****************** Use of AOML CO2 Underway Data ************************** * [Adapted from NOAA Earth System and Diagnostic Laboratory (ESDL) * ************* Carbon Cycle Greenhouse Gasses (CCGG) web site] ************* These data are made freely available to the public and the scientific community in the belief that their wide dissemination will lead to greater understanding and new scientific insights. The availability of these data does not constitute publication of the data. We rely on the ethics and integrity of the user to assure that AOML receives fair credit for our work. Please send manuscripts using this data to AOML for review before they are submitted for publication so we can insure that the quality and limitations of the data are accurately represented. *********************** Reciprocity Agreement ***************************** * [Adapted from NOAA Earth System and Diagnostics Laboratory (ESDL) * ************* Carbon Cycle Greenhouse Gasses (CCGG) web site] ************* Use of these data implies an agreement to reciprocate. Laboratories making similar measurements agree to make their own data available to the general public and to the scientific community in an equally complete and easily accessible form. Modelers are encouraged to make available to the community, upon request, their own tools used in the interpretation of the AOML data, namely well documented model code and any additional information necessary for other scientists to repeat the work and to run modified versions. *************************************************************************** The electronic data in comma delimited ASCII form (.CSV) can be found at: "http://www.aoml.noaa.gov/ocd/gcc/skogafoss_introduction.php" For further information or special requests regarding the Skogafoss data contact: Kevin Sullivan Atlantic Oceanographic and Meteorological Lab 4301 Rickenbacker Causeway Miami, FL 33149 email: Kevin.Sullivan@noaa.gov Phone: 305-361-4382 The data for each year can now be accessed from a drop-down list on this page. Cruise tracks are now color coded to show the approximate partial pressure of CO2 in surface water over the ship's course. The following list gives the data fields with units: COLUMN HEADER EXPLANATION 1. EXPOCODE: AGSKYYYYMMDD (where YYMM is the month/day of the beginning of the cruise). 2. GROUP/SHIP: AOML/Skogafoss 3. CRUISE_DESIGNATION: SKOYNN (where Y is Year and NN is the cruise #). 4. JD_GMT: Decimal year day 5. DATE_DDMMYYYY: UTC Date 6. TIME_HH:MM:SS: UTC Time 7. LAT_DEC_DEGREE: Latitude in decimal degrees (negative values are in southern hemisphere). 8. LONG_DEC_DEGREE: Longitude in decimal degrees (negative values are in western latitudes). 9. xCO2W_PPM: Mole fraction of CO2 in the equilibrator at equilibrator temperature (Teq) in parts per million. 10. xCO2A_PPM: Mole fraction of CO2 in air in parts per million. 11. xCO2A_INTERPOLATED_PPM xCO2 interpolated in parts per million. 12. PRES_EQUIL_hPa: Barometric pressure in the lab in hectopascals (1 hectopascal = 1 millibar). 13. PRES_SEALEVEL_hPa: Barometric pressure from ship's barometer, corrected to sea level in hectopascals (1 hectopascal = 1 millibar). 14. EqTEMP_C: Temperature in equilibrator water in degrees centigade. 15. SST(TSG)_C: Temperature from the ship's thermosalinograph in degrees centigrade. 16. SAL(TSG)_PERMIL: Salinity from the ship's thermosalinograph in [Practical Salinity Scale]. 17. fCO2W@SST_uATM: Fugacity of CO2 in sea water in microatmospheres. 18. fCO2A_uATM: Fugacity of CO2 in air in microatmospheres. 19. dfCO2_uATM: Sea water fCO2 - air fCO2 in microatmospheres. This uses the average value between air measurements. 20. QC_FLAG Quality control flag for seawater fCO2 values with 2 = good value and 3 = questionable. 