IMOS SOOP - Underway CO2 measurements collected on Southern Surveyor Voyages

Brief description

The CO2 measurements group is a research and data collection project carried out through the Integrated Marine Observing System (IMOS) Ship of Opportunity Underway Network facility. The CSIRO CO2 system used for this research is mounted on the Research Vessel Southern Surveyor (IMOS platform code: VLHJ) of the Australian Marine National Facility managed by CSIRO Marine and Atmospheric Research (CMAR). The dataset includes quality controlled data collceted between Jan 2008 and October 2013. The Southern Surveyor voyages where data was collected and processed are listed below in this record. These data are available via the Australian Ocean Data Network AODN and IMOS ocean portal.

Lineage

Progress Code: onGoing

Maintenance and Update Frequency: asNeeded

Statement: The fugacity of carbon dioxide (fCO2) in surface seawater was measured using a General Oceanics Inc. Model 8050 CO2 system (Pierrot et al 2009). Final fugacity data are reported at sea surface temperature and with water saturated air. Seawater from an underway seawater supply is sprayed into an equilibration chamber and CO2 in the headspace gas equilibrates with the seawater. The headspace gas is pumped through a thermoelectric condenser followed by a nafion dryer to remove water vapour before flowing through a Licor 7000 non-dispersive infrared gas analyser used to quantify the CO2 mole fraction (XCO2) of the dried air. The mole fraction measurements are made at atmospheric pressure as gas flow is stopped and the gas line is temporarily vented to the atmosphere. A set of four CO2-in-air standards that cover the range of CO2 values expected in the ocean are analysed about every four hours to calibrate the gas analyser. The standard gas concentrations are on the WMO-X2007 mole fraction scale for CO2-in-air. Atmospheric XCO2 (dry) is measured after the standards by pumping clean outside air from an intake mounted on the forward mast of the ship. The seawater intake is located in the ship bow at about 5.5m below sea level. Sea surface salinity is measured using a thermosalinograph (Seabird Electronics SBE21) located next to the CO2 system and the sea surface temperature is measured using a remote temperature sensor (Seabird Electronics SBE 38) located at the intake is used to measure sea surface temperature (SST). The travel time between the intake and CO2 system is typically about 4 minutes, with warming usually less than 0.6ºC. The thermosalinograph water is from the same intake, but the supply lines separate after the intake and the travel time to the thermosalinograph is about 1.5 minutes. A comparison of thermosalinograph and equilibrator temperature records shows the temperature difference in the two lines is generally less than 0.1ºC. The thermosalinograph water line travels outside the ship and is typically warmer than the equilibrator. The travel time in water line to the thermosalinograph is 2.5 minutes faster than to the equilibrator. Meteorological data, salinity, SST, and ships position and time are taken from the ships logging system. These meteorological parameters and sensors are maintained and calibrated by the Australian Marine National Facility. Quality control and data reduction of CO2 data is carried out by the CMAR ocean carbon group. For data quality control, parameters logged by the CO2 system and ship sensors are first examined for outliers. The data sets are next evaluated for excessive warming of the seawater flowing to the equilibrator, and for contamination of the atmospheric measurements by ship stack gas. After completion of the quality control checks, the measure mole fractions are corrected to final values using measurements of the four CO2-in-air standards. The standards are run about every four hours to bracket the air and equilibrator headspace gas measurements. The offsets between the measured and certified values of each standard are linearly interpolated to the times of measurement of the air and equilibrator headspace measurements and used to correct the mole fractions of these samples. At each measurement time, a linear regression of offset values versus certified standard values is used to calculate the offset to apply to the measured air and equilibrator values. The corrections are typically small (about 1 to 2 ppm) and account for drift of the gas analyser response over time. The corrected mole fractions (dry) for the equilibrator and air samples are flagged as good, questionable, or bad, based on the quality control procedure.

Notes

Credit
SOOP-CO2 data was sourced as part of the Integrated Marine Observing System (IMOS) - an initiative of the Australian Government being conducted as part of the National Collaborative Research Infrastructure Strategy. The science support groups of the Australian Marine National Facility and CSIRO, and in particular Matt Sherlock, Lindsay Pender, Steve Thomas and Drew Mills, are thanked for their expertise in helping to setup and collect data.

Credit
Integrated Marine Observing System (IMOS)

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the CSIRO Marine and Atmospheric Research (CMAR) carbon group

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and relevant CMAR voyage participants.