The purpose of the Pacific Ocean fCO₂ crossover analysis was to determine if any significant systematic offset existed between the various legs of the WOCE/NOAA/JGOFS Pacific Ocean measurements of CO₂ fugacity. Three different types of instruments were used to measure discrete fCO₂ samples. With each an aliquot of seawater was equilibrated at a constant temperature of either 4° or 20°C with a headspace of known initial CO₂ content. Subsequently, the CO₂ concentration in the headspace was determined by nondispersive infrared analyzer (NDIR) or by quantitatively converting the CO₂ to CH₄ and then analyzing the resulting gas composition using a gas chromatograph (GC) with flame ionization detector. The initial fCO₂ in the water was determined after correcting for loss or gain of CO₂ during the equilibration process. This correction can be significant for large initial fCO₂ differences between headspace and water, and for systems with a large headspace to water volume ratio (Chen et al. 1995).

The system used by Takahashi (Chipman et al. 1993; DOE 1994) involved equilibration of a ~50-mL headspace with a ~500-mL sample at either 4°C (T4 = Takahashi @ 4°C) or 20°C (T20 = Takahashi @ 20°C) depending on ambient surface water temperatures. Note that the Takahashi values, reported as partial pressure of CO₂ (pCO₂), were converted to fCO₂ using the correction factor (~ 0.997) given by Weiss (1974). Wanninkhof and co-workers utilized two systems during the Pacific survey cruises. An NDIR-based system (WI20 = Wanninkhof IR @ 20°C) with ~500-mL samples was used for analyses during EQS92 and P18 (Wanninkhof and Thoning 1993). A GC-based system (WG20 = Wanninkhof GC @ 20°C) with samples collected in a closed, septum-sealed bottle having a volume of ~120 mL of seawater and a headspace of ~10 mL was used for P14S15S (Neill et al. 1997).

Detectors were calibrated after every 4 to 12 samples with gas standards traceable to manometrically determined values of C. D. Keeling at SIO.  Assessment of fCO₂ accuracy is difficult to determine because of the lack of aqueous standards. Estimates of precision based on duplicate samples range from 0.1-1% depending on fCO₂ and measurement procedure, with higher fCO₂ levels on the WI20 system (>700 µatm), giving worse reproducibility (Chen et al. 1995).

The stations selected for each crossover were those with carbon data which were close to the crossover point. The number of stations selected was somewhat subjective, but was such that to provide sufficient measurements for the analysis without getting too far away from the crossover location. In all cases the stations were within approximately 1° of latitude or longitude of the crossover point. All potential crossovers, including crossovers where measured values could be compared to fCO₂ values calculated from TCO₂/TALK or TCO₂/pH pairs, were examined. For the crossover comparison all samples run at 4°C were normalized to 20°C by calculating the alkalinity (TALK) from fCO₂ (4°C) and TCO₂, and subsequently calculating fCO₂ (20°C) from the TCO₂and calculated TALK.  The carbonate dissociation constants of Mehrbach et al. (1973) as refit by Dickson and Millero (1987) and ancillary constants listed in DOE (1994) are used for these calculations using the program of Lewis and Wallace (1998). Crossover information is given in the table below.

Click on the last column to see profiles
 
 

Analysis of the calculated fCO₂ values revealed that there may be some problems due to uncertainties as to which carbon dissociation constants to use. This is also a problem for the crossovers which required a temperature conversion. For example, the temperature conversion from 4° to 20°C using the Mehrbach constants yield fCO₂ values in the deep Pacific that are about 50 µatm higher than if the temperature conversion is performed with the Roy constants.  Since the discrepancy in dissociation constants has not been fully resolved, the crossover comparison for fCO₂ data analyzed at different temperatures and for comparisons of measured vs calculated values is problematic.

The crossovers involving calculated values were not considered for the final crossover analysis. Data from deep water (>2000 m) at each of the 15 remaining crossover locations were plotted against the density anomaly referenced to 3000 dB (σ-3) and fit with a second-order polynomial. The difference and standard deviation between the two curves waere then calculated from 10 evenly spaced intervals over the density range common to both sets of crossovers.
 
 

Crossover no.	Cruise 1a	Cruise 2	Density Rangeb	fCO2 Rangec (µatm)	Averaged	Std. Dev.e	Commentsf
34	P14S15S (WG20)	S4P (T4)	41.14-41.49	1090-1110	3.4	2.2	concave/convex
40a	P14S15S (WG20)	EQS92 (WI20)	41.46-41.56	1050-1270	22	4.8	EQS92: 5 points
40i	P14S15S (WG20)	EQS92 (WI20)	41.35-41.52	1080-1320	35	3.3	EQS92: 4 points
41b	P14S15S (WG20)	EQS92 (WI20)	41.45-41.59	1030-1180	29.2	2.9	EQS92: 4 points
44	P14S15S, 94 (WG20)	P14S15S, 93 (WG20)	41.50-41.60	1070-1100	-1	5.4
45	S4P (T4)	P14S15S (WG20)	41.50-41.67	1095-1130	-12	3.5
55	P16S17S (T20)	P16A17A (T20)	41.42-41.59	1050-1180	-5.3	0.9
66b	P16A17A (T20)	P16S17S (T20)	41.40-41.54	1080-1180	1	3.8
67	P17E19S (T4)	P16A17A (T4)	41.23-41.52	1050-1190	-2.4	4.3
68	S4P (T4)	P17E19S (T4)	41.46-41.69	1090-1115	-2.5	0.3
73	P18 (WI20)	EQS92 (WI20)	41.14-41.49	1170-1570	3.4	2.2	EQS92: 6 points
77	P17E19S	P18 (WI20)	41.26-41.61	1050-1200	21.2	0.4
78	S4P (T4)	P18 (WI20)	41.48-41.68	1070-1095	7.6	0.5
82	P19 (T4)	P17E19S (T4)	41.21-41.64	1080-1220	13.6	3.8
83	S4P (T4)	P17E19S (T4)	41.43-41.67	1080-1130	-15	1.4

