Method of underway pCO2 Measurements in Surface waters and the atmosphere during the AESOPS expeditions, 1996-1998 in the Pacific sector of the Southern Ocean and the Ross Sea

                   Taro Takahashi, Colm Sweeney, S. C. Sutherland
                David W. Chipman, John Goddard and Stephany I. Rubin

               Lamont-Doherty Earth Observatory of Columbia University
                               Palisades, NY 10964

                                (April 25, 2000)

This report describes the methods deployed during the AESOPS expeditions for the measurement of pCO2 in surface waters and in the overlying atmosphere in 1994 through 1998. The number of measurements made for carbon chemistry during the AESOPS program and other related programs are summarized in Table 1. The operational procedures of the underway pCO2 measurement system are described in the "pCO2 Equilibrator Users Manual" prepared by the LDEO CO2 Group (1999). The methods used for the discrete measurements made at stations have been reported with the station data.

Table 1 -  Carbon chemistry data from the AESOPS and related cruises submitted to the
           JGOFS Data Center at the Woods Hole Oceanographic Institution.
Cruise Designations       Dates              Number of Measurements
                                         Discrete Station Data   Underway Data___________
                                          (pCO2)sw     TCO2     (pCO2)sw  (pCO2)air  TCO2

R/V Nathaniel B. Palmer
Ross Sea Bio. Study     11/94-12/94         0            0        7,844       *        0

Site Survey             09/96-10/96         0          334        5,529       *        0
Mooring Deployment      11/96-12/96         0            0        3,215       *        0
Ross Sea, ROAVERRS      12/96-01/97         0            0        7,457       *        0
Ross Sea Process II     01/97-02/97       489          490        8,399       *        0
Ross Sea Process III    04/97-05/97         0          400**      5,014       *        0
Ross Sea Process IV     11/97-12/97       661          682       15,411       *        0
Ross Sea, ROAVERRS      12/97-01/98         0          800***    12,788       *        0
Benthic Process         02/98-04/98         0            0       22,474       *        0

R/V Roger Revelle
APFZ Survey II          01/98-02/98       115          115        9,161      505     195
APFZ Process II         02/98-04/98       574          575       10,584      620     120

* Atmospheric CO2 measurements were unreliable due to leakages in the atmospheric sampling system.
** Measured by the RSMAS Group.
*** Measured for the ROAVERRS program, but not yet reported to the JGOFS Data Center.

2-a) The LDEO Underway System for Surface Water pCO2 Measurements:
The system for underway measurements of pCO2 in surface waters consists of a) a water-air equilibrator, b) a non-dispersive infra-red CO2 gas analyzer and c) a data logging system. The measurement system is schematically shown in Fig. 1, and is similar with the one described in Bates et al. (1998). Each of these units and the data reduction procedures used will be described below.

The underway pCO2 system used for the measurements of 
pCO2 in surface  waters during the Southern Ocean JGOFS (AESOPS) Program.

2-b) Water-air Equilibrator:
The equilibrator has a total volume of about 30 liters and is equipped with a specially designed drain which maintains automatically the level of water in the equilibrator at a constant level at about half the height of the equilibrator leaving about 15 liters of headspace. Seawater from the ship's uncontaminated water line is continuously pumped into the equilibrator at a rate of about 10 liters/min, giving a mean residence time of water in the equilibrator of about 1.5 minutes. The headspace above the water serves as an equilibration chamber. A carrier gas (commonly marine air) is drawn into the chamber by a diaphragm pump, and exchanges CO2 with a continuous flow of seawater sprayed into the chamber through a shower head. Because of large gas-water contact areas created by fine water droplets as well as gas bubbles in the pool of water, CO2 equilibration between the carrier gas and seawater is achieved rapidly with a e-folding time of 2 to 3 minutes. Under normal operating conditions, the carrier gas in the equilibration chamber is pumped into the infra-red gas analyzer at a rate of about 50 ml/min. At this rate, the residence time of the carrier gas in the equilibration chamber is about 300 minutes, that is about 100 times as long as the equilibration time. Therefore, the carrier gas in the head space is always in equilibrium with water. The over all response time of the equilibrator system has been estimated to be of an order of several minutes. The large volume of water in the equilibrator is chosen in order to have a large thermal inertia of the equilibrator, so that the effects of room temperature changes on the equilibration temperature may be minimized. The temperature of water in the equilibrator is monitored continuously using a Guildline platinum resistance thermometer (readable to 0.05 deg.C) and recorded on the data logging computer. A calibrated mercury thermometer is also inserted in the equilibrator for testing the performance of the platinum thermometer.
At the gas intake end of the equilibrator, a flow indicator based on U-tube manometer is attached. This gives a visual confirmation for the fact that marine air is taken into the equilibration chamber at a desired flow rate. Since we operate the system with the equilibration chamber at the same pressure as the ambient room pressure, the total pressure, at which the gas was equilibrated, is measured using a precision electronic barometer (Setra Model 270, Action, MA) outside the equilibrator. This equilibration pressure is also logged on the computer.

