Total CO₂ Measurements

 

As on previous cruises, TCO₂ was determined using automated dynamic headspace sample processors (SOMMA) with coulometric detection of the CO₂ extracted from acidified samples. A description of the SOMMA-Coulometry System and its calibration can be found in Johnson et al. (1987), Johnson and Wallace (1992), and Johnson et al. (1993). A schematic diagram of the SOMMA analytical sequence may be found in earlier cruise reports (see Johnson et al. 1995,1996), and further details concerning the coulometric titration can be found in Huffman (1977) and Johnson, King, and Sieburth (1985). The methods used for discrete TCO₂ on WOCE sections have been extensively dealt with in previous reports (Johnson et al. 1998a) and need only be briefly summarized.

The AR24 section required modification of the usual sampling procedures. As noted in Section 3.1.2, 4-L sampling bottles were employed on the rosette, limiting the amount of sample available for the carbonate system analysts to one 500-mL bottle. Hence, the TCO₂ coulometric titration analysis had to be completed before the partially empty 500-mL bottle was passed to the TALK group for the potentiometric alkalinity titration. There was enough sample to complete both measurements, but not enough time or sample for TCO₂ replicate analyses from the same 500-mL sample bottle. The 4-L sampling bottles also made it impossible to draw duplicate samples from the same sampling bottle. Without duplicate samples from the hydrographic stations, standard measures of sample precision (DOE 1994; Johnson et al. 1998b) could not be completed on the AR24 section. Samples were poisoned with 100 L of a 50% solution of HgCl₂ and analyzed for TCO₂ within 24 hours of collection (DOE 1994).

For sections A24, A20, A22, single or duplicate samples were collected in 300-mL biological oxygen demand (BOD) bottles, poisoned with 100 L of a 50% solution of HgCl₂, and analyzed for TCO₂ within 24 hours of collection, according to standard operating procedures (DOE 1994). The samples were stored in a dark refrigerator at 4-6°C until approximately 1-2 hours before analysis, when they were removed and placed in a temperature bath at 18-20°C and thermally equilibrated. The SOMMA sample pipette and sample bath were also kept at approximately 20°C. Duplicate samples were usually collected on each cast at the surface and from the bottom waters. For some casts, three sets of duplicates were taken. The duplicates were analyzed within the run of cast samples from which they originated so that the time elapsed between duplicate analyses was 3-12 hours. As per standard operating procedure (DOE 1994), CRM was routinely analyzed according to DOE (1994) guidelines. The CRM was supplied by Dr. Andrew Dickson of the SIO, and for the North Atlantic cruises, batches 33, 36, and 37 were used. The certified values for these batches were TCO₂ = 2009.85 µmol/kg @ salinity = 33.781 for batch 33; TCO₂ = 2050.21 µmol/kg @ salinity = 35.368 for batch 36; and TCO₂ = 2044.15 µmol/kg @ salinity = 34.983 for batch 37. The CRM TCO₂ concentration was determined by vacuum-extraction/manometry in the laboratory of C. D. Keeling at SIO.

An accurately known volume of seawater was injected from an automated to-deliver (TD) pipette into a stripping chamber. Following acidification, the resultant CO₂ from continuous gas extraction was dried and coulometrically titrated on a model 5011 UIC coulometer with a maximum titration current of 50 mA in the counts mode (the number of pulses or counts generated by the coulometers VFC during the titration was displayed). In the coulometer cell, the acid (hydroxyethylcarbamic acid) formed from the reaction of CO₂ and ethanolamine is titrated coulometrically (electrolytic generation of OH^-) with photometric endpoint detection. The product of the time and the current passed through the cell during the titration (charge in coulombs) is related by Faradays constant to the number of moles of OH^- generated and thus to the moles of CO₂ that reacted with ethanolamine to form the acid. The age of each titration cell is logged from its birth (time that electrical current is applied to the cell) until its death (time when the current is turned off). The age is measured in minutes from birth (chronological age) and in mgC titrated since birth (carbon age).

