Discrete pCO2
Measurements
The discrete
measurements of pCO2 were performed by the LDEO group on three of
four sections of the
An automated equilibrator-IR gas analyzer system
was used during the expedition for the determination of partial pressure of CO2
in the seawater samples. Its design is similar to that described by Chipman, Marra, and Takahashi
(1993) with the exception that the gas chromatograph was replaced with an IR
gas analyzer. The equilibrator-IR system is shown schematically in Fig. 4.
The system consists of a circulation pump
plumbed to recirculate air in a closed system through
porous plastic gas dispersers immersed in a 250-mL seawater sample. The
seawater sample is contained in a 250-mL Pyrex reagent bottle with a standard
taper-ground glass stopper that serves as an equilibration vessel. A Pyrex
extension tube (~20 mL), which has a standard
taper-ground glass male-joint to form an airtight seal with the reagent bottle,
is connected to the mouth of the reagent bottle to provide an extra headspace
to prevent seawater from entering the gas circulation line. Four sets of flasks
and circulation pumps are used so that four water samples can be processed
concurrently. Because the partial pressure of CO2 is sensitive to
temperature, the equilibration flasks are kept immersed in a water bath
maintained at 20C. The temperature at which the water sample is equilibrated
with circulating gas is measured with a precision of 0.01C and is recorded.
An electrically driven Valco
10-port valve (the equilibrator selection valve in Fig.
4) is used to isolate each of the equilibrators during the initial
equilibration. Manually operated 2-way and 3-way Whitey valves allow the
headspace in each equilibrator to be filled with a calibration gas of known CO2
concentration, creating a known initial condition for the headspace (about 40 mL) before equilibration. The equilibrator is open to the
laboratory air through isolation coils attached to the low-pressure side of the
equilibrator, keeping the total pressure of equilibration the same as the
ambient atmospheric pressure. The atmospheric pressure is measured with a
high-precision electronic barometer with an accuracy of better than 0.05% and
is recorded. It takes about 20 minutes for each water sample to be thermally
equilibrated with the constant-temperature water bath, and the headspace gas is
recirculated through the water sample throughout the
period to ensure CO2 equilibration.
An electrically driven Valco
6-port valve (the sample selection valve in Fig. 4)
is connected to the equilibrator selection valve and to the calibration gas
selection valve. This allows selection of the gas sample to be analyzed for CO2:
the equilibrated sample gas or one of the four calibration gases. A 2-way
normally-closed Skinner solenoid valve on the output of the calibration gas
selection valve controls the flow of the calibration gases to the sample selection valve. It also
provides a necessary second means of stopping the flow of the calibration gases
to prevent their accidental loss in case of a control malfunction. The concentration of CO2 in the
gas equilibrated with the seawater sample is determined using an IR gas
analyzer (LICOR Model 6125) in a flow-through mode. A 0.5-mL aliquot of
equilibrated headspace gas, representing less than 1% of the circulating gas,
is isolated using a gas pipette (attached to the sampling valve in Fig. 4) and swept with CO2-free air (or
pure nitrogen gas) flowing at a constant rate of about 50 mL/min. For low-pCO2 samples, a 1-mL gas pipette
(attached to the sampling valve) is used. The sample gas is passed through a
permeation drying tube for the removal of water vapor and injected into the IR
gas analyzer cell (about 7 mL in volume) filled previously
with CO2-free air. The displaced CO2-free air is
discharged out of the cell into the laboratory. The small volume of the gas
sample ensures that all of the CO2 from the gas pipette is found in
the analyzer cell at the same time, so that the peak height is proportional to
the amount of CO2 present in the gas pipette. Drying of the sample
gas avoids the effects of pressure-broadening of the CO2 absorption
spectra and of dilution caused by water vapor. The amount of CO2 in
the sampling pipette is a function of the loop volume, temperature, and
pressure. The temperature is held constant and measured, and the pressure of
the sample gas is same as the barometric pressure, which is measured with an
accuracy of better than 0.05%. The peak height, which represents the number of
moles of CO2 in the sample gas, is calibrated every 1.5 hours using
a quadratic equation fitted to three calibration gas mixtures (366.52, 788.8
and 1211.4 ppm mole fraction in dry air).
