INTRODUCTION: The information presented in this file is applicable to all the data sets collected on the F.G. Walton Smith that are presented at: www.aoml.noaa.gov/ocd/ocdweb/wsmith/wsmith_introduction.html. 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 ensure that the AOML ocean carbon group receives fair credit for its work. Please consult with us prior to use so we can ensure that the quality and limitations of the data are accurately represented. Platform Information: In October 2011, the Ocean Carbon Cycle Group at NOAA's Atlantic Oceanographic and Meteorological Laboratory (AOML) in collaboration with the Marine Physical Chemistry Group at RSMAS installed an instrument to measure CO2 levels in surface water and air on the F.G. Walton Smith. The Smith operates in the Gulf of Mexico, the Florida Keys, the Bahamas, and surrounding regions. Vessel Name: F.G. Walton Smith Class of Data: Surface ocean carbon dioxide concentrations Scientists responsible for the technical quality of this pCO2 dataset: Frank Millero University of Miami/RSMAS 4600 Rickenbacker Causeway Miami, FL 33149 FMillero@rsmas.miami.edu Rik Wanninkhof NOAA/AOML/Ocean Chemistry and Ecosystems Division 4301 Rickenbacker Causeway Miami, FL 33149 Rik.Wanninkhof@noaa.gov Contact persons for this dataset: Ryan Woosley University of Miami/RSMAS 4600 Rickenbacker Causeway Miami, FL 33149 RWoosley@rsmas.miami.edu Kevin Sullivan NOAA/AOML/Ocean Chemistry and Ecosystems Division 4301 Rickenbacker Causeway Miami, FL 33149 Kevin.Sullivan@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 (October 2011 - 10 June, 2012; March, 2013 - present) Licor_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 840 (11 June, 2012 - February, 2013) Licor_840_Manual.pdf CO2 resolution: 0.01 umol/m CO2 accuracy: better than ± 1.5% of reading Pressure resolution: 0.1 hPa Internal pressure transducer accuracy: ±15 hPa (manufacturer specifications: ±1.5% FS, where FS = 150-1150 hPa) Equilibrator Pressure Transducer: Setra model 270, absolute pressure at analyzer exit (11 June, 2013 - present) http://www.setra.com/ProductDetails/270_Baro.htm Resolution: 0.015 hPa Accuracy: ±0.15 hPa (manufacturer specifications: ±0.05% FS, where FS = 80-110 kPa) Setra model 239, differential pressure at equilibrator headspace (October 2011 - present) 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 differential pressure measured between the headspace and the surrounding laboratory space plus the absolute pressure measured by the infrared analyzer. For the first twenty-one months the internal transducers of the analyzers were employed. Since June 2013, an external transducer was used to measure the absolute pressure at the analyzer exit to the space surrounding the instrument during stopped flow conditions. Equilibrator Temperature: Hart model 1523 (October 2011 - present) http://www.testequipmentdepot.com/fluke-calibration/pdfs/1523-1524_data.pdf Accuracy: ±0.021°C Resolution: 0.001°C Sea Surface Salinity and Temperature (maintained by ship): SeaBird model SBE-45 http://www.seabird.com/pdf_documents/manuals/45_017.pdf Temperature resolution: 0.0001°C Temperature accuracy: ±0.002°C Salinity resolution: 0.0002‰ Salinity accuracy: ±0.005‰ SeaBird model SBE-38 http://www.seabird.com/pdf_documents/manuals/38_013.pdf Temperature resolution: 0.00025°C Temperature accuracy: ±0.001°C Atmospheric Pressure (maintained by ship): RMYoung model 61302 http://www.youngusa.com/products/3/22.html Resolution: 0.1 hPa Accuracy: ±0.3 hPa Due to restrictions on its configuration and data logging, the resolution of the ship's barometer was 1.0 hPa from 2011 until Oct 2013. 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 sample analyses are kept to a minimum by removing as much water as possible. The water is first condensed out of the sample gas stream by cooling to ~5 °C using a thermoelectric device. Then water is further removed using Nafion® gas dryers before the sample gas reaches the IR analyzer. The counterflow gas in the dryer is pre-dried outside air. Typical water content of the analyzed gas is less than 3 millimoles/mole with approximately 90% of the water being removed. The infrared analyzer is calibrated regularly using four standard gases (200 - 1500 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 D. Keeling, which are directly traceable to the WMO scale. A zero gas of ultra-high purity air is analyzed regularly. 