Biogeochemical Sensors
Instrumentation
pCO2 Underway System
CO2 System Overview:
The fugacity of carbon dioxide (fCO2) in surface seawater is measured using a General Oceanics Inc. automated system (Model 8050; figure 1, 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 (Table 1) 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.
Table 1. CO2-in-air standard values
Cylinder no. | Cylinder number | CO2 (ppm) |
1 | CA06070 | 0.00 |
2 | CA06072 | 249.88 |
3 | CA04597 | 373.08 |
4 | CA06096 | 448.42 |
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 (figure 2). 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.
- Figure 2. Wet box with spray equilibration chamber mounted in CTD room of RV Southern Surveyor.
Quality control and data reduction:
Parameters logged by the fCO2 system and ship sensors are quality controlled after each voyage.
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.
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 warming of seawater in the system used on RV Southern Surveyor is typically less than 0.6°C and usually less than 0.3°C. Higher values tend to occur 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.
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 (Table 1). 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 (see Pierrot et al 2009). 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 includedt in the final data set.
- Figure 1. Schematic of underway pCO2 system (from Pierrot et al 2009).
References
Copin-Montegut, C. (1988) A new formula for the effect of temperature on the partial pressure of CO2 in sea water, Marine Chemistry, 25, p29-37 (incl. Corrigedum, Marine Chemistry (1989) 27, pp143-144).
Pierrot, D., C. Neill, K. Sullivan, R. Castle, R. Wanninkhof, H. Lüger, T. Johannessen, A. Olsen, R. A. Feely, C. E. Cosca (2009) Recommendations for Autonomous Underway pCO2 Measuring Systems and Data Reduction Routines, Deep-Sea Research II, 56, 512-522, doi:10.1016/j.dsr2.2008.12.005
Weiss, R. F (1974) Carbon Dioxide in water and sea water: the solubility of a non-ideal gas, Marine Chemistry, 2, pp.203-215
Weiss, R.F. and B. A. Price (1980) Nitrous oxide solubility in water and seawater. Marine Chemistry 8, 347–359.
