**OVIDE 2002**

**Contract number: 01/ 2 210 557**

**MESURES DE pH ET D'ALCALINITÉ LORS DE LA CAMPAGNE OVIDE 2002.**

**Fiz F. Pérez**

**Marta Álvarez**

Instituto de Investigaciones Marinas, (CSIC)

C/ Eduardo Cabello, Nº 6, 36208 VIGO.

**FINAL SCIENTIFIC REPORT**

**CO~2~ parameters report.**

## Fiz F. Pérez and Marta Álvarez

## Instituto de Investigaciones Marinas, CSIC, C/Eduardo Cabello Nº 6, 36208, Vigo, Spain.

The carbon system is defined by four variables: pH, Total Alkalinity
(TA), partial pressure of carbon dioxide (pCO~2~) and Total Inorganic
Carbon (C~T~). The knowledge of two of these variables allows
calculating the other two by means of a set of equations deduced from
thermodynamic equilibrium. During the OVIDE 2002 cruise carried out
between 11^th^ June and 11^th^ July on board R/V THALASSA, pH and TA
measurements were sampled from bottle depths at selected stations
(Table 1) and analysed on board. Moreover, pCO~2~ has been continuously
determined in surface waters along the vessel track.

**a) pH analysis.**

pH was measured spectrophotometrically following Clayton and Byrne
(1993). Roughly, this method consists on adding a dye solution to the
seawater sample, so that the ratio between two absorbances at two
different wavelengths is proportional to the sample pH.

[Sampling and analytical methods.]{.underline}

Seawater samples for pH were collected after oxygen samples from depth
using cylindrical optical glass 10-cm pathlength cells, which were
filled to overflowing and immediately stoppered.

Seawater pH was measured using a double-wavelength spectrophotometric
procedure (Byrne, 1987). The indicator was a solution of m-cresol purple
prepared in seawater.

After sampling all the samples were estabilised at 25C. All the
absorbance measurements were obtained in the thermostatted (250.2 ºC)
cell compartment of a CECIL 3041 spectrophotometer.

After blanking with the sampled seawater without dye, 75 l of the dye
solution were added to each sample using an adjustable repeater pipette.
The absorbance was measured at three different fixed wavelenghts (434,
578 and 730 nm), pH, on the total hydrogen ion concentration scale, is
calculated using the following formula (Clayton and Byrne, 1993):

pH~T~=1245.69/T + 3.8275 +(2.11.10^-3^)(35-S) +
log((R-0.0069)/(2.222-R\*0.133))

where R is the ratio of the absorbances of the acidic and basic forms of
the indicator corrected for baseline absorbance at 730 nm
(R=A~578~/A~434~), T is temperature in Kelvin scale and S is salinity.

DelValls and Dickson (1998) in a revision of the pH values initially
assigned to the 'tris' buffers used to characterise the indicator, have
suggested an increase of 0.0047, which translate into a comparable
increase in the pH~T~ values finally calculated.

***Table 1.*** Number of samples taken in each station for pH and Total
Alkalinity (TA) analysis.

  --------- -------- -------- -- --------- -------- -------- -- --------- -------- -------- -- --------- -------- -------- -- --------- ---------- ---------
  **St.**   **pH**   **TA**      **St.**   **pH**   **TA**      **St.**   **pH**   **TA**      **St.**   **pH**   **TA**      **St.**   **pH**     **TA**

  **0**     28       28          **21**    28                   **42**    28       18          **61**    28                   **81**    28         

  **1**     28                   **22**    21       12          **43**    28                   **62**    28       18          **82**    28         18

  **2**     28       24          **23**    28                   **44**    28       18          **63**    28                   **83**    28         

  **3**     28       24          **24**    16       15          **45**    28                   **64**    28       18          **84**    28         18

  **4**                          **25**    16                   **46**    28       18          **65**    28                   **85**    28         

  **5**                          **26**    18       15          **47**    28                   **66**    28       18          **86**    28         18

  **6**     7        6           **27**    14                   **48**    28       18          **67**    28                   **87**    28         

  **7**     13                   **28**    17       15          **49**    28                   **68**    28       18          **88**    28         18

  **8**     20       20          **29**    15                   **50**    28       18          **69**    28                   **89**    28         

  **9**     15                   **30**    22       13          **51**    28                   **70**    28       18          **90**    27         18

  **10**    19       15          **31**    21                   **52**    28       18          **71**    28                   **91**    20         

  **11**    19                   **32**    15       15          **53**    28                   **72**    28       18          **92**    21         13