Each system was controlled with an IBM-compatible PC equipped with two RS232 serial ports (coulometer and barometer), a 24-line digital input/output card (solid state relays and valves), and an analog-to-digital card (temperature, conductivity, and pressure sensors). Real Time Devices (located in State College, PA 16803) manufactured the cards. The SOMMA temperature sensors (model LM34CH, National Semiconductor, Santa Clara, CA) with a voltage output of 10 mV/F were calibrated against thermistors certified to 0.02°F prior to the cruise using a certified mercury thermometer. These sensors monitored the temperature of SOMMA components, including the pipette, gas sample loops, and coulometer cell. The SOMMA software was written in GWBASIC Version 3.20 (Microsoft Corp., Redmond, WA), and the instruments were driven from an options menu appearing on the PC monitor. With the coulometers operated in the counts mode, conversions and calculations were made using the SOMMA software rather than the programs and the constants hardwired into the coulometer circuitry.

The SOMMA-coulometry systems were calibrated with pure CO₂ (calibration gas) using hardware consisting of an 8-port gas sampling valve (GSV) with two sample loops of known volume [determined gravimetrically by the method of Wilke, Wallace, and Johnson (1993)] connected to the calibration gas through an isolation valve; the vent side of the GSV was plumbed to a barometer. When a gas loop was filled with CO₂ at known temperature and pressure, the mass (moles) of CO₂ contained therein was calculated, and the ratio of the calculated mass to that determined coulometrically was the calibration factor (CALFAC); the CALFAC was used to correct the subsequent sample titrations for small departures from 100% recoveries (DOE 1994). The standard operating procedure was to make gas calibrations daily for each newly prepared titration cell [normally, one cell per day and three sequential calibrations per cell at a carbon age of 39 mgC (mean age @ 6 mgC), with the result of the third calibration taken as the CALFAC if it was consistent with the second (i.e., agreement to ± 0.1% or better)]. Daily gas calibrations were made on both systems throughout the cruises.

The "to-deliver" volume (V_cal) of the sample pipettes was determined (calibrated) gravimetrically prior to the cruise to ± 0.02% or better in October of 1996. The calibration was checked periodically during all cruises by collecting aliquots of deionized water dispensed from the pipette into pre-weighed serum bottles. The serum bottles were crimp-sealed and weighed immediately during the on-shore laboratory calibrations, or returned to shore where they were reweighed on a model R300S balance (Sartorius, Gttingen, Germany) as soon as possible. The apparent weight (g) of water collected (W_air) was corrected to the mass in vacuum (M_vac) with the to-deliver volume being M_vac divided by the density of the calibration fluid at the calibration temperature. After the AR24 section in 1996, the system pipettes were dismounted and replaced with chemically cleaned pipettes in March, 1997. For the 1997 sections, the calibration volumes (V_cal) at the calibration temperature (t_cal) of the sample pipettes were redetermined to ± 0.01% from a set of calibration samples taken on July 3, 1997, on board the Knorr at the completion of section A24 and were weighed on September 17. The TCO₂ pipette volumes for the four North Atlantic sections are summarized in Table 2.

 

 

The sample volume (V_t) at the pipette temperature was calculated from the expression:

 

V_t = V_cal [1 + a_v (t - t_cal)]

 

where a_v is the coefficient of volumetric expansion for pyrex-type glass (1 x�10^-5/°C), and t is the temperature of the pipette at the time of a measurement. The mean pipette temperature on the AR24 section in 1996 was 20.32 ± 0.51°C (n = 948), and on the 1997 North Atlantic Sections it was 19.55  0.52°C (n = 4666).