The temperature and salinity of seawater at the in situ conditions were measured using a SeaBird Model SBE-21 thermosalinograph aboard the N. B. Palmer and a SIO/ODF thermosalinograph unit based on Neil Brown sensors aboard the R. Revelle. The precision of the report temperature data has been estimated to be about 0.005 deg. C.

2-c) Infra-red CO2 Gas Analyzer:
The equilibrated gas was passed through a water trap (to collect aerosols and condensates), mass flow controller and a reverse flow naphion dryer (PermaPure flushed with pure nitrogen gas) to remove water vapor (to a level of -20 deg. C), and was introduced into the IR sample cell at a rate of about 50 ml/min for CO2 determinations. A LI-COR infra-red gas analyzer (Model 6251, Lincoln, NB) was used. After about 3 minutes of purging period, the gas flow was stopped and readings were recorded on the computer. Although an electronic circuit was provided by the manufacturer in order to linearize the CO2 response, it exhibited a few inflexions that deviated from linearity by a few ppm. Therefore, we chose not to use the outputs from the linearization circuit supplied by the manufacturer. Instead, we used five standard gas mixtures (one pure nitrogen and four CO2-air mixtures) during the expeditions, and established response curves using the raw output from the analyzer. The CO2 concentrations in the gas mixtures were calibrated using the SIO standards determined by C.D. Keeling's group using the manometric method. The concentrations of CO2 in the standard gas mixtures are summarized in Table 2.


Table 2 - Concentrations of CO2 in the CO2-air gas mixtures using during the AESOP Expeditions,
          1996-1998. The values are in ppm mole fraction of CO2 in dry air, and have a precision
          of about +/- 0.1 ppm. Std. 1 is pure nitrogen gas and has a CO2 concentration of 0.00 ppm.

Ship/Cruise   Designation         Dates              CO2 concentrations (ppm)
                                                 Std. 2    Std. 3   Std. 4   Std. 5
Palmer-94/6   Ross Sea            Nov-Dec 94     149.22    249.15   360.12   450.92
Palmer-96/4   Site Survey         Aug-Sep 96     149.22    247.43   360.12   450.92
Palmer-96/5   Mooring Deploy.     Nov 96         149.22    247.43   360.12   450.92
Palmer-96/6   ROAVERRS            Dec 96-Jan 97  149.22    247.43   360.12   495.18
Palmer-97/1   Process II          Jan-Feb 97     149.22    247.43   360.12   495.18
Palmer-97/3   Process III         Apr-May 97     149.22    247.43   360.12   495.18
Palmer-97/8   Process IV          Nov-Dec 97     149.22    247.43   363.02   495.18
Palmer-97/9   ROAVERRS            Dec 97-Jan 98  149.22    247.43   363.02   495.18
Palmer-98/2   Benthic Proc.       Feb-Mar 98     109.95    236.39   363.02   495.18

Revelle-KIWI8 Survey II           Jan-Feb 98     150.19    252.00   353.77   448.56
Revelle-KIWI9 Process II          Feb-Mar 98     150.19    252.00   353.77   448.56

During normal operations, each of the standard gas mixtures was passed through the analyzer for 70 to 90 seconds at a rate of about 60 ml/min. This replaced the IR analyzer cell completely with the new gas. The flow was stopped for 5 seconds and then a millivolt reading from the analyzer was taken and recorded. Samples of equilibrated air and marine air were pumped through the analyzer for 180 seconds (3 minutes) at a rate of about 50 ml/min to purge the previous sample in the IR cell. The flow was stopped for 5 seconds and a reading for the analyzer output was recorded. This procedure was intended to eliminate errors due to fluctuations of the dynamic pressure within the IR cell by irregular gas flow rates. The slow flow rates used for samples were required for the removal of water vapor using the PermaPure membrane dryer. Between two sets of calibration runs using the five standard gases, 6 to 20 samples were analyzed depending upon the stability of the IR analyzer.

2-d) Data Logging System:
The following values were recorded on a laptop computer. The sample locations were derived from a GPS positioning unit that is a part of our surface water pCO2 system. The CO2 readings for samples were recorded once every 3 minutes (180 seconds), and those for the standard gas mixtures once every 1.5 minutes.

Time (GMT),
Sample ID (standard gas cylinder numbers, seawater CO2, atmospheric CO2) Barometric pressure in the laboratory (to 0.1 mb)
IR cell temperature,
Gas flow rate in the IR cell (to 0.1 ml/min),
Temperature of equilibration (to 0.01 deg.C),
Analyzer output (millivolts to 0.1 mv)
CO2 concentration in dry gas sample (preliminary based on the last response curve), and pCO2 (preliminary value based on the last response curve).

