Cloud-Processed Aerosol Size Distribution Signatures in Coastal Marine Stratocumulus during EPCAPE

Coastal regions experience an ever-changing mixture of highly polluted and nearly pristine air that can be used to characterize a dynamic range of modes in the size distribution of aerosol acting as cloud condensation nuclei (CCN) and subsequently to constrain aerosol-cloud interactions in global climate models. Measurements from the DOE ARM Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) in La Jolla, California, at Mt. Soledad (<2 km inland from the coast, 250 m ASL) included aerosol aerodynamic and mobility size distributions sampled from an isokinetic inlet for non-cloud periods and from a ground-based counterflow virtual impactor for in-cloud conditions. February-June 2023 measurements were separated into multi-hour in-cloud periods (5 hour average in-cloud duration) as well as pre-cloud and post-cloud periods. Pre-cloud size distributions were primarily characterized by a broad accumulation mode with the appearance of a lower concentration coarse-mode shoulder that was representative of sea spray aerosol during clean marine periods. In-cloud droplet residual and post-cloud dried size distributions were bimodal with a number concentration minimum typically at approximately 80 nm that separated the accumulation-mode particles from the Aitken-mode particles. An analysis of the critical CCN activation diameter represented by the “Hoppel Minimum” diameter before, during, and after cloud provided the sensitivity of droplet activation to the number and size of particles. During clean marine aerosol conditions, the relationship of the number of particles acting as CCN (indicated by the accumulation mode) with the cloud critical diameter (indicated by the Hoppel Minimum diameter) shows evidence of aerosol-limited cloud drop activation. These results provide aerosol characterization that connects aerosol sources and cloud properties in this complex coastal environment.