  **12**    28       22          **33**    15                   **54**    28       18          **73**    28                   **93**    12         

  **13**    28                   **34**    15       14          **55**    28                   **74**    28       18          **94**    8          8

  **14**    28       22          **35**    18                   **56**    28       18          **75**    28                   **95**    8          

  **15**    28                   **36**    28       16          **57**    28                   **76**    28       18          **96**    6          5

  **16**    28       15          **37**    28                   **58**    28       18          **77**    28                                        

  **17**    28                   **38**    28       18          **59**    28                   **78**    28       18                               

  **18**    28       16          **39**    28                   **60**    28       18          **79**    28                                        

  **19**    28                   **40**    28       18                                         **80**    28       18                               

  **20**    15       12          **41**    28                                                                                 **Tot**   **2337**   **831**
  --------- -------- -------- -- --------- -------- -------- -- --------- -------- -------- -- --------- -------- -------- -- --------- ---------- ---------

As the injection of the indicator into the seawater perturbs the sample
pH slightly, the absorbance ratios measured in the seawater samples
(R~m~) should be corrected to the R values that would have been observed
in an unperturbed analysis (R~real~). In order to do this, we obtain the
correction in the absorbance ratio of every sample as a function of the
absorbance ratio measured (R~m~). This linear function was calculated
from second additions of the indicator over samples with a wide range of
pH:

R~real~ = R~m~ -- (-0.0120.002·R~m~ + 0.0130.003); r^2^= 0.50, n=35

This function also corrects for deviations in the linear relationship
between absorbance and the indicator concentration; this is, deviations
from the Beer Law in the spectrophotometer.

All the pH measurements are referred to 25ºC and corrected for the
addition of the indicator using the former formula. The magnitude of
that correction over our range of pH is small ranging from 0.009 to
0.002 pH units.

[Quality Control.]{.underline}

In order to check the accuracy of the pH measurements, samples of CO~2~
reference material (CRM, batches 55 and 54, distributed by A.G. Dickson
from the Scripps Institution of Oceanography) were analysed during the
cruise.

Figure 1 shows the deviation from the mean value of the pH25~T~
measurements on the CRM batches. The mean pH25~T~ values obtained over
62 measurements done on samples from batch 55 was 7.9170 0.0023, and
over 20 samples of batch 54 was 7.9191 0.0016. The corresponding
theoretical pH25~T~ values for these batches using the dissociation
constants from Lueker *et al.* (2000) were 7.9172 and 7.9184,
respectively. Therefore, our pH measurements are in agreement with the
theoretical values.

![](media/image1.wmf){width="5.110416666666667in"
height="3.5909722222222222in"}

[Reproducibility.]{.underline}

Regarding the reproducibility of our measurements, at station 73 all the
bottles were closed at the same depth, about 4236 meters. We took and
analysed 26 samples taken from 26 different bottles. Besides, we
analysed 6 samples taken from two bottles. Table 2 shows the results of
these measurements.

> ***Table 2**. Summary of the reproducibility on pH25~T~ measurements
> (station 73).*

  ---------- --------------------- --------------- ------------------------------
             [Samples taken at the                 [Samples taken at different
             same Niskin                           Niskin bottles]{.underline}
             bottle]{.underline}                   

  Bottle     22                    5               from 3 to 30

  Mean       7.7364                7.7356          7.7357

  STD        0.0005                0.0013          0.0011

  N          6                     6               26
  ---------- --------------------- --------------- ------------------------------

In several stations two Niskin bottles were closed at the sampe depth.
Figure 2 shows the absolute pH difference between replicates. The mean
and standard deviation of these differences is 0.0016 0.0015 (n=32).

![](media/image2.wmf){width="5.110416666666667in"
height="2.7916666666666665in"}

> ***Figure 2**. Absolute difference in the pH values for the duplicates
> taken at each station during the cruise. The line is the mean value of
> the differences.*

From the former series of analysis we conclude that pH was determined
during the OVIDE cruise with an uncertainty of 0.0014 pH units, this is
the mean of the standard deviations obtained in the CRM and
reproducibility analyses.

**b) Alkalinity analysis.**

[Sampling and analytical methods.]{.underline}

Following the sampling sequence proposed during WOCE (World Ocean
Circulation Experiment), seawater samples for Total Alkalinity (TA) were
collected after pH samples, in 600 ml glass bottles. Samples were filled
to overflowing and immediately stopped. TA profiles were usually sampled
and analysed every other station (Table 1). Eighteen samples were taken
at each profile, all the bottle levels were analysed from bottom up to
500 meters, and every two levels from 500 meters up to the surface.

