TITLE OF THE DATA SET Surface Water and Atmospheric Carbon Dioxide and Nitrous Oxide Observations by Shipboard Automated Gas Chromatography: Results from Expeditions between 1977 and 1990. DATA CONTRIBUTORS R. F. Weiss F. A. Van Woy P. K. Salameh Scripps Institution of Oceanography University of California, San Diego La Jolla, California DOI: 10.3334/CDIAC/otg.ndp044 SOURCE AND SCOPE OF THE DATA Source Information The surface water and atmospheric carbon dioxide (CO2) and nitrous oxide (N2O) data reported here were obtained by direct shipboard gas chromatographic measurement. These data include results from 11 different oceanic surveys for a total of 41 expedition legs. The represented oceanic surveys include the following: (1) the Indomed expedition, 1977-1979 [Indomed legs 4 and 5 are also part of the Geochemical Ocean Sections (GEOSECS) Indian Ocean expedition]; (2) the North Pacific Experiment (NORPAX) Hawaii-Tahiti Shuttle Experiment, 1979-1980; (3) and (4) the Transient Tracers in the Ocean, North Atlantic and Tropical Atlantic Studies (TTO/NAS, TTO/TAS), 1981-1983; (5) the Hudson 82-001 expedition, 1982; (6) the Ajax expedition, 1983-1984; (7) and (8) the Trans-Pacific Sections expeditions along 24 degrees North and 47 degrees North (TPS24 and TPS47), 1985; (9) the fifth "Antarktis" expedition (Ant V) of the R/V Polarstern, as part of the Winter Weddell Sea Experiment, 1986; (10) the South Atlantic Ventilation Experiment (SAVE), 1987-1989 (SAVE legs 4-6 are also designated as legs 2-4 of R/V Melville's Hydros expedition); and (11) the 1990 expedition of the National Oceanic and Atmospheric Adminis- tration's Climate and Global Change series (CGC-90). The file TRACK.LST (File 2 on the magnetic tape) presents a track list showing the dates, ports of departure and arrival, regions surveyed, and cruise ship names for each of the 41 expedition legs that contributed data. In addition, a series of maps showing the tracks of the expeditions and the N2O and CO2 results for each expedition leg is presented in Figs. 1-83 of the documen- tation that accompanies this tape/diskettes. Methodology This document describes the results of surface water and atmospheric CO2 and N2O measurements carried out by shipboard gas chromatography over the period 1977-1990. The measurements were made by an automated high- precision shipboard gas chromatographic system developed during the late 1970s and used extensively over the intervening years. This instrument, which is described by Weiss (1981), measures CO2 by flame ionization after quantitative reaction to methane in a stream of hydrogen using a reduced nickel catalyst preceded by a palladium catalyst to protect the nickel from oxygen and other atmospheric oxidants. Nitrous oxide is measured by a separate electron capture detector. (Methane is also measured by the flame ionization detector, although the system is not optimized for this gas. The methane results are not included in this data set because methane's equilibration time constant is long, and the results are there- fore subject to contamination by biological activity in the ship's sea- water pumping system.) The chromatographic system measures 196 dry-gas samples a day, divided equally among the atmosphere, gas equilibrated with surface water, a low-range gas standard, and a high-range gas standard. Thus, the atmo- sphere and the ocean are each measured every half-hour, or 48 times a day. This corresponds to a spatial resolution of about 10 km when the ship is under way and gives several replicate measurements at each hydrographic station. The typical relative standard deviation of a single determina- tion is about 0.04% for CO2 and 0.3% for N2O, but precision is occasion- ally affected adversely by shipboard operating conditions. The measure- ments are calibrated with dry-air secondary standards stored in Spectra- Seal aluminum cylinders. These standards are periodically calibrated for CO2 against the Scripps Institution of Oceanography (SIO) manometric scale in the laboratory of C. D. Keeling and for N2O against the calibration scale developed by Weiss et al. (1981). In addition