INTRODUCTION: The information presented in this file is applicable to all the data sets collected on the R/V Gordon Gunter that are presented at: www.aoml.noaa.gov/ocd/gcc/rvgunter_introduction.php. Any temporary changes in this information will be noted in the readme files for the individual expeditions. Statement for use of data: These data are made available to the public and the scientific community in the belief that their wide dissemination will lead to greater understanding and new insights. The availability of these data does not constitute publication of the data. We rely on the ethics and integrity of the user to insure that the AOML ocean carbon group receives fair credit for its work. Please consult with us prior to use so we can insure that the quality and limitations of the data are accurately represented. Platform Information: In 2008, the Ocean Carbon Group at NOAA's Atlantic Oceanographic and Meteorological Laboratory (AOML) installed an instrument to measure CO2 levels in surface water and air on the NOAA Ship Gordon Gunter. The Gordon Gunter primarily operates in the Gulf of Mexico, Atlantic and Caribbean waters. Vessel Name: R/V Gordon Gunter Class of Data: Surface ocean carbon dioxide concentrations Scientists responsible for the technical quality of this pCO2 dataset: Rik Wanninkhof and Denis Pierrot NOAA/AOML/Ocean Chemistry Division 4301 Rickenbacker Causeway Miami, FL 33149 Rik.Wanninkhof@noaa.gov Denis.Pierrot@noaa.gov Contact persons for this dataset: Kevin Sullivan and Denis Pierrot NOAA/AOML/Ocean Chemistry Division 4301 Rickenbacker Causeway Miami, FL 33149 Kevin.Sullivan@noaa.gov Denis.Pierrot@noaa.gov COMPONENT SPECIFICATIONS and ACCURACIES: The accuracies of all components, when operating optimally, are such that the calculated seawater fCO2 has an accuracy of 2 uatm or better and the calculated mole fraction of CO2 (XCO2) in air has an accuracy of 0.1 uatm. Infrared Analyzer: LI-COR model 6262 (2008-August 2010) ftp://ftp.licor.com/perm/env/LI-6262/Manual/LI-6262_Manual.pdf CO2 resolution: 0.01 umol/m CO2 accuracy: ± 1 ppm at 350 ppm Internal pressure transducer accuracy: ± 1.2 hPa (manufacturer specifications: ±0.1% FS, where FS = 0-1150 hPa) LI-COR model 7000 (August 2010 - current) ftp://ftp.licor.com/perm/env/LI-7000/Manual/LI-7000Manual.pdf CO2 resolution: 0.01 umol/m CO2 accuracy: ± 1% nominal Internal pressure transducer accuracy: ± 1.2 hPa (manufacturer specifications: ±0.1% FS, where FS = 0-1150 hPa) Equilibrator Pressure Transducer: Setra model 239, differential pressure (2008 ? current) http://www.setra.com/ProductDetails/model_239.htm Resolution: 0.01 hPa Accuracy: ± 0.052 hPa (manufacturer specifications: ± 0.14% FS, where FS = ± 7.5 inches WC) The absolute pressure of the equilibrator headspace reported in data files is the sum of the infrared analyzer pressure and the differential pressure from the pressure transducer attached to the equilibrator. Equilibrator Temperature: Hart model 1521 (August 2010 - current) http://www.testequipmentdepot.com/hart/pdfs/1521_1522.pdf Resolution: 0.001°C Accuracy: ± 0.025°C Atmospheric Pressure: Druck model RPT 350 (2008 - August 2010) http://www.ge-mcs.com/en/pressure-and-level/transducerstransmitters/rtp350.html Resolution: 0.01 hPa Accuracy: ± 0.08 hPa (manufacturer specifications: ± 0.02% FS, where FS = 400 hPa) RMYoung model 61201 (maintained by ship) (August 2010 - present) http://jsinstruments.com/files/Model%2061201%20Barometric%20Pressure%20Sensor.pdf Resolution: 0.01 hPa Accuracy: ± 0.5 hPa Sea Surface Salinity (maintained by ship): SeaBird model SBE-21 (2008 - present) http://www.seabird.com/pdf_documents/manuals/21_027.pdf Temperature Accuracy: ± 0.01 °C Salinity Accuracy: 0.05 ‰ Sea Surface Temperature (maintained by ship): Furuno model T2000 (2008 - 2012) Furuno T2000 Manual pdf Temperature Resolution: 0.01 °C Temperature Accuracy: ± 0.02 °C Sea Surface Temperature (maintained by ship): SeaBird model SBE-38 (2013 - present) http://www.seabird.com/pdf_documents/manuals/38_013.pdf Temperature Resolution: 0.00025°C Temperature Accuracy: ± 0.001°C