Unsaturated aliphatic and sulfur-containing organic matter as surfactants in the surface microlayer

The surface microlayer (SML) is a 10s–100s μm thick layer which mediates fluxes across the air-sea interface. Organic matter (OM) enrichments at the SML are known to influence SML physical properties and air-sea exchanges, but the role of detailed molecular level OM composition in influencing those processes hasn't been fully explored. SML and subsurface (SUB, 8–15 cm) water at four stations encompassing different influences (marine/fluvial/salt marsh) on the Delaware Bay system were sampled and examined for relationships between SML/SUB OM composition and surface tension. Samples collected December 2018–October 2019 show SML dissolved organic carbon (DOC) enrichments of 0.87 to 4.42 times the SUB concentration. Excitation-emission matrix spectroscopy (EEMs) and negative electrospray ionization (-ESI) Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) show marine samples have higher contributions from photobleached material and higher relative abundances of CHON compounds relative to inner bay sites, respectively. Principal component analyses further reveal consistent differences in SML OM composition relative to SUB. The SML contains higher abundances of compounds with H/C > 1.7 and O/C < 0.2, including sulfur-containing compounds - compositions suggestive of surfactant-like molecules, able to depress surface tension at the air-sea interface. Surface tension depressions were significantly correlated with unsaturated aliphatic and sulfur-containing compounds identified from FT-ICR MS data, yet showed no relationship with DOC abundances or enrichments, highlighting the need for compositional assessments for understanding OM influences on SML properties and air-sea exchanges. The sources and structures of SML surfactant molecules should be a focus of future work.