to its accuracy, the chromatographic method for CO2 offers the benefits over other commonly used techniques of being independent of oxygen concentration and using small amounts of sample and standard. Surface seawater is pumped continuously from the bow of the ship (nominal depth approximately 3 m) at a rate of about 100 liters per min. This high pumping rate and the use of plastic polyvinylchloride (PVC) piping assure a minimal change in temperature and a minimal opportunity for chemical alteration of the water. The equilibrator is constructed of heavy acrylic plastic (for visibility and temperature insulation) and has an internal gas volume of about 20 liters. The equilibrator design consists of 2 concentric cylindrical stages, with a drain at the center to minimize volume changes as a result of ship motion. The water "rains" through the 2 stages of the equilibrator at a combined rate of about 20 liters/min, and a low 0.2-atm pressure head minimizes spraying, bubble entrapment, and other dynamic pressure effects. The 20-liter gas space is circulated by an air pump through a closed loop which provides the pressurized gas required by the chromatograph. Each half-hourly analysis removes about 75 ml of gas from this pumped loop. The first stage of the equilibrator is vented to the outside atmosphere so that the gas used for the analysis is replaced by clean marine air. The temperature of the equilibrator is monitored and compared with the surface ocean temperatures measured at hydrographic stations to deter- mine the thermal effect of the ship's pumping system as a function of in- take temperature. The maximum amount of change, found for the coldest surface waters, is typically a warming of <1 C. As expected, this temper- ature difference decreases to zero when the water temperature reaches the mean inside temperature of the ship. The measured CO2 values are corrected for this thermal effect (roughly 4% per degree) using an empirical equation (Weiss et al. 1982) which is dominated by the temperature dependence of the CO2 solubility coefficient (Weiss 1974). The measured N2O values are also corrected for the solubility effect (Weiss and Price 1980). The response time of the equilibrator has been evaluated theoretically and experimentally. For unbuffered gases such as nitrous oxide, oxygen and nitrogen, the theoretical response time (assuming complete exchange between water and gas) is given as FS/V, where F is the flux of water through the system, V is the volume of the equilibrated gas phase, and S is the Ostwald solubility coefficient. For the equilibrator used in the measurements presented here, this gives a characteristic (1/e) response time of about 1 min for N2O, about 0.5 hr for oxygen and about 1 hr for nitrogen. For CO2 the response time would be similar to N2O if there were no chemical buffering, but with chemical buffering (see gas exchange dis- cussion in Broecker and Peng 1982) the response time is enhanced by an order of magnitude to about 0.1 min. Laboratory experiments by Weiss et al. (1982) and by scientists at the National Oceanic and Atmospheric Adminis- tration (NOAA), Pacific Marine Environmental Laboratory (PMEL) and Climate Monitoring and Diagnostics Laboratory (CMDL), who have adopted this equili- brator design, have confirmed that the actual equilibration times are close to these theoretical values. These differences in exchange times are important in understanding the performance of an equilibrator that is vented to atmospheric pressure. Since the major components of equilibrated gas -- nitrogen, oxygen, and argon -- have equilibration times that are much longer than those of the measured species, N2O and CO2, the effect will be for the equilibrium partial pressures of these latter two gases to be present in a water-saturated gas phase at a total pressure equal to the barometric pressure. This is exactly the condition that is satisfied by the actual atmosphere when it is in equili- brium with the ocean, since the gas-phase boundary layer is always saturated with water vapor and at the total barometric pressure. Since the chromato- graphic