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)
1. INTRODUCTION
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.
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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
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* 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. MEASUREMENTS OF pCO2 IN SURFACE WATERS
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.
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
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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.
Date,
Time (GMT),
Latitude,
Longitude,
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. MEASUREMENTS OF pCO2 IN THE ATMOSPHERE
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.
4. REFERENCES CITED
[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.