Methane and carbon dioxide concentration data, environmental data, and calculations used to determine sea-air flux on the northern Greenland margin

The following text is partially taken and then modified from the Supporting Information for Pohlman and others, 2017 (see cross-reference). The navigational data, meteorological parameters, water environmental parameters, and gas concentrations were collected by different instruments and correlated based on time stamping or acquisition of the data on a single computer with a program that provides a single, continuous time stamp based on a Network Time Protocol (NTP). Navigational data (latitude and longitude) and meteorological data (true wind speed) were recorded from IB Oden digital log files; instrument data were recorded at intervals ranging from three to seven seconds, conditional upon the data latency of the Picarro CRDS data stream. The ship's position is determined using global positioning system (GPS) receivers.
The ship's meteorological sensors are located on top of the bridge. Because the standard method for determining sea-air flux depends on the square of the windspeed, small oscillations or quality problems in the true windspeed data are magnified by the flux calculation. Near-surface water temperature and salinity were measured with a YSI EXO2 sonde in a flow-through chamber using water pumped into the ship's laboratory. The chamber was located upstream of the equilibrator that forms the front-end to the USGS Gas Analysis System (USGS-GAS), which consisted of a G-2301f CRDS that received headspace gas from a Weiss-type equilibrator.
The shipboard laboratory component of the surface water analysis element of the USGS Gas Analysis System consists of a seawater feed, a YSI EXO2 sonde, a Weiss-type equilibrator, a single-channel air-handler and a Picarro G-2301f CRDS. The intake of the seawater pump was located 3 meters below the waterline of the bow. Based on the characteristics of the shipboard plumbing and a pumping rate of 20 liters of seawater per minute to the wet lab, the water samples required time to traverse the distance between the pump intake at the bow and equilibrator in the shipboard laboratory. A split of the water pumped through the tubing from the seawater intake was directed to the YSI EXO2 sonde, which measured water temperature, salinity, and other parameters (e.g., pH, dissolved oxygen and fDOM). A second split of the flow sprayed into the equilibrator. Gas in the equilibrator was circulated continuously at a rate of 2 liters per minute through the single-channel air-handler, where it was dried and a fraction (80 milliliters per minute) fed continuously through the G-2301f. At the beginning of the expedition and prior to initiating the key surveys, the leak rate of the closed-loop gas analysis system that includes the Picarro G-2301f CRDS was measured. Tests were conducted by injecting 50 milliliters of a 1000 part per million methane certified gas standard into the 1.5 liter analytical loop with the equilibrator sealed at the base. The change in methane concentration over time (i.e., the leak rate) was then monitored. The much greater first order turnover time based on the measured leak rate means that system leakage did not meaningfully affect the methane concentrations measured in the equilibrator during this expedition.
Concentrations of methane and carbon dioxide were measured with a Picarro G-2301f CRDS for near-surface water and the Los Gatos direct absorption spectrometer for air in the atmospheric marine boundary layer. The Los Gatos direct absorption spectrometer was operated by Brett Thornton of Stockholm University. Concentrations are determined from the 12C absorption spectrum for those species. The measured concentration of methane and carbon dioxide for the Los Gatos instrument and the G-2301f, as appropriate, were corrected using a slope and offset correction based on a linear best-fit regression between the measured values and standards of known concentration. The slopes and offsets for the concentration calibration were determined from Air Liquide certified gas standards that contained 1.21, 2.01 and 30 parts per million methane, and 198, 348 and 500 parts per million carbon dioxide (± 5%). Concentration standards were analyzed at least once per day during the expedition.
Processing took place between 2019 and 2022.

Data files created by the LabVIEW program were divided into sections of maximum length of one day, with additional smaller sections corresponding to continuous records between operational and calibration interruptions. The data were processed using Excel_2018 to filter out bad data, using the following thresholds: Beginning on day 3 of the expedition (08/10/2019), when the vessel slowed to less than 1 knot, data were not recorded from the CRDS, sonde, or Los Gatos spectrometer. Records with non-physical YSI sonde values were removed. Non-physical YSI measurements were those outside the following ranges: Salinity between 0 and 35 (consistent with Stranne and others, 2021); Temperature between -2 and 10 C (consistent with Stranne and others, 2021); DO between 0 and 17 (DO values outside this range are not environmentally reasonable); pH between 7 and 9 (pH outside this range is considered not physically possible); fDOM between -5 and 20 (values outside this range are beyond the sensor's calibration and not considered physically plausible for oceans); ORP between 0 and 250 (values outside the range are not physically possible for the oceans); Chlorophyll between 0 and 20 (values outside this range are not physically possible for the ocean). In addition, water depths recorded as 0 meters were removed and left as empty cells. Missing YSI values were interpolated from values collected one minute before and prior. In 6 instances, there were no data available to perform the interpolation. Those records (6 out of 47317) were deleted. Processing took place from 2019 to 2022.

The data resulting from the Excel processing were read into Matlab (version 2021A). The data for each continuous period were linearly interpolated at 30 second intervals. The 30-second interpolation was done on discrete time snippets of data (polylines) split up into segments during which data were being collected between icebreaking or other activities. The interpolated data were written to spreadsheets using Matlab.

In Excel 365, the interpolated data that had been written to the spreadsheet in the previous process step were used to calculate point-by-point intermediate and final values related to gas saturations in near-surface waters, sea-air fluxes of carbon dioxide and methane, and other parameters. The details of the calculations are given in the Supporting Information for Pohlman and others (2017), which documented operations on similar data for the West Spitsbergen margin.
YSI EXO2 sonde temperature and salinity were used to determine (a) Bunsen solubility coefficients used to calculate dissolved gas concentrations from headspace measurements made by the CRDS and (b) the Schmidt number used to calculate gas fluxes. In previous data releases (Ruppel and others, 2017), surface water temperatures from the ship’s hull-mounted temperature sensor were used to calculate these parameters. The hull-mounted temperatures could not be used for the calculations for this data release because the values exceeded the known range of surface water temperatures present during the survey.
The data were exported as a CSV file.