README for underway fCO2 files for Equatorial Pacific. The electronic data comma delimited ASCII form (.CSV) can be obtained via anonymous FTP from: or via the World Wide Web: The data files are called: EQPACS94.CSV: Underway fCO2 data obtained during the boreal spring of 1994. EQPACF94.CSV: Underway fCO2 data obtained during the boreal fall of 1994. DISCO94.CSV: Data obtained during the Discoverer cruise along nominally 110oW in the boreal spring of 1994. DIC94.CSV: Select surface water data for DIC, O2, Chl, and NO3, NO2, and SiO3 obtained during the spring and fall cruises. For further information or special requests regarding data format contact: Ms. Betty Huss Huss@aoml.noaa.gov phone: 305-361-4395 The methodology and data given here is described in detail in the data report: NOAA Data Report ERL AOML-## MEASUREMENTS OF UNDERWAY fCO2 IN THE EASTERN EQUATORIAL PACIFIC ON NOAA SHIPS MALCOLM BALDRIGE AND DISCOVERER FROM FEBRUARY TO SEPTEMBER, 1994 by: David T. Ho* Rik Wanninkhof Jason Masters Ocean Chemistry Division Atlantic Oceanographic and Meteorological Laboratory Richard A. Feely Cathy E. Cosca Ocean Climate Research Division Pacific Marine Environmental Laboratory *Current Address: Lamont-Doherty Earth Observatory and Department of Earth and Environmental Sciences Columbia University Palisades, NY 10964 A short synopses is as follows: ABSTRACT: From February through September, 1994 underway measurements of the fugacity (partial pressure) of carbon dioxide (fCO2) were performed in the Eastern Equatorial Pacific as part of the Ocean Atmosphere Carbon Exchange Study (OACES) of the National Oceanographic and Atmospheric Administration (NOAA). The measurements were performed with semi-autonomous instruments which measured the fugacity in the air and in the headspace of an equilibrator drawing water from the bow of the ship, from which the fCO2 of the surface water is calculated. From the difference in fugacity in air and water, the CO2 flux from the Equatorial Pacific can be estimated. On the NOAA ship MALCOLM BALDRIGE the system measured three reference standards, three air values, and eight water values per hour. The system on the DISCOVERER measured three standards, one 19-minute average air sample and one 20-minute average water sample per hour. This report contains a description of the methodology and reduction of the fCO2 and ancillary measurements. The full data sets and support measurements can be obtained either via anonymous FTP from or via the World Wide Web: . Calculations: The mixing ratios of ambient air and equilibrated headspace air are calculated by fitting a second-order polynomial through the hourly averaged response of the detector versus mixing ratios of the 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 deltafCO2. For ambient air and equilibrator headspace the fCO2a, or fCO2eq is calculated assuming 100 % water vapor content: fCO2a/eq = xCO2a/eq(P-pH2O)exp(B11+2d12)P/RT where fCO2a/eq is the fugacity in ambient air or equilibrator, pH2O is the water vapor pressure at the sea surface temperature, P is the atmospheric pressure (in atm), and T is the SST or equilibrator temperature (in K) and R is the ideal gas constant (82.057cm cubed * atm / deg * mol). The exponential term is the fugacity correction where B11 is the second virial coefficient of pure CO2 B11 = -1636.75+12.0408T-0.032795T^2+3.16528E-5 T^3 and d12(=57.7 - 0.118 T) is the correction for an air-CO2 mixture in units of 1/cm3mol (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 0.2 degrees C warmer than sea surface temperature. The empirical temperature correction from equilibrator temperature to SST is outlined in Weiss et al. (1982) . Dln(fCO2)=(teq-SST)(0.0317-2.7851E-4 teq - 1.839E-3 ln(fCO2eq)) where Dln(fCO2) is the difference between the natural logarithm of the fugacity at teq and SST, and teq is the equilibrator temperature in degrees C. Explanation of column headers in EQPACS94.CSV, EQPACF94.CSV: JD Julian Day Date Date (month, day, year) Time Greenwich Mean Time Lat Latitude (decimal degrees) Long Longitude (decimal degrees) Distance Distance traveled (in km) since start of cruise xCO2,w Mixing ratio of CO2 (dry) in headspace of equilibrator. Water comes from a bow intake 5 m below the water line (in ppm) xCO2,a Mixing ratio of CO2 (dry) from the bow of the ship (15 m above water) Eq Temp Temperature in equilibrator measured with a calibrated thermistor (in degrees C) Pressure Pressure in OCEA laboratory (in mB) SST (TSG) Sea surface temperature measured at the water intake (5 m below water line) (degrees C) Sal (TSG) Salinity measured at the water intake (5 m below water line) fCO2w, eq Fugacity of water in equilibrator calculated according to DOE (1994) (in microatm) fCO2w, In situ fugacity of water at SST calculated from algorithm of Weiss et al. (1982) fCO2a Fugacity of CO2 in air deltafCO2 Water- air fugacity difference Note Lat etc. Comment on the particular cell value, indicating extrapolation, deletion etc. NOTES ON DATA: Columns have a default value of -999.99 in case of instrument malfunction or erroneous readings. Furthermore, if a suspicious xCO2 value, pressure or temperature value is encountered which cannot be readily extrapolated, the fCO2 is not calculated For the spring cruise the air pump failed during the 140 W transit. The xCO2 values from 125 W at the appropriate latitude were used instead. The 125 W transect occurred 2-weeks to a month earlier and air values show a seasonal cycle, particularly in the Northern hemisphere. To assess the possible error introduced by this extrapolation the air values at 125 W were compared with the northern hemisphere values along 110 W at the same latitude but occupied a month later, immediately after the 140 W transect. The agreement from 0 to 5 N is within 1 ppm. The correspondence between xCO2 at 125 W and at 140 W when the air pump was working is good as well. At 21 N, the xCO2a value was 362.82 at 140 W and 363.96 at 125 W; at 8 S the 140 W value was 358.21 and at 125 W it was 357.79. The in situ SST, salinity, and pressure values were used to calculate the corresponding fCO2a values along 140 W. Explanation of column headers in DISCO94.CSV file: DATE (GMT) Fractional Julian Day (in GMT time) Latitude Decimal Latitude Longitude Decimal Longitude SST (C) Temperature at bow intake (in degrees C) EqT (C) Temperature in equilibrator (in degrees C) Salinity Salinity from thermosalinograph at bow intake Pressure (mbar) Pressure (in millibar) XCO2 air (ppm) Mole fraction of CO2 in dried air XCO2 sw (ppm) Mole fraction of CO2 (dry) in headspace of equilibrator Point Warm (C) Eq T - SST (in degrees C) f(CO2) air (uatm) fugacity of CO2 in air (in microatmosphere) f(CO2) sw (uatm) fugacity of CO2 in surface seawater (at SST) deltafCO2 (uatm) f(CO2) sw - f(CO2) air Comments Comments Comments Explanation of column headers in DIC94.CSV file: Date Mo./Day/Year Time (GMT) Hour:minute (Greenwich mean time) Latitude Decimal degrees Longitude Decimal degrees Salinity Temp. (degrees C) Temperature in degree centigrade O2 (umol/l) Oxygen concentration in micromol/l AOU (umol/l) Apparent oxygen utilization using solubility equation of Weiss (1972) NO2 (umol/l) Nitrite concentration in micromol/l NO3 (umol/l) Nitrate concentration in micromol/l SiO3 (umol/l) Silicate concentration in micromol/l PO4 (umol/l) Phosphate concentration in micromol/l (fall only) DIC (umol/kg) Total dissolved inorganic carbon in micromol/kg Chl a (ug/l) Chloropyll-a in microgram/l (spring only) phao (ug/l) Phaopigments in microgram/l (spring only) total Chl (ug/l) Total chlorophyll (= chl-a + phao) (spring only) References: DOE, Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water; version 2, DOE, 1994. Weiss, R.F., The solubility of nitrogen, oxygen and argon in water and seawater, Deep-Sea Research, 17, 721-735, 1970. Weiss, R.F., Carbon dioxide in water and seawater: the solubility of a non-ideal gas, Mar. Chem., 2, 203-215, 1974. Weiss, R.F., R.A. Jahnke, and C.D. Keeling, Seasonal effects of temperature and salinity on the partial pressure of CO2 in seawater, Nature, 300, 511-513, 1982. ---------------------------------------------------- Rik Wanninkhof Phone: (305) 361-4379 OCD/AOML/NOAA FAX: (305) 361-4392 4301 Rickenbacker Causeway Miami FL 33149 -----------------------------------------------------