This data was collected in July and August 2009 by the IMOS Ship of Opportunity Underway CO2 Measurement research group on RV Southern Surveyor (IMOS platform code: VLHJ) voyage ST022009.
Departed: Noumea, New Caledonia, July 29, 2009
Arrived: Hobart, Tasmania, August 6, 2009
CO2 System Overview:
The fugacity of carbon dioxide (fCO2) in surface seawater was measured using a General Oceanics Inc. automated system (Model 8050; Pierrot et al 2009). Seawater 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 drying tube before flowing through a Licor 7000 non-dispersive infrared gas analyser used to measure the CO2 mole fraction (XCO2) of the dried air. The gas flow is stopped temporarily for the CO2 measurements, which are made at atmospheric pressure. A set of four CO2 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 on the forward mast of the ship.
Seawater intake and ancillary data:
The seawater intake is located at about 5.5m depth in the bow of the ship. Sea surface salinity is measured using a thermosalinograph (Seabird Electronics SBE21) located next to the CO2 system. 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. 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 parameters and the data quality are maintained by the Australian Marine National Facility.
Statement: Parameters logged by the fCO2 system and ship sensors are quality controlled after each voyage.
1. Data with missing parameters or obvious outliers for the ship or fCO2 system parameters are marked as missing and removed from the calculations. Parameter values are flagged as good (flag=2), questionable (flag=3), or bad (flag=4), depending on the range of values expected. Many of the ship and CO2 system parameters are not reported in the final dataset, but are used to establish that the system is functioning correctly. For example, water flow rates to the equilibrator below 2 LPM are flagged as questionable and the cause investigated with the flag value changed to 4 if the flow has been interrupted or is insufficient. Similar checks are made to ensure the gas flow through the infrared gas analyser is in a suitable range (50 to 120ml/min). The list of parameters checked are:
CO2 system data quality controlled:
GPS date and Time
Latitude and Longitude
Water flow rate
Gas flow rates through licor analyser
Equilibrator water temperature
Mole fraction CO2
Water vapour in gas stream
Licor NDIR temperature
Ship's data quality controlled:
GPS date and time
Latitude and Longitude
Sea surface temperature
Sea surface salinity
Relative wind speed and direction
True wind speed and direction
2. 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.
The fCO2 value in the water is sensitive to warming between the ship intake and equilibrator. The travel time between the ship intake and equilibrator is first checked by comparing the timing of rapid changes in surface water temperature for the intake (SST) and the equilibrator temperatures. The travel time or lag time is usually about 4 minutes, although this can vary due to ship engineers altering the flow rates through the water line and other users removing water. The lag is accounted for in the warming calculation. High lags cause some smearing of the equilibrator temperature signal, relative to the SST, are also expected to cause some smoothing of the fCO2 signal. The warming in the system used on RV Southern Surveyor is typically less than 0.6°C, with higher values expected in cooler regions, or when water flow problems occur. Data with excessive warming (>0.6°C) is examined to evaluate the cause. The higher lags can result in greater warming when the ship is in cooler waters. Low water flow rates are typically associated with anomalously high warming and these data are flagged as bad.
Atmospheric CO2 values can be influenced by contamination from industrial and population centres and from contamination with ship stack gas. The intake on the forward mast of the ship is within about 20m of the ship stacks. The relative wind speed and direction recorded by the ship meteorological sensors are used to evaluate if anomalous atmospheric measurements could be due to stack gas contamination. High XCO2ATM_PPM values due to stack gas is often observed at relative wind speeds below about 5 knots and relative wind direction less than ±70 degrees over the bow. The data with likely stack gas contamination are flagged as bad (flag = 4) and not included in the calculations outlined below.
3. 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 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 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 flagged as good are then used to calculate the fugacity of CO2. Only data flagged as good or suspect are report in the final data set.
Ships underway thermosalinograph, sea surface temperature and meteorological data were collected, calibrated and quality controlled by the Australian Marine National Facility Data Centre. To improve the TSG salinity relative to the CTD, a 32 second lag was applied to the TSG conductivity. Then an averaged scale factor of 0.999748856709685 was applied to the TSG data (Sarraf, 2010).
Raw SST values were replaced by the calibrated SST values with a time lag of 400 seconds (time lag between the intake temperature or SST and the equilibrator temperature on this particular cruise). Raw salinities were also replaced with corrected salinities.
Please see the voyage dataset report for additional processing details and fugacity of carbon dioxide calculations (fCO2SW and fCO2ATM)..
Australia’s Integrated Marine Observing System (IMOS) is enabled by the National Collaborative Research Infrastructure Strategy (NCRIS). It is operated by a consortium of institutions as an unincorporated joint venture, with the University of Tasmania as Lead Agent.
Australian Marine National Facility (MNF)
CSIRO Marine and Atmospheric Research (CMAR)