21. QC_SUBFLAG Quality control flag for seawater fCO2 values with 1 = outside standard range, 2 = bad SST, 3 = bad EqT, 4 = bad ΔT, 5 = excess warming, 6 = bad salinity, 7 = bad pressure, 8 = low gas flow, 9 = bad air value, 10 = interpolated standard and 11 = miscellaneous. ********************************************************************************* CALCULATIONS: The mixing ratios of ambient air and equilibrated headspace air are calculated by using a linear fit of the time-weighted CO2 values of the preceding and following standards. Mixing ratios of dried equilibrated headspace and air are converted to fugacity of CO2 in surface seawater and water saturated air in order to determine the fCO2. For ambient air and equilibrator headspace the fCO2a, or fCO2eq is calculated assuming 100% water vapor content: fCO2 = xCO2 P (1-pH2O) exp[(B11+2d12)P/RT] where fCO2/eq is the fugacity in ambient air or equilibrator, pH2O is the water vapor pressure at the sea surface temperature, P is the equilibrator or atmospheric pressure (in atm), T is the SST or equilibrator temperature (in K) and R is the ideal gas constant (82.057 cm^3·atm·deg^-1·mol^-1). The exponential term is the fugacity correction where B11 is the first virial coefficient of pure CO2 B11 = -1636.75 + 12.0408 T - 0.0327957 T^2 + 3.16528E-5 T^3 and d12 = 57.7 - 0.118 T is the correction for an air-CO2 mixture in units of cm^3·mol^-1 (Weiss, 1974). The calculation for the fugacity at SST involves a temperature correction term for the increase of fCO2 due to heating of the water from passing through the pump and through 5 cm ID PVC tubing within the ship. The water in the equilibrator is typically 1 °C warmer than sea surface temperature. The empirical temperature correction from equilibrator temperature to SST is outlined in Takahashi et al. (1993). fCO2(SST) = fCO2(teq)exp[0.0423(SST-teq)] where fCO2(SST) is the fugacity at the sea surface temperature and fCO2(teq) is the fugacity at the equilibrator temperature. SST and teq are the sea surface and equilibrator temperatures in degrees C, respectively. NOTES ON DATA: Columns have a default value of -9 in case of instrument malfunction or erroneous readings. Furthermore, if a suspicious xCO2 value, pressure or temperature value is encountered which cannot readily be extrapolated, the fCO2 is not calculated. INSTRUMENT DESCRIPTION: The general principle of operation of the instrument can be found in Wanninkhof and Thoning (1993), Ho et al. (1995), and Feely et al. (1998). The concentration of CO2 in the headspace gas is measured using the adsorption of infrared (IR) radiation, which results from changes in the rotational and vibrational energy state of the CO2 molecule. The LI-COR 6262 analyzer passes IR radiation through two 6" cells. The reference cell is flushed with a gas that passes through beds of soda lime and magnesium perchlorate to remove CO2 and water. The sample cell is flushed with the standard or sample gas. A vacuum-sealed, heated filament is the broadband IR source. The IR radiation alternates between the two cells via a chopping shutter disc spinning at 500 Hertz. Two bandpass optical filters select adsorption bands optimized for CO2 (4.62 micron) and water (2.59 micron) to reach the detectors. The solid state (lead selenide) detectors is kept at -5 degrees C for excellent stability and low signal noise. To reduce the size of corrections done because of the presence of water vapor, the outside air and equilibrator headspace gas pass through Naphion gas dryers before reaching the IR analyzer. The counter flow gas used in the dryer is ambient air at reduced pressure, which also reduces the partial pressure of water vapor. When operating well, the gas dryer reduces the water content of the gas streams to less than 3 millimole/mole (volume). The infrared analyzer is calibrated regularly with four standard gases (200 - 450 ppm CO2 in air) from Scott-Marrin Inc. (Riverside, CA). Before and after use in the field, the standards are calibrated using primary reference gases from the laboratory of Dr. Charles D. Keeling, which are directly traceable to the WMO scale. After the sample cell is flushed with standard or sample gas, the flow is stopped and multiple readings of the detector response is averaged for the result. The outside air is drawn from an inlet on the 70-foot bow mast. Outside air is constantly being pulled (6 liter/min maximum flow) through ~400 feet of tubing (3/8" OD Dekoron) to