The factory-calibrated coulometers were electronically calibrated independently in the laboratory before the cruise as described in Johnson et al. (1993, 1996) and DOE (1994), and the terms INT_ec and SLOPE_ec were obtained and entered into the software for each system. The micromoles of carbon titrated (M), whether extracted from water samples or the gas loops, was

 

M = [Counts / 4824.45 - (Blank �xT_t ) - (INT_ec � x T_i)] / SLOPE_ec

 

where 4824.45 (counts/µmol) is a scaling factor obtained from the factory calibration; T_t is the length of the titration in minutes; Blank is the system blank in mol/min; INT_ec is the intercept from electronic calibration in mol/min; T_i is the time in minutes during the titration where current flow was continuous; and SLOPE_ec is the slope from electronic calibration. Note that the slope obtained from the electronic calibration procedure applied for the entire length of the titration, but the intercept correction applied only for the period of continuous current flow (usually 34 min) because the intercept can be calculated only from calibrated levels of current flowing continuously.

Unfortunately, the coulometer system 030, which was electronically calibrated prior to the AR24 cruise and again in March 1997, had to be replaced at the start of section A24 in May 1997. However, the replacement coulometer (S/N CBE-9010-V) was calibrated at the factory on March 20, 1997. Hence we assumed that the replacement coulometer was properly calibrated, and we entered the default calibration coefficients into the software (SLOPE_ec = 1.0 and INT_ec = 0.0). The system 004 was also recalibrated in March 1997 following the AR24 cruise with nearly identical results to those obtained in October 1996, and it was not recalibrated during the 1997 WOCE sections. The electronic calibration coefficients, along with the mean gas calibration factors determined for the North Atlantic section discrete TCO₂ coulometers, are given in Table 3.

Table 3 illustrates an advantage of the independent laboratory electronic calibration procedure. The mean CALFAC for systems 004 and 030 using the laboratory-determined electronic calibration coefficients was approximately 1.0036 (or 99.64% recovery of the theoretical mass of CO₂ calibration gas measured coulometrically) vs 1.0053 (99.47% recovery) for the factory-calibrated coulometer. Hence, a small percentage (0.17%) of the less than 100% recovery for known masses of CO₂ coulometrically

 

 

 

titrated can be explained by a factory-calibration procedure that is apparently slightly less accurate than the laboratory calibration. This difference has been consistent throughout the CO₂ survey.

For water samples, the discrete TCO₂ concentration in µmol/kg was calculated from

 

TCO₂ = M x�CALFAC x�[1 / (V_t x �p)] x�d_Hg

where p is the density of sea water in g/mL at the measurement temperature and sample salinity calculated from the equation of state given by Millero and Poisson (1981), and d_Hg is the correction for sample dilution with bichloride solution (for the AR24 section in 1996 dHg = 1.0002 and for the 1997 sections d_Hg = 1.000333 ).

One of the SOMMA-Coulometry Systems (S/N 004) was equipped with a conductance cell (Model SBE-4, Sea-Bird Electronics, Inc., Bellevue, WA) for the determination of salinity measurement as described by Johnson et al. (1993). Whenever possible SOMMA and CTD salinity were compared to identify mistrips or other anomalies, but the bottle salinity (furnished by the chief scientist) was used to calculate TCO₂.

Quality control-quality assurance (QC-QA) was assessed from the results of the 275 CRM analyses made using systems 004 and 030 during the four North Atlantic sections. These data are summarized in Table 4, and the temporal distribution of the differences is plotted in Fig. 2 for section AR24 (1996) and in Fig. 3 for sections A24, A20, and A24 (1997).

 

 

Table 4. The mean analytical difference (ΔTCO2 = measured-certified) and the standard deviation of the differences between measured and certified TCO2 on WOCE Sections AR24, A24, A20, and A22
Section	System S/N	Δ TCO2 (µmol/kg)	St. dev. (µmol/kg)	n
AR24	004	1.42	2.10	16
AR24	030	1.54	1.88	49
Mean/total		1.51	1.92	65
A24	004	0.04	1.10	49
A20	004	0.23	1.20	42
A22	004	0.06	0.69	17
Mean/total		0.10	1.08	108
A24	030	0.79	1.00	48
A20	030	0.44	1.43	35
A22	030	0.26	1.22	19
Mean/total		0.57	1.21	102
Overall mean/total		0.61	1.47	275