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. The system also measures the CO2 content of the atmospheric air from an inlet ~13 meters above the sea surface on the instrument tower above the bridge. Air is constantly being pulled (6 liters/min maximum flow) through ~30 meters of tubing (1 cm OD Dekoron) from the inlet to the analytical system located in the dry lab. The flushing rate of the LI-COR analyzer during ATM analyses is 60 - 150 ml/min. The dedicated scientific seawater system draws water through an inlet ~1.5 meters below the sea surface on the inboard side of the starboard hull. A remote temperature sensor (SBE38) is located near the sea water pump in the engine room for in-situ sea surface temperature (SST) measurements. A thermosalinograph (SBE45) are located near to the CO2 instrument in the dry lab. The seawater travels from the inlet to the CO2 instrument within approximately two minutes Seawater is pushed through a spray head into an equilibration chamber that includes a water jacket for better thermal stability (optional on the General Oceanic model 8050 system). The chamber had a 0.6 L water reservoir and a 0.8 L gaseous headspace. Water flow rate is 1.2 - 1.8 L/min. The rate that the headspace gas is recirculated through the analyzer during EQU analyses is 60 - 150 ml/min. The ship provides a serial connection for GPS locations and times that are logged in real time. An array of oceanographic and meteorological parameters (e.g. SST, salinity, atmospheric pressure) are recorded by the ship and merged with the CO2 data after the cruise. A typical sequence of continuous analyses was: STEP TYPE REPETITIONS 1 Standards (all four) 1 2 ATM 5 3 EQU 100 A complete set of standards and the atmospheric analyses are done every 4.5 hours and a full day contains over 500 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 headspace, 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. The amount of time between the sea water passing by the SST sensor 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. DATA FILE STRUCTURE: List of variables included in this dataset: COLUMN HEADER EXPLANATION 1. EXPOCODE Expedition code, where '33WA' is the NODC ship identifier, and YYYYMMDD is the UTC date that the ship starts the expedition 2. Group_Ship RSMAS_F.G.WaltonSmith, (if present) 3. Cruise_ID Dependent upon expedition's name, (if present) 4. YD_UTC Decimal year day 5. DATE_UTC_ddmmyyyy UTC Date 6. TIME_UTC_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. xCO2_EQU_ppm Mole fraction of CO2 in the equilibrator headspace (dry) at equilibrator temperature, in parts per million 10. xCO2_ATM_ppm Mole fraction of CO2 in outside air (dry), in parts per million 11. 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 12. PRES_EQU_hPa Barometric pressure in the equilibrator headspace, in hectopascals (1 hPa = 1 millibar) 13. PRES_ATM@SSP_hPa Pressure measured by outside barometer, corrected to sea level, in hectopascals 14. TEMP_EQU_C Water temperature in equilibrator, in degrees centigrade 15. SST_C Sea surface temperature from the ship's temperature sensor, in degrees centigrade [interpolated, see note above] 16. SAL_permil Salinity from the thermosalinograph (SBE45), on the Practical Salinity Scale 17. fCO2_SW@SST_uatm Fugacity of CO2 in sea water, in Microatmospheres (100% humidity) 18. fCO2_ATM_interpolated_uatm Fugacity of CO2 in air corresponding to the interpolated xCO2, in microatmospheres (100% humidity) 19. dfCO2_uatm Sea water fCO2 minus interpolated air fCO2, in microatmospheres 20. WOCE_QC_FLAG Quality control flag for fCO2 values (2 = good value, 3 = questionable value) 21. 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. Stringent minimum and maximum values for numerous parameters (e.g.temperature difference between the equilibrator temperature and SST) have been established by CO2 researchers (see Pierrot et al., 2009). These ranges were chosen so that if each parameter were within their stringent range, the resulting CO2 data would almost certainly be good. If a parameter is outside its range or if a parameter is estimated from surrounding good values, the quality flag of that data record is set to 3 (questionable value). The resulting CO2 data could be good; however, investigators should determine whether these data are valid for their purposes. 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.
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