Twelve Months of Aerosol and Cloud Microphysics during the Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE)

LYNN M. RUSSELL, Abigail Williams, Jeramy Dedrick, Veronica Berta, Christian Pelayo, Nattamon (Jeep) Maneenoi, Sanghee Han, Atsushi Osawa, Louise Tibia, Karoline Braga, Elavarasi Ravichandran, Markus Petters, Lauren Robinson, Rachel Chang, Michael Wheeler, Jeremy Wentzell, John Liggio, Israel Silber, EPCAPE Science Team, Scripps/UCSD

     Abstract Number: 80
     Working Group: Coast to Coast Campaigns on Aerosols, Clouds, Chemistry, and Air Quality

Abstract
Coastal cities provide the opportunity to characterize marine clouds and the substantial effects of manmade particles on cloud properties and processes. The Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) at La Jolla sampled air masses at the Scripps Pier and the Mt. Soledad that were usually more than 90% northwesterly, roughly following the southern California coast for the prior two days bring a mixture urban and marine emissions. DOE ARM cloud, aerosol, and precipitation measurements at the Scripps Pier from February 2023 to February 2024 combined with a multi-institution, international collaboration at Mt. Soledad to characterize aerosol particle size distributions at both sites and cloud properties. Twelve months of sampling from an isokinetic aerosol inlet in clear air and more than 800 hr of measurements behind a counterflow virtual impactor in cloud provide a detailed climatology of the activated and unactivated phases associated with cloud processing and microphysics. This overview summarizes the variability in the aerosol size distributions and properties at both sites associated with clear and cloudy conditions. In-cloud size distributions show clear evidence of Hoppel minima, while clear-air submicron distributions were bimodal or only single, broad lognormal modes with concentrations typically <3000 cm^-3. Cloud condensation nuclei concentrations at 0.2% supersaturation typically ranged between 200 and 800 cm^-3. Low cloud liquid water content (<0.3 g m^-3) and small mean volume diameters (5-30 μm) reveal many hygroscopic particles that constitute only part of the ambient submicron mass concentration. Non-refractory submicron mass concentration was 1-4 μg m^-3, usually consisting of more than half non-refractory organic components with sulfate then nitrate and ammonium making up most of the remaining submicron non-refractory mass concentration. Sea salt and trace metals contributed less than 20% of submicron mass concentration and refractory black carbon accounted for <0.1 μg m^-3. Hygroscopicity parameters derived from size-resolved cloud condensation nuclei show particles <0.1 μm diameter being less hygroscopic with higher organic fraction than larger ones. Rain events with more than 0.4 mm/hr exceeded 30 min duration on 29 days and provided episodic particle sinks, with local vehicle sources contributing intermittent emissions at both sites. Together these results show cloud effects in a variety of aerosol conditions, providing important constraints for modeling aerosol indirect effects.