^acruise designation and system used in brackets
^bdensity range (σ-3) over which the fit was performed
^cfCO₂ range over which fit was performed
^daverage difference between 2nd order polynomial fits of data for Cruise1 and Cruise 2
^estandard deviation between second-order polynomial fits
^fconcave/convex = curve shape for Cruise 1 is concave while for Cruise 2 it is convex. The EQS92 cruises had few samples taken within depth range. Other cruises had more than 10 points over appropriate density range.
 

The standard deviation for the 15 fCO₂ crossover comparisons was 16.0 µatm. The average of the absolute value of the differences was 10.3±13.7 µatm. Notable offsets were observed for crossovers 82 and 83, with P19 showing a positive offset and S4P showing a negative offset relative to P17E19S. These two crossovers are both in the southern Pacific Ocean within 15° of each other.  If this is systematic throughout the cruises, it would imply that the fCO₂ for S4P and P19 differ by about 30 µatm, which is roughly comparable to an offset of ~4-5 µmol/kg in TCO₂ or TALK. The largest offsets (35 µatm) were observed for EQS92. We suspect that the large offset observed on EQS92 is caused by a bias in the analytical system used during this cruise although biases in the other crossovers involving the infrared (IR) system at 20°C (WI20) were less pronounced.  Crossover 73 shows excellent agreement where both cruises used the WI20 technique. The large head space-to-water volume of the IR system may be the cause of the error. When fCO₂ data obtained using the different types of instruments are compared with the calculated fCO₂ values using TALK and TCO₂, a bias between the IR and small-volume GC systems becomes apparent. The GC-based system (WG20) yielded significantly higher fCO₂ values than calculated values using the recommended constants, while the IR based system did not show a clear trend, but rather increased scatter with increased fCO₂.

Summary

Based on careful laboratory studies, it appears that the IR-based measurements may give low results at fCO₂ values >700 µatm. The deep water data with WI20 are low by about 20-30 µatm in the range of 1000-1100 µatm. This result is in accordance with the recent findings of Lee et al. (2000). As suggested by Lee and co-workers, the trend in the calculated values of fCO₂ from TALK and TCO₂ most likely results from a thermodynamic inconsistency with the Merbach et al. (1973) constants. Until this has been resolved, there is insufficient information to warrant further analysis of the fCO₂ data. For a summary table of analytical techniques, PIs, sample volumes, and shorebased analysis for fCO₂ Click Here.

Chen, H., R. Wanninkhof, R. A. Feely and D. Greeley (1995). Measurement of fugacity of carbon dioxide in sub-surface water: an evaluation of a method based on infrared analysis, NOAA technical report ERL AOML-85, 52 pp. NOAA/AOML.

Chipman, D. W., J. Marra and T. Takahashi (1993) Primary production at 47N and 20W in the North Atlantic Ocean: A comparison between the ¹⁴C incubation method and mixed layer carbon budget observations. Deep-Sea Res. II 40: 151-169.

Dickson, A.G., and F.J. Millero (1987) A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Res., 34, 17331743.

DOE (1994) Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water; version 2, A.G. Dickson and C. Goyet, eds. ORNL/CDIAC-74 Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tenn. Read here

Lee, K., F. J. Millero, R. H. Byrne, R. A. Feely and R. Wanninkhof (2000) The Recommended Dissociation Constants of Carbonic Acid for Use in Seawater. Geophys. Res. Lett. 27: 229-232.

Lewis, E. and D. W. R. Wallace (1998). Program developed for CO₂ system calculations. Oak Ridge, Oak Ridge National Laboratory. Read here

Merhbach, C., C.H. Culberson, J.E. Hawley, and R.M. Pytkowicz (1973): Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnol. Oceanogr., 18, 897907.

Neill, C,  K.M. Johnson, E. Lewis, and DWR Wallace (1997). Accurate headspace analysis of fCO₂ in discrete water samples using batch equilibration. Limnol. Oceanogr. 42(8), 1774-1783.

Wanninkhof, R. and Thoning, K. (1993) Measurement of fugacity of CO₂ in surface water using continuous and discrete sampling methods.  Mar. Chem., 44, 189-204.

Weiss, R. F. (1974) Carbon dioxide in water and seawater: the solubility of a non-ideal gas. Mar. Chem. 2: 203-215.