Data for: Chemical composition and formation of secondary marine aerosol during the Sea Spray and Particle Evolution (SeaSCAPE) Experiment

Marine aerosol can impact atmospheric radiative balance and cloud formation. The formation mechanisms, growth pathways, and composition of ultrafine (<100 nm diameter) marine aerosol formed through secondary processes generally remain uncertain. Here, we present measurements of the composition of ultrafine secondary marine aerosol (SMA) during the Sea Spray Chemistry and Particle Evolution (SeaSCAPE) experiment, conducted at Scripps Institution of Oceanography in  summer 2019. SMA was produced through the equivalent of one day of hydroxyl radical exposure to gaseous precursors emitted from coastal sea water from Scripps Pier in La Jolla, CA. The composition of ultrafine SMA was measured using Thermal Desorption Chemical Ionization Mass Spectrometry and mass fractions of calibrated species were determined. These composition measurements were compared to concentrations of the potential gaseous precursors dimethyl sulfide, dimethyl disulfide, methanethiol, monoterpenes and isoprene. At times of high sulfur-containing gaseous precursors, more ultrafine SMA particles were generated and they contained higher fractions of particulate sulfate and methanesulfonate. The mass fractions of calibrated species were more substantial early in the measurement period, when seawater heterotrophic bacteria and sulfur-containing gas concentrations were highest. Significant fractions of dimethylaminium and ammonium were measured early in the measurement period and we infer that the conjugate bases were neutralized by organic acids as well as sulfuric and methanesulfonic acids. We also detected organic compounds in ultrafine SMA. These common organic compounds consist of oxidized organics with low volatilities and were always present in SMA even during periods of low ocean water biological activity.