the analytical system in the engine room. The seawater is drawn from the sizable flow towards the engine heat exchangers. One stream of seawater goes through a Seabird thermosalinograph that contains temperature and salinity sensors. Another stream of seawater goes to the CO2 equilibrator. The CO2 equilibrator was fabricated using a filter housing (ColeParmer, U-010509-00) with ~0.5 L water reservoir and ~0.8 L gaseous headspace. The seawater enters via a spiral spray head at a flow rate between 1 and 1.5 L/min. The headspace gas passes through the IR analyzer and returns to the equilibrator at a flow rate of ~80 ml/min. The headspace is maintained at ambient pressure via a vent connected to a smaller equilibration chamber. This chamber has a seawater spray head that helps ensure that any gas entering the main equilibrator's headspace will be close to its CO2 concentration. SAMPLING CYCLE: The basic sampling sequence for the first two years of operation was a single analysis of each of the 4 standard gases, 3 analyses of outside air, 20 analyses of the equilibrator headspace, a single analysis of the standard gas (#3) closest to outside air, and 20 analyses of the equilibrator headspace. The single analysis of standard gas #3 was done to monitor any changes in the detector response while minimizing consumption of gas standards. After 7 December, 2005, the sampling sequence was changed to single analysis of the 4 standard gases, 5 analyses of outside air, and then 45 analyses of the equilibrator headspace. After 4 or 5 cycles of these 48 analyses, the ~200 um filter before the equilibrator is backflushed with fresh water. The minimum flush time with a particular gas for the first two years of operation was 170, 170, and 250 seconds for standard, outside air, and equilibrator headspace respectively. An additional 10 seconds of flush time was done when the preceding gas type is different than the current gas type. After 7 December, 2005, the initial flush time of any gas was at least 240 seconds while the later analyses of the same gas had flush times of at least 60 seconds for standards and air and 150 seconds for equilibrator headspace. After flushing the sample cell with the selected gas, the gas flow is stopped for 10 seconds and then 5 readings are taken and averaged. This pattern results in average analysis time of 3.5 minutes for standard gases, 3.3 minutes for outside air, and 4.6 minutes for equilibrator headspace. The average day contains 265-270 analyses of the equilibrator headspace. UNITS: All xCO2 values are reported in parts per million (ppm) and fCO2 values are reported in microatmospheres (uatm) assuming 100 % humidity at the equilibrator temperature. COMMON PORTS OF CALL: CITY LATITUDE LONGITUDE Reykjavik, Iceland 64.150 -21.855 Argentia, Newfoundland 47.296 -53.984 Shelbourne, Nova Scotia 43.753 -65.322 Boston, Massachusetts 42.390 -71.054 Philadelphia, Pennsylvania 39.850 -75.341 Newport News, Virginia 36.963 -76.417 Richmond, Virginia 36.517 -77.417 REFERENCES: DOE (1994). Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water; version 2. DOE. Feely, R. A., R. Wanninkhof, H. B. Milburn, C. E. Cosca, M. Stapp and P. P. Murphy (1998) A new automated underway system for making high precision pCO2 measurements onboard research ships. Analytica Chim. Acta 377: 185-191. Ho, D. T., R. Wanninkhof, J. Masters, R. A. Feely and C. E. Cosca (1997). Measurement of underway fCO2 in the Eastern Equatorial Pacific on NOAA ships BALDRIGE and DISCOVERER, NOAA data report ERL AOML-30, 52 pp. , NTIS Springfield, Wanninkhof, R. and K. Thoning (1993) Measurement of fugacity of CO2 in surface water using continuous and discrete sampling methods. Mar. Chem. 44(2-4): 189-205. Weiss, R. F. (1970) The solubility of nitrogen, oxygen and argon in water and seawater. Deep-Sea Research 17: 721-735. Weiss, R. F. (1974) Carbon dioxide in water and seawater: the solubility of a non-ideal gas. Mar. Chem. 2: 203-215. Weiss, R. F., R. A. Jahnke and C. D. Keeling (1982) Seasonal effects of temperature and salinity on the partial pressure of CO2 in seawater. Nature 300: 511-513.