TA was measured using an automatic potentiometric titrator \"Titrino
Metrohm\", with a Metrohm 6.0233.100 combination glass electrode and a
Pt-100 probe to check the temperature. Potentiometric titrations were
carried out with hydrochloric acid (\[HCl = 0.1 N) to a final pH of 4.40
(Pérez and Fraga, 1987). The electrodes were standardised using a buffer
of pH 4.42 made in CO~2~ free seawater (Pérez *et al.*, 2002). Table 3
shows the value of the asymetrical pH (pHas), which is the value of the
electrode pH after its calibration. The 0.1 N hydrochloric acid was
prepared mixing 0.5 mol (18.231 g) of commercially HCl supplied by
Riedel-deHaën (Fixanal 38285) with mili-Q water into a graduated 5-L
beaker at controlled temperature conditions. The HCl normality (Table 3)
is exactly refereed to 20ºC. The variation of salinity after the
titration is lower than 0.1 units, which is taken into account in the
final TA calculation.

[Quality control.]{.underline}

Usually, each sample is analysed twice for alkalinity. Table 3 shows the
average standard deviation of the replicates analysed during each batch
of analysis. This difference was about 1.0 µmol·kg^-1^.

CRM analyses were performed in order to control the accuracy of our TA
measurements. Accordingly, the final pH of every batch of analyses was
corrected to obtain the closest mean TA on the CRM analyses to the
certified value. Table 3 shows the pH (pH) correction applied to each
batch and the mean value of the CRM determinations after applying the
former correction.

In order to check the precision of the TA measurements, surface seawater
was used as a "quasi-steady" seawater substandard (SB). It consists in
surface seawater taken from the non-toxic supply and stored in the dark
into a large container (25 liters) during 2 days before use. This
substandard seawater was analysed at the beginning and at the end of
each batch of analyses. The reproducibility of these substandard
measurements is better than 0.05% (Table 3) and the estimated drift for
each day was very low.

***Table 3.** Daily calibrations of the pH electrode during the TA
analyses. pHas is the asymetrical pH, T is the temperature at which the
electrode was calibrated with the buffer solution which has a very
stable pH of 4.42 despite the temperature variation. N~HCl~ is the
normality of the hydrochloric solution used. pH is the pH correction
applied to each set of measurements to refer the TA determinations on
the CRM to the corresponding nominal value: batch 55 has a certified TA
value of 2227.9 µmol·kg^-1^ and batch 54, 2342.1 µmol·kg^-1^. The mean
value of the TA measurements on the CRM samples is also shown (Fitted
TA). At the beginning and the end of each batch of measurements a series
of substandard (SB) analyses was done, the mean and the drift obtained
from those analyses are also shown. The average of the difference (Av.
Dif.) in the duplicate's analyses is shown.*