INSTRUMENT DESCRIPTION and CONFIGURATION: The general principle of operation of the instrument can be found in Wanninkhof and Thoning (1993), Ho et al. (1995), Feely et al. (1998), and Pierrot et al. (2009). Seawater flows through an equilibrator chamber where CO2 exchanges between water and the air above it. Small changes in seawater CO2 concentration are rapidly translated into changes in CO2 concentration in the air of the chamber (headspace). The mole fraction of CO2 in the headspace gas is measured using a non-dispersive infrared (NDIR) analyzer from LICOR®. The effects of water vapor on the signal can be compensated for by the analyzer (LiCOR model 6262) but are also kept to a minimum by removing as much water as possible. The water is first condensed out of the gas stream by cooling. Then it is further removed using Nafion® gas dryers before reaching the IR analyzer. Typical water content of the gas streams are less than 3 millimoles/mole with approximately 90% of the water removed. The infrared analyzer is calibrated regularly using four standard gases (200 - 450 ppm CO2 in air) from Scott-Martin Inc. (Riverside, CA). Before and after use in the field, the standards are calibrated using primary reference gases from the laboratory of Dr. Charles P. Keeling, which are directly traceable to the WMO scale. Any value outside the range of the standards should be considered approximate (+/-5 ppm). While individual data points above the highest standard or below the lowest standard may be less accurate, the general trends would be indicative of the seawater chemistry. The exact concentrations of the standards used on a particular cruise are listed in the individual readme file. The system also measures the CO2 content of the outside air, which is drawn from an inlet on the box mast approximately 15 meters above the water. Atmospheric air was constantly being pulled (6 liters/min maximum flow) through ~200 feet of tubing (3/8" OD Dekoron) to the analytical system located located in the web lab. The seawater is drawn from the ship's flowing seawater line, which also feeds a thermosalinograph and Turner fluorometer. The three CO2 standard cylinders come from Scott-Marrin, Inc., and are calibrated with primary standards that are directly traceable to the WMO scale. The zero gas is ultra-high purity air. Any value outside the range of the standards should be considered approximate (+/-5 ppm). While individual data points above the highest standard or below the lowest standard may less accurate, the general trends would be indicative of the seawater chemistry. The standards used on a particular cruise are listed in the individual readme file. A 'Deck Box' containing a high precision pressure transducer, a GPS and Iridium satellite modem is located outside and several decks above the system on part of the ship's superstructure protected from severe weather. The instrumental system via the deck box records the atmospheric pressure and the position of the ship. The measured pressure is corrected for the height of the barometer above the sea surface with the addition of dgh/u - where d is atmospheric density (1.2 kg/m3), g is gravitational acceleration (9.8 m/sec2), h is height of the barometer above the sea surface, and u is the conversion factor from pascals to desired pressure units. The estimated height of 15 meters resulted in a change in the barometric pressure of approximately of 1.8 mbar. A typical sequence of continuous analyses was: STEP TYPE REPETITIONS 1 Standards (all four) 1 2 ATM 5 3 EQU 60 The amount of time between analyses depends on whether the analyses are of the same type of gas (e.g., STD, ATM, EQU) or not. When switching between different gases, the connecting tubes and analyzer are flushed for an initial interval called the 'PRE-FLUSH' time plus an interval called the 'REGULAR FLUSH' time. Between successive measurements of the same type of gas, the system is flushed for only the 'REGULAR FLUSH' time. The gas flow is then stopped. After the 'STOP FLOW' time interval, which is 10 seconds for all analyses, the output of the NDIR analyzer is read. The pre-flush time is set to 180 seconds and the regular flush time is set to 60 seconds for standard and air analyses. Both the pre-flush and regular flush times are 120 seconds for equilibrator headspace analyses. With these settings, a complete set of standards and the atmospheric analyses are done every 3 hours and a full day contains about 480 analyses of the equilibrator headspace. CALCULATIONS: The measured xCO2 values are linearly corrected for instrument response using the standard measurements (see Pierrot et al., 2009). 