Organic Signatures of Surfactants and Organic Molecules in Surface Microlayer and Subsurface Water of Delaware Bay

Abstract

Surface-active species are present in marine waters and can accumulate in the surface microlayer (SML). Surfactants are amphiphilic organic compounds that reduce surface tension at an interface. Current work is investigating the link between the molecular composition of surfactants in the SML and those in subsurface waters, as well as the differences in surfactant molecular composition across different water types. In this study, subsurface water and SML samples were collected on three sampling days at three sites in the Delaware Bay: the Mouth of the Bay, Mid-Bay, and the marsh-influenced Broadkill River. Organic matter was extracted from the SML and subsurface water using two solid-phase extractions (graphitized carbon and C18) and then analyzed using tensiometry and high-resolution mass spectrometry, in positive and negative ionization modes. Here, we show that molecules with high H/C are preferentially enriched in the SML compared to the subsurface waters. We demonstrate that the measured organic extracts contribute to lower surface tensions in the SML. A rainfall event led to increased terrestrial runoff and mixing that altered the composition of the organic molecules in the SML and subsurface waters of the Broadkill River site, and the composition of the extracted organic molecules varied across sampling days. These results imply that the surfactant compositions are not uniform across sampling regions or from the subsurface to the SML.

Influence of Air Mass Source Regions on Signatures of Surface-Active Organic Molecules in Size Resolved Atmospheric Aerosol Particles

“…The percentage of molecular formulas that were identified as aliphatic and aromatic compounds varied for the three air mass regions. Aromatic formulas were identified as those with a calculated aromaticity index value of greater than 0.5. ,,,, In total across all three samples, almost 17% of the organic formulas were classified as aromatic (Table ). The organic formulas from the Marine Influenced sample had the highest average percentage of aromatic compounds (20%) out of the three samples (Table ), which may be due to local and transported ship emissions .…”

“…This process was repeated to obtain triplicate droplet measurements for each sample. The surface tension of the 40 μL hydrated extract is referred to herein as the surface tension minimum. ,,,, Surface tension minimums are reported for the May 26–27 and May 28–29 samples. Surface tension minimums were not measured for the June 4–5 sample.…”

“…Previous work has also identified surfactants with similar H/C and O/C formulas. 34,35,58 Using this classification for surfactant-like organic molecules, more than 60% of formulas identified in the Marine Influenced samples were classified as surfactant-like. A negative correlation (R = −0.68, n = 10) was observed between the percentage of formulas in the surfactant region and the minimum surface tension of the Marine Influenced samples for the different particle size ranges (Figure 5).…”

“…Surfactants are organic molecules that reduce the surface tension at the interface of a solution. , Surfactants have been measured in seawater − and in marine aerosol particles. , Other studies have observed surfactants in anthropogenic-influenced particles, , ambient urban particles, and biogenic terrestrial particles. , Surfactants can impact particle hygroscopic growth , and their potential to act as cloud condensation nuclei. − Recent work has used high-resolution mass spectrometry to identify fatty acids and other surfactants in aerosol particles. , These results can be combined with surface tension measurements to confirm the presence of surfactants …”

“…34,35 These results can be combined with surface tension measurements to confirm the presence of surfactants. 58 In this study, high-resolution mass spectrometry was used to identify and characterize surfactants extracted from sizeseparated aerosol particles collected from three different air mass source regions sampled from Skidaway Island, GA in May and June 2018. Particle number size distributions were collected simultaneously and varied in number concentrations, depending on the air mass source region.…”