<table>
<colgroup>
<col style="width: 5%" />
<col style="width: 6%" />
<col style="width: 5%" />
<col style="width: 6%" />
<col style="width: 10%" />
<col style="width: 6%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 4%" />
<col style="width: 8%" />
</colgroup>
<tbody>
<tr class="odd">
<td><p>Day</p>
<p>2002</p></td>
<td>pHas</td>
<td>T ºC</td>
<td>pH buffer</td>
<td>N<sub>ClH</sub></td>
<td>CRM #</td>
<td>pH</td>
<td>Fitted TA</td>
<td>SB1 TA</td>
<td>SB2 TA</td>
<td>Drift</td>
<td><p>Av.</p>
<p>Dif.</p></td>
<td>Station</td>
</tr>
<tr class="even">
<td>14-6</td>
<td>6.90</td>
<td>24.8</td>
<td>4.42</td>
<td>0.100008</td>
<td>55</td>
<td>0.010</td>
<td>2227.9</td>
<td>2336.9</td>
<td></td>
<td>0.002%</td>
<td>0.8</td>
<td>0</td>
</tr>
<tr class="odd">
<td>17-6</td>
<td>6.91</td>
<td>24.0</td>
<td>4.42</td>
<td>0.100008</td>
<td>55</td>
<td>0.010</td>
<td>2227.9</td>
<td>2333.1</td>
<td></td>
<td>0.000%</td>
<td>0.6</td>
<td>2 – 3</td>
</tr>
<tr class="even">
<td>19-6</td>
<td>6.92</td>
<td>22.2</td>
<td>4.42</td>
<td>0.100008</td>
<td>55</td>
<td>-0.010</td>
<td>2229.6</td>
<td>2334.1</td>
<td></td>
<td>0.000%</td>
<td>1.0</td>
<td>6 – 10</td>
</tr>
<tr class="odd">
<td>19-6</td>
<td>6.93</td>
<td>22.2</td>
<td>4.42</td>
<td>0.100008</td>
<td>55</td>
<td>-0.025</td>
<td>2228.1</td>
<td>2334.7</td>
<td></td>
<td>-0.001%</td>
<td>1.4</td>
<td>12 - 14</td>
</tr>
<tr class="even">
<td>20-6</td>
<td>6.91</td>
<td>24.0</td>
<td>4.42</td>
<td>0.100008</td>
<td>55</td>
<td>-0.010</td>
<td>2227.9</td>
<td>2334.1</td>
<td></td>
<td>0.001%</td>
<td>1.0</td>
<td>16 - 18</td>
</tr>
<tr class="odd">
<td>21-6</td>
<td>6.93</td>
<td>23.4</td>
<td>4.42</td>
<td>0.100008</td>
<td>55</td>
<td>-0.015</td>
<td>2228.4</td>
<td>2334.1</td>
<td></td>
<td>0.000%</td>
<td>0.9</td>
<td>20 - 24</td>
</tr>
<tr class="even">
<td>22-6</td>
<td>6.93</td>
<td>21.9</td>
<td>4.42</td>
<td>0.100008</td>
<td>55</td>
<td>-0.015</td>
<td>2228.5</td>
<td>2333.5</td>
<td></td>
<td>0.000%</td>
<td>1.1</td>
<td>26 - 30</td>
</tr>
<tr class="odd">
<td>23-6</td>
<td>6.93</td>
<td>22.0</td>
<td>4.42</td>
<td>0.100008</td>
<td>55</td>
<td>-0.015</td>
<td>2227.9</td>
<td>2333.4</td>
<td></td>
<td>0.000%</td>
<td>1.0</td>
<td>32 - 36</td>
</tr>
<tr class="even">
<td>24-6</td>
<td>6.94</td>
<td>21.9</td>
<td>4.42</td>
<td>0.100008</td>
<td>55</td>
<td>-0.015</td>
<td>2227.7</td>
<td>2335.0</td>
<td></td>
<td>0.000%</td>
<td>1.0</td>
<td>38 - 40</td>
</tr>
<tr class="odd">
<td>25-6</td>
<td>6.93</td>
<td>22.5</td>
<td>4.42</td>
<td>0.100008</td>
<td>55</td>
<td>-0.005</td>
<td>2228.0</td>
<td>2335.4</td>
<td></td>
<td>0.000%</td>
<td>0.9</td>
<td>42 - 46</td>
</tr>
<tr class="even">
<td>26-6</td>
<td>6.94</td>
<td>22.0</td>
<td>4.42</td>
<td>0.099978</td>
<td>55</td>
<td>0.021</td>
<td>2226.2</td>
<td>2335.0</td>
<td></td>
<td>0.000%</td>
<td>0.7</td>
<td>48 - 50</td>
</tr>
<tr class="odd">
<td>28-6</td>
<td>6.97</td>
<td>17.2</td>
<td>4.42</td>
<td>0.099978</td>
<td>55</td>
<td>-0.007</td>
<td>2227.9</td>
<td>2334.4</td>
<td></td>
<td>0.012%</td>
<td>0.9</td>
<td>52 - 56</td>
</tr>
<tr class="even">
<td>29-6</td>
<td>6.97</td>
<td>23.1</td>
<td>4.42</td>
<td>0.099978</td>
<td>55</td>
<td>-0.004</td>
<td>2227.8</td>
<td></td>
<td>2347.9</td>
<td>0.005%</td>
<td>0.7</td>
<td>58 - 62</td>
</tr>
<tr class="odd">
<td>30-6</td>
<td>6.98</td>
<td>24.5</td>
<td>4.42</td>
<td>0.099978</td>
<td>55</td>
<td>-0.040</td>
<td>2228.4</td>
<td></td>
<td>2348.6</td>
<td>0.000%</td>
<td>0.9</td>
<td>64 - 66</td>
</tr>
<tr class="even">
<td>1-7</td>
<td>6.97</td>
<td>26.0</td>
<td>4.42</td>
<td>0.099978</td>
<td>55</td>
<td>-0.001</td>
<td>2227.8</td>
<td></td>
<td>2348.8</td>
<td>0.000%</td>
<td>0.9</td>
<td>68 - 70</td>
</tr>
<tr class="odd">
<td>2-7</td>
<td>6.99</td>
<td>25.7</td>
<td>4.42</td>
<td>0.099978</td>
<td>55</td>
<td>-0.013</td>
<td>2227.9</td>
<td></td>
<td>2347.9</td>
<td>0.000%</td>
<td>1.2</td>
<td>72</td>
</tr>
<tr class="even">
<td>6-7</td>
<td>6.99</td>
<td>25.7</td>
<td>4.42</td>
<td>0.099978</td>
<td>55</td>
<td>0.001</td>
<td>2227.9</td>
<td></td>
<td>2348.4</td>
<td>0.000%</td>
<td>0.8</td>
<td>74 - 76</td>
</tr>
<tr class="odd">
<td>7-7</td>
<td>7.00</td>
<td>23.0</td>
<td>4.42</td>
<td>0.099978</td>
<td>54</td>
<td>-0.016</td>
<td>2342.1</td>
<td></td>
<td>2346.9</td>
<td>0.000%</td>
<td>1.1</td>
<td>78 - 80</td>
</tr>
<tr class="even">
<td>8-7</td>
<td>6.99</td>
<td>25.6</td>
<td>4.42</td>
<td>0.099978</td>
<td>54</td>
<td>-0.016</td>
<td>2342.3</td>
<td></td>
<td>2346.4</td>
<td>-0.003%</td>
<td>1.1</td>
<td>82 - 84</td>
</tr>
<tr class="odd">
<td>9-7</td>
<td>6.99</td>
<td>25.7</td>
<td>4.42</td>
<td>0.099978</td>
<td>54</td>
<td>-0.019</td>
<td>2342.2</td>
<td></td>
<td>2347.9</td>
<td>0.000%</td>
<td>1.4</td>
<td>86 - 90</td>
</tr>
<tr class="even">
<td>10-7</td>
<td>7.00</td>
<td>22.2</td>
<td>4.42</td>
<td>0.099978</td>
<td>54</td>
<td>-0.016</td>
<td>2349.2</td>
<td></td>
<td>2346.5</td>
<td>0.000%</td>
<td>1.2</td>
<td>92 - 96</td>
</tr>
<tr class="odd">
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>Average</td>
<td>2334.2</td>
<td>2347.8</td>
<td></td>
<td>1.0</td>
<td></td>
</tr>
<tr class="even">
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>Std</td>
<td>0.7</td>
<td>0.8</td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>0.03%</td>
<td>0.04%</td>
<td></td>
<td></td>
<td></td>
</tr>
</tbody>
</table>