system measures the dry-gas mole fractions of these constituents, xCO2 and xN2O, in both the atmosphere and the equilibrated gas, and the total pressure is the same in both cases, the differences in xCO2 between these phases are a close measure of the differences in CO2 partial pressure (pCO2), as long as the total pressure is near 1 atm: delta(pCO2) = delta(xCO2)(P - pH2O), where delta signifies the difference between sea and air, P is the barometric pressure, and pH2O is the water vapor pressure. Because the temperature and the barometric pressure are routinely recorded, the system is effectively completely constrained, but even without these variables delta(xCO2) is a very good approximation of delta(pCO2). The argument is, of course, the same for the partial pressure of N2O. Another question which is more difficult to answer is whether the equilibrium reached by the equilibrator is the true thermodynamic equili- brium that we wish to measure. This type of question is always very difficult to answer, but there is indirect evidence that it is very close. The discrete equilibrator pCO2 measurements carried out by T. Takahashi's group at Lamont-Doherty Geological Observatory during the SAVE expeditions have shown agreement with the equilibrator values presented here to within 1 or 2 ppm (T. Takahashi, personal communication), even though their measurements are made at a fixed temperature and must be corrected to the surface water temperature. Also, the measurements of pN2O in the central gyres of the major oceans presented here are generally within 1% of atmo- spheric saturation. If this were not the correct equilibrium value, one could not explain the constancy of these values over many thousands of kilometers in many different central gyres. Through continued use of the same equilibrator design during the World Ocean Circulation Experiment (WOCE), it is hoped to obtain further verification that the measurements are being made at true equilibrium through comparisons with the discrete pCO2 and carbon system measurements being carried out by other laboratories. Concentrations of CO2 and N2O are calculated by fitting detector peak area response to a quadratic polynomial forced through the origin (zeroth order term is zero). The calculation is performed with the assumption that the linearity of the detector varies slowly compared with changes in detector sensitivity. Accordingly, the second order term of the quadratic polynomial (linearity of the detector) is determined from a running mean of the high standard to low standard response ratio over a range of plus and minus 20 runs, and the first order term (sensitivity of the detector) is determined from the immediately bracketing high and low standard runs. Further details concerning the methods of sample measurement and analysis are provided in Weiss (1981), a copy of which is provided in the appendix of the documentation that accompanies this tape/diskettes. DATA FORMAT Eighty-seven files are provided on this magnetic tape or these floppy diskettes, including (1) this documentation file -- File 1 on the magnetic tape or NDP044.TXT on the floppy diskettes; (2) a file containing a list of expedition legs on which measurements were made -- File 2 on the magnetic tape or TRACK.LST on the floppy diskettes; (3) a file containing a list of the data filenames corresponding (by number) to the expedition legs listed in File 2 (or TRACK.LST) -- File 3 on the magnetic tape or DATA.LST on the floppy diskettes; (4) a FORTRAN-77 retrieval program to read and print any of the data files -- File 4 on the magnetic tape or NDP044.FOR on the floppy diskettes; (5) a SAS input/output routine to read and print any of the data files -- File 5 on the magnetic tape or NDP044.SAS on the floppy diskettes; and (6)-(87) 82 files containing the surface water and atmospheric CO2 and N2O data -- Files 6-87 on the magnetic tape or *.AIR and *.H2O on the floppy diskettes [with full filenames as listed in File 3 (or DATA.LST)]. Table 2 (located in the documentation that accompanies this tape/dis- kettes) presents a partial listing of one of the surface water