Aerosol Size Distribution Modes and their Potential Source Contribution Functions in the Southern California Coastal Region during EPCAPE 2023

Aerosol particles can act as Cloud Condensation Nuclei (CCN) on which cloud droplets form. The DOE ARM Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) in La Jolla, California, in 2023 measured the properties, extent, and interactions of aerosols with stratocumulus clouds. By merging hourly size distribution measurements from the Scanning Electrical Mobility Spectrometer (SEMS) and Aerodynamic Particle Sizer (APS) collected during EPCAPE (10 to 800 nm and from 0.5 to 20 microns in diameter, respectively), log-normal fits for 3 submicron modes were retrieved for 6 weeks in February and March 2023 (with number-weighted mean diameter, 0.059 µm). The aerosol size mode retrievals reveal high contributions of Aitken and Accumulation mode number concentrations across the first 6 weeks of EPCAPE data. HYSPLIT back trajectories were used to trace the origin of air masses, using GDAS 1-degree data as input. Five-day back trajectories from the Scripps Pier (32°52'N, 117°16'W) indicate air flow primarily from the northwest at the surface for the first 3 months of EPCAPE. Potential Source Contribution Function (PSCF) calculations pinpoint probable source locations, utilizing the upper quartile to identify high concentrations. High Aitken mode number concentrations were mostly associated with urbanized northwest areas like Los Angeles whereas low aerosol-mode concentrations were notably associated with western and southern regions. Collectively the results spotlight the influence of urban-generated pollution on San Diego air quality, impacting clouds, climate, and human health.