[Reproducibility.]{.underline}

![](media/image3.wmf){width="5.677083333333333in"
height="3.188888888888889in"}In the test station (station 0) 28 bottles
were closed at 4000 dbars. Twenty-four of the them were sampled for
alkalinity but one of them was rejected because of a likely leakage in
the Niskin bottle. Figure 3 shows the analyses made in each Niskin
bottle (in some of them two analyses were done, so the mean and standard
deviation are shown), labelled consecutively from 3 to 30. The standard
deviation of all the TA determinations for these 23 bottles was 0.8
µmol·kg^-1^.

> ***Figure 3**. TA values obtained in the test station (station 0). All
> samples were taken at 4000 dbars. Most of the samples from the same
> bottle were analysed twice, so the mean and standard deviation of both
> analyses is shown by vertical bars. The standard deviation of all
> samples was 0.8 µmol·kg^-1^.*

Additionally, in many stations two Niskins bottles were closed at the
same depth. Figure 4 shows the absolute difference between the
duplicates. The mean difference was 0.5 µmol·kg^-1^.

> ![](media/image4.wmf){width="5.110416666666667in"
> height="2.795138888888889in"}***Figure 4**. Absolute difference of the
> TA values for the duplicates taken at each station during the cruise.
> The line is the mean value of the differences.*

From the former series of analyses we conclude that TA was determined
during the OVIDE cruise with an uncertainty of 1 µmol·kg^-1^.

