Pacific fCO2 Crossovers

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).

Initial Crossover Analysis

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

Crossing # old # Latitude Longitude Cruise 1 Cruise 1 Sta Cruise 2 Cruise 2 Sta fCO2
34 1 66°S 171°E P14S15S 32 S4P 783,787 MvsM
40a 16 0 170°W P14S15S 174 EQS92 56 MvsM
40b 14 0 170°W P14S15S 174 P15N 112 MvsC
40c   0 170°W P15N 112 EQS92 56 CvsM
40d 42 1°S 170°W P14S15S 173 P15N 114 MvsC
40e 43 2°S 170°W P14S15S 172 P15N 116 MvsC
40f 44 3°S 170°W P14S15S 171 P15N 118 MvsC
40h 45 4°S 170°W P14S15S 170 P15N 120 MvsC
40i 13 5°S 170°W P14S15S 169 EQS92 63 MvsM
40j 11 5°S 170°W P14S15S 169 P15N 122 MvsC
40k   5°S 170°W P15N 122 EQS92 63 CvsM
40l 46 6°S 170°W P14S15S 167 P15N 124 MvsC
40m 47 7°S 170°W P14S15S 165 P15N 126 MvsC
40n 48 8°S 170°W P14S15S 163 P15N 128 MvsC
40o 50 12°S 170°W P14S15S 155 P15N 134,136 MvsC
41a 9 10°S 170°W P14S15S 157,159,161 P15N 130,132 MvsC
41b 8 10°S 170°W P14S15S 157,159,161 EQS92 66 MvsM
41c 10 10°S 170°W P14S15S 157,159,161 P31 54,57,61 MvsC
41d   10°S 170°W P15N 130,132 EQS92 66 CvsM
41e   10°S 170°W EQS92 66 P31 54,57,61 MvsC
42 6 17°S 170°W P14S15S 141,142,144 P21 193,195,197 MvsC
44   40°S 173°W P14S15S/1 93 P14S15S/2 94 MvsM
45 4 67°S 169°W P14S15S 33 S4P 755 MvsM
53a 52 17°S 150°W P16C 222 P16S17S 220 CvsM
53d 54 17°S 150°W P16S17S 220 P31 2,5 MvsC
53e 53 17°S 150°W P16S17S 220 P21 157,160 MvsC
55 18 37°S 150°W P16S17S 180 P16A17A 3 MvsM
64 26 6°S 135°W P17C 121 P16S17S 124 CvsM
65 25 16°S 133°W P16S17S 148 P21 131 MvsC
66b 57 33°S 135°W P16S17S 179 P16A17A 119 MvsM
67 23 53°S 135°W P16A17A 77 P17E19S 128 MvsM
68 29 66°S 126°W P17E19S 163 S4P 723,727 MvsM
73 32 5°N 110°W P18 155,159 EQS92 6 MvsM
74 36 17°S 103°W P18 105,106 P21 77 MvsC
77   52°S 103°W P18 37 P17E19S 194 MvsM
78 34 67°S 103°W P18 10,11 S4P 711,712,713 MvsM
80 40 16°S 86°W P19 333 P21 49 MvsC
82 39 53°S 88°W P19 256 P17E19S 206 MvsM
83 37 67°S 88°W S4P 703 P17E19S 229 MvsM

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.

Final Crossover Analysis

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.

Cruise 1a Cruise 2 Density Rangeb fCO2 Rangec
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.



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. Here is a summary table of analytical techniques, PIs, sample volumes, and shorebased analysis for fCO2.


  • 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.
Last modified: 2021-03-17T18:30:24Z