Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • BCO-DMO Processing Description
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Sea surface microlayer (SML) samples were collected using the rosette-based glass plate sampler aboard the R/V Hugh R. Sharp. Subsurface seawater (SSW) was collected from the starboard side of the ship using a modified pump profiler system (Hudson et al. 2019) from ~0.5 meters (m) depth, immediately after each SML sampling. The samples were filtered using 0.7 micrometer (µm) pre-combusted glass fiber filters. An aliquot of filtered were frozen at -20 degrees Celsius (C) for DOC measurements, while another portion was stored in the fridge at 4 degrees C for colored dissolved organic matter (CDOM) and fluorescent dissolved organic matter (FDOM) analysis.

To measure dissolved organic carbon (DOC) concentrations, ~20 milliliters (mL) filtered water samples were acidified to a pH of ~2 using 4 drops of 2 molar (M) HCl. DOC was then quantified using the non-purgeable organic carbon method on a high temperature combustion instrument (Shimadzu TOC-V) that has a low blank (<5 micromolar (µM) C) and high precision (~1% RSD; (Qian and Mopper 1996)), following published studies (e.g., (Wozniak et al. 2012)). Sample responses were calibrated and validated against consensus surface seawater reference material from the Hansell laboratory at the University of Miami. DOC enrichment factors were calculated as a ratio of DOC concentration in the SML to DOC concentration in the paired SSW sample.

To characterize the optical properties of the SML and SSW DOM, both absorbance and excitation emission matrix (EEMs) fluorescence spectra were collected using a Horiba Aqualog spectrofluorometer. Absorbance spectra were acquired from 230 to 700 nanometers (nm) at 2 nm intervals. DOM aromaticity was estimated using SUVA254, which was calculated by dividing the absorption coefficient at 254 nm (a254) by DOC concentration (milligrams carbon per liter) (mg C L⁻¹). EEMs were obtained by scanning excitation wavelengths at 2 nm intervals between 230 and 700 nm, with emission spectra recorded at 4.65 nm intervals from 254 to 822 nm. The humification and biological indices (HIX and BIX) were calculated from the EEMs data using the staRdom and eemR R packages following standard procedures (Ohno 2002; Ouyang et al. 2024).

Additional measurements of the physical properties of seawater and meteorological conditions were made with the ship's instrumentation. Sea surface temperature and salinity were measured and recorded continuously with the shipboard instrumentation (SBE 45, Sea-Bird, at 3.7 m depth). Chlorophyll a concentrations were continuously measured with the shipboard fluorometer (AU10, Turner). Discrete water samples were also collected and used for chlorophyll a concentration measurements in the lab, using the spectrophotometric method described by Aminot and Rey (2000), to calibrate the shipboard fluorometer measurements. Wind velocity and air temperature were measured and recorded continuously with the ship's meteorological station (RM Young 26700).

- Imported original file "SOAPI_1-3_DOC_CDOM.xlsx" into the BCO-DMO system.
- Renamed fields to comply with BCO-DMO naming conventions.
- For all columns except Latitude and Longitude, if more than 3 decimal places in numerical values, values were rounded to 3 decimal places.
- Saved the final file as "986840_v1_soapi_doc_and_cdom.csv".

NSF abstract:
The surface microlayer (SML), the thin layer of water at the interface between the ocean and the atmosphere, controls the exchange of materials to and from the ocean. As a result, it can profoundly influence biogeochemical cycles and global climate. One type of chemical species that accumulates at this interface are surfactant molecules, which influence the surface tension of and the rate of material exchange at air-water interfaces. Biological and chemical production and degradation processes represent surfactant sources and removal pathways, but the relative importance of those processes for determining surfactant quantities and molecular composition remains unclear. Similarly, the relationship between surfactant molecule composition and surface tension at the air-water interface has not been established. As a result, their effects on material exchange at the interface cannot currently be predicted. This work will use measurements at sea, laboratory experiments, and high-resolution analyses to measure the chemical and physical characteristics of surfactants and their properties at the air-sea interface. An improved understanding of surfactant processes and surface ocean will benefit society by improving our understanding of the exchange of climate-relevant gases and particles. Two early career PIs will advance their established collaboration and gain further experience leading research projects and mentoring students. Students will receive valuable hands-on training in oceanographic field collections, state-of-the-science analytical techniques, data interpretation, and data dissemination. The results and methodologies from this work will be featured in courses at the University of Georgia and the University of Delaware and will be developed into content for K-12 students, enhancing infrastructure for education.

This work includes the unique pairing of state-of-the-science measurements across time and spatial scales to assess the influence of oceanographic processes on surfactant chemical composition and physical air-sea relevant properties. SML and subsurface waters will be collected from estuarine, coastal ocean, and open ocean sites during high and low productivity conditions to establish surfactant molecular characteristics over a range of space, time, and ocean biological activity. The effects of light will be assessed via diurnal sampling efforts and laboratory experiments. Samples will be analyzed for their detailed chemical, biological, and physical characteristics. The surface tension of the SML is expected to be inversely correlated with the abundance of lipid-like compounds (low O content, high H/C ratios, e.g., sulfur-containing lipids) produced during periods of high biological activity. Prolonged exposure to light is hypothesized to result in photo-oxidation of surfactant compounds, higher abundances of oxygenated and lower molecular weight aliphatic compounds, and increased surface tension. Multivariate statistical approaches will be used to reveal a mechanistic understanding of the links between biological and photochemical processes and the resulting surfactant and SML chemical and physical characteristics. This new knowledge will represent a first step toward improved models of the air-sea exchange of climate relevant gases which currently have large uncertainties. It will inform future work on the exchange of volatile and aerosol organics with significant potential impacts for our understanding of the climate system.