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 is the fugacity in ambient air or equilibrator, pH2O is the water vapor pressure at the sea surface or equilibrator temperature, P is the equilibrator or outside 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 fugacity as measured in the equilibrator is corrected for any temperature difference between sea surface temperature and equilibrator chamber using the empirical correction 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. DATA FILE STRUCTURE: List of variables included in this dataset: COLUMN HEADER EXPLANATION 1. Group_Ship AOML_GGunter 2. Cruise_ID dependent upon expedition's name 3. JD_GMT Decimal year day 4. DATE_UTC_ddmmyyyy UTC Date 5. TIME_UTC_hh:mm:ss UTC Time 6. LAT_dec_degree Latitude in decimal degrees (negative values are in southern hemisphere) 7. LONG_ dec_degree Longitude in decimal degrees (negative values are in western latitudes) 8. xCO2_EQU_ppm Mole fraction of CO2 in the equilibrator headspace (dry) at equilibrator temperature, in parts per million 9. xCO2_ATM_ppm Mole fraction of CO2 in outside air (dry), in parts per million 10. xCO2_ATM_interpolated_ppm xCO2 in outside air associated with each water analysis. These values are interpolated between the bracketing averaged good xCO2_ATM analyses, in parts per million 11. PRES_EQU_hPa Barometric pressure in the equilibrator headspace, in hectopascals (1 hPa = 1 millibar) 12. PRES_ATM@SSP_hPa Pressure measured by outside barometer, corrected to sea level, in hectopascals 13. TEMP_EQU_C Water temperature in equilibrator, in degrees centigrade 14. SST_C Sea surface temperature from the ship's remote temperature sensor, in degrees centigrade [interpolated, see note below] 15. SAL_permil Salinity from the thermosalinograph (SBE45), on the Practical Salinity Scale 16. fCO2_SW@SST_uatm Fugacity of CO2 in sea water, in Microatmospheres (100% humidity) 17. fCO2_ATM_interpolated_uatm Fugacity of CO2 in air corresponding to the interpolated xCO2, in microatmospheres (100% humidity) 18. dfCO2_uatm Sea water fCO2 minus interpolated air fCO2, in microatmospheres 19. WOCE_QC_FLAG Quality control flag for fCO2 values (2 = good value, 3 = questionable value) 20. QC_SUBFLAG Quality control sub flag for fCO2 values provides explanation for atypical data, when QC_FLAG = 3 The quality control flags are provided as an aid to the interpretation of the CO2 data. The amount of time between the sea water entering the ship and the water flowing through the equilibrator is estimated before assigning an SST value to each analysis. The patterns in the temperature records for the equilibrator and for SST over time are compared, and a time offset that optimizes the match of these patterns is determined. The time offset is applied to the SST measurements. A linear interpolation between the time-adjusted SST data yields the SST value assigned to each CO2 analysis and used in the fugacity calculations. 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, Pierrot, D., C. Neill, K. Sullivan, R. Castle, R. Wanninkhof, H. Luger, T. Johannessen, A. Olsen, R. A. Feely, and C. E. Cosca (2009). Recommendations for autonomous underway pCO2 measuring systems and data-reduction routines. Deep Sea Research II, 56: 512-522. 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. Takahashi, T., J. Olafsson, J. G. Goddard, D. W. Chipman, and S. C. Sutherland (1993). Seasonal variation of CO2 and nutrients in the high-latitude surface oceans: a comparative study, Global Biogeochem. Cycles, 7, 843-878.