2-e) Data Deduction Procedures:
The concentration of CO2 in the sample was computed by the following way based on the millivolt reading and time of the reading. The millivolt reading taken for each of the five standard gases at the time of sample measurement was computed by linearly interpolating as a function of time using the readings taken before and after the respective standard gases were analyzed. This yields millivolt reading for each of the five standard gases at the time when the sample was analyzed. These five values were fit to a fourth-order polynomial equation (with five constants to be determined). This serves as the response curve. The CO2 concentration in the sample was computed using the response curve that was established at the time of each sample analysis. This method has been demonstrated to yield more reliable CO2 values compared with those computed, for example, using a least-squares fit of a quadratic or cubic functions to the five calibration points. The method described above yields atmospheric CO2 values that are consistent with those reported for the South Pole and the Cape Grim by the Climate Monitoring and Diagnostics Laboratory/NOAA in Boulder, CO.

The partial pressure of CO2 in seawater, (pCO2)sw, at the temperature of equilibration, Teq, in the unit of microatmospheres (uatm) was computed using the expression:

            (pCO2)sw @ Teq = (Vco2)eq  x (Pb - Pw) .............................[1]

(Vco2)eq  =  the mole fraction concentration (ppm) of CO2 in the dried equilibrated carrier gas;
      Pb  =  the barometric pressure (that is equal to the total pressure of equilibration)
               in atmospheres; and
      Pw  =  the equilibrium water vapor pressure at Teq ( deg.C) and salinity.

The water vapor pressure was computed using the following formulation;

     Pw (atm) = (1/760)x(1 - 5.368x10-4x Sal) x EXP{[0.0039476 - (1/TK)]/1.8752x10-4}...... [2]

where Sal is salinity in PSU measured using the ship's thermosalinograph, and TK is the
temperature of equilibration in deg.K.

     The (pCO2)sw at the in situ temperature, Tin-situ, was computed using a constant value
of 0.0423 % per deg.C for the effect of temperature (Takahashi et al., 1993):

       (pCO2)sw @ Tin-situ  = (pCO2)sw @ Teq x EXP[0.0423 x (Tin-situ - Teq)].
The value for Tin-situ is taken to be the seawater temperature measured by the ship's thermosalinograph at the time of pCO2 measurements. Teq is generally warmer than Tin-situ by 0.5 ~ 0.8 deg.C. Hence the temperature correction is normally less than 3% of pCO2 values.

The over all precision of the reported pCO2)sw values has been estimated to be about +1.5 uatm.


3-a) Measurements:
The air measurement system is shown schematically in Fig. 1. Uncontaminated marine air samples were collected about 10 m above the sea surface using a DEKORON tubing (1/4" i.d., Calco Inc., PA), a thin-wall aluminum tubing protected by plastic casing. The intake was located at the middle of the foremast about 10 m above the sea surface. A KNF Neuberger air pump that was located near the IR analyzer was used to pump air through the tubing and into the IR analyzer. Even when air samples were not analyzed, the pump was on all the time to keep the air flowing through the sampling line. For the analysis, the air sample was passed through a water trap and a drying column to remove water vapor (the same PermaPure column as used for the equilibrated gas) and introduced into the IR cell for CO2 analysis at a rate of about 50 ml/min. After 3 minutes of purging the cell, the flow was stopped for 5 seconds and the IR millivolt output reading was recorded.

3-b) Data Processing:
The partial pressure of CO2 in the air, (pCO2)air, was computed in the unit of microatmospheres (microatm) in the same way as that for seawater using Eq. [3] below:

             (pCO2)air  = (Vco2)air  x  (Pb - Pw) .............................[3]

(Vco2)air   =  the mole fraction concentration (ppm) of CO2 in the dried air sample; 
       Pb   =  the barometric pressure at sea surface in atmospheres; and 
       Pw   =  the equilibrium water vapor pressure at Tin-situ (deg. C) and salinity given
               by Eq. [2].

    The precision of the atmospheric pCO2 values have been estimated to be about 
+/- 1 uatm.


[1] Bates, N. R., Takahashi, T., Chipman, D. W. and Knapp, A. H. (1998). Variability of pCO2 on diel to seasonal time scales in the Sargasso Sea. Jour. Geophys. Res., 103, 15567-15585.

[2] CO2 Group, Lamont-Doherty Earth Observatory. (1999) "pCO2 Equilibrator Users Manual", LDEO of Columbia University, Palisade, NY, pp.10.

[3] Takahashi, T., Olafsson, J., Goddard, J., Chipman, D. W. and Sutherland, S. C., (1993). Seasonal variation of CO2 and nutrients in the high-latitude surface oceans: A comparative study. Global Biogeochemical Cycles, 7, 843-878.