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Abstract
Sea spray aerosol particles (SSA) constitute the largest global source of natural aerosol and therefore contribute significantly to Earth’s radiative balance. These particles are comprised of inorganic sea salt and a complex organic fraction which includes surface active compounds, known as surfactants. Surfactants, even when present in small quantities, influence particle interfacial tension and consequently the process of cloud droplet formation from SSA particles. The properties and concentrations of surfactants in SSA, therefore, are a defining factor in the role of SSA in global climate change. Here, the properties of individual surfactants were probed in connection to their effects on the hygroscopic growth of model SSA particles comprised of sea salt, a saccharide, and a surfactant. In submicron particles, ionic surfactants facilitated water uptake from 50% to 70% relative humidity and decreased the deliquescence relative humidity of sea salt/glucose particles by about 5%, while nonionic surfactants had minimal effects. The incorporation of a more complex saccharide dampened the effect of ionic surfactants on the deliquescence of sea salt particles but with cationic surfactants, particle water uptake was enhanced at high relative humidity conditions. To characterize the true composition of surfactants from ambient seawater and SSA particles, recently developed targeted solid phase extractions (SPE) were coupled with the colorimetric quantification of anionic, cationic, and nonionic surfactants. Investigations into method improvements showed that modifying the final acetonitrile elution step to 8 mL and performing separate colorimetric analyses on the two SPE extracts improved surfactant quantification and minimized the effects of matrix interferences. The full extraction and analysis method was directly applied to determine the seasonal variability of the surfactant composition and enrichment in SSA particles compared to seawater. Anionic surfactants were consistently measured and comprised most of the total surfactant fraction in both seawater (41% to 100%) and SSA (41% to 85%). Winter SSA particles exhibited the most diverse surfactant composition, 41% anionic, 19% cationic, and 40% nonionic surfactants, whereas anionic surfactants were the only class measured in the associated parent seawater. SSA surfactants have lower surface activity, but higher overall enrichment than those in seawater, with more enrichment during the summer.