Size-Resolved Submicron Aerosol Organic Functional Group Measurements during Low-Cloud Conditions in EPCAPE 2023

As part of the Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) in La Jolla, California, aerosol particles were collected on teflon filters for 23 hours daily at Mt. Soledad and Scripps Pier in 2023 (Russell et al., 2021). Particles were sampled at both sites with a <1 micron cyclone. At Mt. Soledad, particles were also collected with <0.18 micron and <0.5 micron Berner impactors for smaller particles and with a Counterflow Virtual Impactor for cloud droplet residuals during cloud events. The organic functional group mass composition was analyzed using a Bruker Tensor 27 Fourier Transform Infrared (FTIR) Spectrometer for alcohol, alkane, amine, acid, and non-acid carbonyl group mass concentration (Russell, 2003). At both Mt. Soledad and Scripps Pier, the submicron fractional composition of alcohol, alkane, and acid groups were similar to those reported for polluted and clean marine conditions (Frossard et al., 2011; Hawkins et al., 2008; Lewis et al., 2022). To characterize the regional background conditions from February to April at Mt. Soledad, 74% of <1 micron samples from 71 days of sampling were selected based on the criteria that the organic functional group mass concentrations were within two standard deviations of the medians. For low cloud days (cloud base height <1000m), the <0.5 micron and <1 micron particles have a higher mean fraction of alcohol groups (>20%) compared to the <0.18 micron size-cut filters (<10%), consistent with the reported association of the alcohol group with sea spray mode particles (Russell et al., 2010). In contrast, the <0.18 micron particles have a higher mean fraction of acid groups (>40%) compared to the <0.5 micron and <1 micron size-cut particles (<30%), consistent with secondary organic aerosol formation contributing a larger mass fraction at smaller sizes (Maria et al., 2004). In addition, mean carboxylic acid group mass concentrations more than doubled during low cloud days relative to high cloud days, possibly indicating substantial organic mass concentration increases from in-cloud aqueous reactions. These size-resolved organic functional group measurements provide distinctive chemical signatures of primary and secondary aerosol sources associated with sea spray, accumulation, and Aitken modes of the aerosol distribution and the contributions of cloud processing, providing process-specific constraints for climate modeling.

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