Water-Soluble Organic Composition of the Arctic Sea Surface Microlayer and Association with Ice Nucleation Ability

Organic matter in the sea surface microlayer (SML) may be transferred to the atmosphere as sea spray and hence influence the composition and properties of marine aerosol. Recent work has demonstrated that the SML contains material capable of heterogeneously nucleating ice, but the nature of this material remains largely unknown. Water-soluble organic matter was extracted from SML and underlying seawater from the Arctic and analyzed using a combination of mass spectrometric approaches. High performance liquid chromatography-ion trap mass spectrometry (LC-IT-MS), and Fourier transform ion cyclotron resonance MS (FT-ICR-MS), showed seawater extracts to be compositionally similar across all stations, whereas microlayer extracts had a different and more variable composition. LC-IT-MS demonstrated the enrichment of particular ions in the microlayer. Ice nucleation ability (defined as the median droplet freezing temperature) appeared to be related to the relative abundances of some ions, although the extracts themselves did not retain this property. Molecular formulas were assigned using LC-quadrupole time-of-flight MS (LC-TOF-MS2) and FT-ICR-MS. The ice nucleation tracer ions were associated with elevated biogenic trace gases, and were also observed in atmospheric aerosol collected during the summer, but not early spring suggesting a biogenic source of ice nuclei in the Arctic microlayer.