Aerosol Organic and Inorganic Composition on Sunny and Cloudy Days during EPCAPE

SANGHEE HAN, Abigail Williams, Veronica Berta, Jeramy Dedrick, Christian Pelayo, Nattamon (Jeep) Maneenoi, Lynn Russell, University of California, San Diego

     Abstract Number: 775
     Working Group: Aerosol Chemistry

Abstract
Aerosol composition in the atmosphere can be significantly affected by two distinct secondary processes: photochemically-driven reactions on sunny days and aqueous reactions on cloudy days. Sunlight drives photochemical reactions that initiate atmospheric oxidation processes of reactive organic gases. On cloudy days, both organic and inorganic atmospheric trace gases are absorbed by cloud droplets, allowing in-droplet chemical reactions. These secondary processes closely relate to the physicochemical properties of both gas and particle compounds in the ambient air. The Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) provides Aerosol Mass Spectrometer and Fourier Transform Infrared measurements of aerosol composition in the coastal area of La Jolla, CA, from February 2023 to February 2024. This year-long study included frequent northwesterly winds that provided an opportunity to investigate sunny and cloudy conditions with similar upwind aerosol sources.For the first two months of the campaign, there were 17 sunny days and 18 cloudy days. Overall, organic aerosols were the largest fraction of non-refractory mass concentration, constituting 60%, followed by nitrate (18%) and sulfate (16%). On sunny days, organic aerosol fragments accounted for approximately 67% of non-refractory mass concentration, while on cloudy days, this fraction decreased to 52%. Nitrate and sulfate contributed higher fractions of non-refractory submicron mass concentration (21% each) on cloudy days compared to sunny days (14% and 11%, respectively). These analyses are important for characterizing the aerosol interactions with cloud processes, which contribute to the improvement of climate and air quality models.