The analytical procedure begins with water samples
being drawn from the 10-L Niskin bottles off a
rosette directly into 250-mL Pyrex reagent bottles. These served as both sample
containers and equilibration vessels. The samples were immediately inoculated
with 100 µL of 50% saturated mercuric chloride
solution, sealed airtight with ground glass stoppers to prevent biological
modification of the pCO2, and stored in the dark until analysis.
Measurements were normally performed within 24 hours of sampling. A headspace of
3 to 5 mL was left above the water to allow for
thermal expansion during storage. Prior to analysis, the sample flasks were
brought to the water bath temperature of 20°C in the constant-temperature bath.
The equilibrator headspace, including the extension tube and the gas
circulation tubings, was filled with a calibration
gas of known CO2 concentration. The gas in the equilibrators, and in
the tubing that connects them to the gas pipette loop, was recirculated
continuously for about 20 minutes through a gas disperser immersed in the
water. This provided a large surface area for gas exchange between the sample
water and circulating gas, and equilibrium for CO2 was attained in
15 min. The temperature of the bath water was assumed to be that of the
sample water and was measured at the time of equilibration with a precision of
0.01°C using a thermometer calibrated against a NIST-certified thermometer.
This temperature is reported in the data tables as TEMP_PCO2 and showed no
variation at a limit of 0.01°C.
The equilibrated air samples were saturated with
water vapor at the temperature of equilibration and had the same pCO2
as the water. By injecting the air aliquot into the IR analyzer after the water
vapor was removed, the concentration of CO2 was measured. Therefore,
the effect of water vapor must be taken into consideration for computing pCO2
as follows:
pCO2 (atm) = [Cmeas
(ppm)] x [total press. of equilibration (atm) - water vapor press. (atm)]
where Cmeas is the
mole fraction concentration of CO2 in dried equilibrated air. The
total pressure of equilibrated air is measured by having the headspace in the
equilibrator flask always at atmospheric pressure. The latter was measured with
an electronic barometer at the time each equilibrated air sample was injected
into the IR analyzer for CO2 determination. The water vapor pressure
was computed at the equilibration temperature, and salinity of the seawater. Cmeas was determined by using a quadratic
equation fit to three of the calibration gas mixtures.
The concentrations for standard gases used are
traceable to the WMO reference scale through analysis in the laboratories of C.
D. Keeling of SIO (
Corrections were made to account for the change
in pCO2 of the sample water due to the transfer of CO2
between the water and circulating air during equilibration. We know the pCO2
in equilibrated, perturbed water and the TCO2 by coulometry
before the equilibration. We can also calculate the change in TCO2
in the water based on the change in pCO2 between the
post-equilibrium value and the known concentration in the pre-equilibrium
condition. With the pre-equilibrium TCO2 plus the perturbation in
TCO2 during equilibration, the post-equilibrium TCO2
value was obtained. Using the post-equilibrium TCO2 and measured pCO2
values, TALK at the end of the equilibration was calculated, using the
temperature, salinity, phosphate, and silicate data. Since the perturbation
does NOT change the TALK, the pre-equilibrium pCO2 from the
pre-equilibrium TCO2, the calculated TALK, and the temperature,
salinity, etc., were calculated. This is the value that was reported as pCO2,
the pre-equilibrium calculated value. The magnitude of this correction is
generally less than 2 atm. Details of the
computational scheme are presented in a DOE technical report by Takahashi, et
al. (1998).
The pCO2 values reported in this data
set are expressed as micro-atmospheres at the temperature of equilibration. The
precision of the pCO2 measurement for a single hydrographic station
was estimated to be about 0.15% based on the reproducibility of replicate
equilibrations. The station-to-station reproducibility was estimated to be
about 0.5%.