concentrations and predicted CCN (using κ-Köhler theory and measurement-constrained aerosol properties, including their mixing state) revealed that the effective supersaturations of shallow cumuli ranged between 0.06% and 0.24%. This study highlights the importance of measuring particle-by-particle variability in size and composition, and mixing state of aerosol population to accurately represent their activation into shallow cumuli cloud droplets, even at a background site like SGP.