Aerosol-Correlated Activation and Supersaturation Variability in Low-level Coastal Marine Stratus during EPCAPE

The complex mixture of highly polluted and nearly pristine air that influences cloud condensation nuclei (CCN) in coastal regions serves as a unique experiment to explore process-specific constraints on coastal fog forecasting and highly uncertain aerosol-cloud interactions in global climate models. Measurements at Scripps Pier and Mt. Soledad during the DOE ARM Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) in La Jolla, California were used to characterize the in- and sub-cloud size distribution of aerosol and cloud droplets during fog and low-cloud events, signatures of processing following these events, and the aerosol conditions preceding them. Three lognormal modes were superimposed onto the measured size distributions (10 nm-10 µm) representing Aitken, accumulation, and coarse modes using an automated fitting procedure. Typical pre-cloud aerosol size distributions were characterized by a single, broad accumulation mode that developed into a bimodal submicron distribution following cloud development and dissipation. The bimodality, represented by a signature of the in-cloud critical activation diameter, known as the “Hoppel Minimum”, correlated (r=0.4) with the total aerosol concentration for low aerosol conditions (<500 cm^-3) and were associated with small activation diameters (<75 nm). Based on in-cloud hygroscopicity determined from submicron composition and cloud condensation nuclei measurements, higher effective cloud supersaturations were estimated for low aerosol conditions than more polluted conditions (>500 cm^-3). The relationship of the number of particles acting as CCN (indicated by the accumulation mode number) with the Hoppel Minimum diameter shows evidence of aerosol-correlated feedback on supersaturation and cloud drop activation that varied with the aerosol conditions and turbulence. These results provide activation characterization that connects aerosol sources and low-level stratus properties in this complex coastal environment.