and atmo- spheric CO2 and N2O data files. The data files are formatted in the follow- ing way: CHARACTER SAMPTYP, HEADER*77, DATEMO*3, LATHEM, LONHEM INTEGER DATEDA, DATEYR, TIME REAL LAT, LON, PRESS, H2OTMP, XN2O, XCO2 READ (5,500) SAMPTYP, HEADER 10 READ (5,600,END=800) DATEDA, DATEMO, DATEYR, TIME, 1 LAT, LATHEM, LON, LONHEM, PRESS, H2OTMP, XN2O, XCO2 GOTO 10 500 FORMAT (A1,2X,A77) 600 FORMAT (I2,1X,A3,1X,I2,3X,I4,3X,F7.3,1X,A1,3X,F8.3, 1 1X,A1,3X,F6.1,3X,F5.2,3X,F5.1,3X,F5.1) 800 STOP where SAMPTYP is a one-character code describing the type of samples being presented in the data file: A = atmospheric samples, S = surface seawater (i.e., gas equilibrated with surface seawater) samples; HEADER is a descriptive character string consisting of (1) the name of the expedition (e.g., AJAX Leg 1) and (2) the name of the research vessel (e.g., R/V Knorr); DATEDA is the numeric day of the month on which the sample was collected; DATEMO is the three-letter abbreviation (Jan, Feb, etc.) for the month in which the sample was collected; DATEYR is the final two digits of the year (since 1900) in which the sample was collected; TIME is the Greenwich Mean Time at which the sample was collected, expressed in 24-hour time from 0000 to 2359; LAT is the latitude (in decimal degrees) at which the sample was collected, with possible values from -90.000 to 90.000 (north latitudes are represented as positive); LATHEM is the latitudinal hemisphere in which the sample was taken: N = Northern Hemisphere, S = Southern Hemisphere; LON is the longitude (in decimal degrees) at which the sample was collected, with possible values from -180.000 to 180.000 (east longitudes are represented as positive); LONHEM is the longitudinal hemisphere in which the sample was taken: E = Eastern Hemisphere, W = Western Hemisphere; PRESS is the approximate sea level barometric pressure in mBar, interpolated from discrete values recorded on the ship at hydrographic stations; H2OTMP is the approximate surface water temperature in degrees Celsius, interpolated from discrete values recorded on the ship at hydrographic stations; XN2O is the dry gas mole fraction of nitrous oxide (N2O) in the sample, measured in parts per billion (ppb); XCO2 is the dry gas mole fraction of carbon dioxide (CO2) in the sample, measured in parts per million (ppm); Stated in tabular form, the contents include the following: ______________________________________________________________ Variable Variable Starting Ending Variable type width Line column column ______________________________________________________________ SAMPTYP Character A1 1 1 1 HEADER Character A77 1 4 80 DATEDA Numeric I2 2...n 1 2 DATEMO Character A3 2...n 4 6 DATEYR Numeric I2 2...n 8 9 TIME Numeric I4 2...n 13 16 LAT Numeric F7.3 2...n 20 26 LATHEM Character A1 2...n 28 28 LON Numeric F8.3 2...n 32 39 LONHEM Character A1 2...n 41 41 PRESS Numeric F6.1 2...n 45 50 H2OTMP Numeric F5.2 2...n 54 58 XN2O Numeric F5.1 2...n 62 66 XCO2 Numeric F5.1 2...n 70 74 ______________________________________________________________ Missing values are represented as follows -- PRESS: -999.9; H2OTMP: 99.99; XN2O: -99.9; XCO2: -99.9. Values for variable width are entered as FORTRAN 77 format codes. REFERENCES Broecker, W. S., and T.-H. Peng. 1982. Tracers in the Sea. Eldigio Press, Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York. Weiss, R. F. 1974. Carbon dioxide in water and seawater: The solubility of a non-ideal gas. Marine Chemistry 2:203-215. Weiss, R. F., and B. A. Price. 1980. Nitrous oxide solubility in water and seawater. Marine Chemistry 8:347-359. Weiss, R. F. 1981. Determinations of carbon dioxide and methane by dual catalyst flame ionization chromatography and nitrous oxide by electron capture chromatography. Journal of Chromatographic Science 19:611-616. Weiss, R. F., C. D. Keeling, and H. Craig. 1981. The determination of tropospheric nitrous oxide. Journal of Geophysical Research 86:7197-7202. Weiss, R. F., R. A. Jahnke, and C. D. Keeling. 1982. Seasonal effects of temperature and salinity on the partial pressure of carbon dioxide in seawater. Nature 300:511-513.