Properties of Seawater Surfactants Associated with Atmospheric and Primary Marine Aerosol Particles

Surfactants account for a minor fraction of the total organic carbon in the ocean but may have a major impact on the surface tension of bursting bubbles at the sea surface and the production of primary marine aerosol particles (PMA). Surfactants associated with PMA may significantly reduce the surface tension of water and increase the potential for particle growth into cloud droplets. The transfer of surfactants from seawater and enrichment in PMA remains unconstrained. We compare the composition and strength of surfactants extracted from atmospheric aerosol particles and estuarine water from Skidaway Island, GA in Summer 2018 to those of surfactants extracted from model PMA (mPMA) and seawater from the North Atlantic in Fall 2016. We used solid phase extraction to separate the surfactant fraction and characterized concentrations and compositions using UV-Vis spectroscopy or mass spectrometry. Surfactant strengths were inferred from critical micelle concentrations (CMCs) and surface tension curves. The surface tension depression of bulk estuarine waters was less than that of the productive and oligotrophic seawater, consistent with an additional sink or a different mixture of surfactants in the estuarine waters. Surfactant CMCs in biologically productive seawater were lower than those in the oligotrophic seawater suggesting surfactant mixtures in the two regions were chemically distinct. Surface tension curves indicate surfactants in mPMA were weaker than those in the corresponding seawater, suggesting preferential partitioning during bubble bursting. The presence of surfactants with lower CMCs in seawater resulted in mPMA with larger relative concentrations of surfactants, demonstrating a direct link between seawater and aerosol surfactants. Photochemical reactions in the surface ocean may influence surfactant properties and drive the observed variations in surfactant concentrations over diel cycles. In both estuarine seawater and biologically productive seawater, surfactant strengths were lower during the day, compared to the night. Surface tension of irradiated estuarine water confirmed a change in surfactant properties due to photochemical processing. Mass spectral signatures demonstrate similarity of surfactants in ambient aerosol particles to those extracted from estuarine water.