[Comparison with silicate.]{.underline}

Silicate and salinity normalized alkalinity (NTA = TA/S·35) in the North
Atlantic present a strong covariation. Figure 5 shows the high
correlation of NTA versus silicate for samples taken below 200 meters,
presumably not affected by biological processes. The standard deviation
of the NTA residuals as a function of silicate is 2.9 µmol·kg^-1^. This
figure can be considered as the likely maximum error incurred in
alkalinity if no error is assumed in the silicate analyses. Other likely
sources of uncertainty are the error in the silicate determination and
no linear natural processes, which could contribute to explain the
difference between the analytical error in the alkalinity and the
standard deviation of these residuals.

> ![](media/image5.wmf){width="5.905555555555556in"
> height="4.15in"}***Figure 5**. Relationship between normalized
> alkalinity (NTA) and silicate (Si(OH)~4~) for samples below 200
> meters, both variable in µmol·kg^-1^ .*

**c) Underway CO~2~ measurements.**

A system designed by the IIM group of Vigo was used to measure the mole
fraction of CO~2~ in air and surface seawater. Atmospheric CO~2~ was
measured by the system from an air intake mounted in the mast of the
ship and surface seawater was pumped from the ship\'s keel. This system
is very similar to the one developed in the University of Kiel by
Körtzinger *et al.* (1996) and uses a non-dispersive infrared detector
(LICOR 6262) for CO~2~ and H~2~O. The equilibrator combines two types of
equilibration concepts, the bubble and laminar type flows, the first one
describes the water chamber constantly renewed with water (appr. 1500
ml) and bubbled with air, and the latter one describes the flow of
entering seawater from the top as a laminar flow. Therefore, the
counter-current flow direction of seawater and air as well as the large
surface area facilitate the establishment of equilibrium.

The equipment was calibrated with two standards, CO~2~-free air and high
CO~2~ standard gas. Surface seawater partial pressure of CO~2~ (pCO~2~
atm) at 100% humidity was calculated based on molar fraction of CO~2~
(xCO~2~, directly measured by the LICOR) ambient pressure p (atm),
recorded by the system, and saturation water vapour pressure w (atm).

pCO~2~ = xCO~2~ (p - w)

pCO~2~ is corrected for the temperature shift between in-situ
temperature and equilibrator temperature using an empirical equation
(DOE, 1994) which was originally proposed by Takahashi *et al.* (1993).
The non-ideal behaviour of CO~2~, although small, has to be taken into
account. The calculation of CO~2~ fugacity was done after Weiss (1974).

The equipment also included a probe (SBE micro TSS) for measuring
seawater temperature and salinity, a probe for measuring surface oxygen
(SBE-43) and a fluorometer (wetstar from Wetlab) to determine the
fluorescence of surface water.

The molar fraction of CO~2~ is corrected according to the standards run
during each calibration. Table 4 shows the small corrections applied at
320 and 370 ppm.

> ***Table 4.** Date of the LICOR calibration, and corrections applied
> over the molar fraction of CO~2~* at 320 and 370 ppm.

+----------------------------+--------------------+--------------------+
| Calibration                | Cor. at 320 ppm    | Cor. at 370 ppm    |
|                            |                    |                    |
| date                       |                    |                    |
+----------------------------+--------------------+--------------------+
| 11-6-02 19:57              | 0.4                | 0.6                |
+----------------------------+--------------------+--------------------+
| 12-6-02 21:17              | 0.5                | 0.7                |
+----------------------------+--------------------+--------------------+
| 13-6-02 7:28               | -0.2               | -0.1               |
+----------------------------+--------------------+--------------------+
| 13-6-02 21:51              | -0.2               | -0.1               |
+----------------------------+--------------------+--------------------+
| 14-6-02 11:59              | -0.1               | 0.0                |
+----------------------------+--------------------+--------------------+
| 14-6-02 14:05              | -0.3               | -0.2               |
+----------------------------+--------------------+--------------------+
| 15-6-02 6:04               | -0.1               | 0.1                |
+----------------------------+--------------------+--------------------+
| 16-6-02 6:05               | 0.1                | 0.2                |
+----------------------------+--------------------+--------------------+
| 16-6-02 8:44               | 0.2                | 0.2                |
+----------------------------+--------------------+--------------------+
| 17-6-02 1:52               | -0.1               | -0.1               |
+----------------------------+--------------------+--------------------+
| 18-6-02 2:34               | -0.1               | -0.1               |
+----------------------------+--------------------+--------------------+
| 18-6-02 6:17               | -0.3               | -0.2               |
+----------------------------+--------------------+--------------------+
| 20-6-02 3:19               | 1.6                | 1.8                |
+----------------------------+--------------------+--------------------+
| 22-6-02 4:13               | 2.0                | 2.2                |
+----------------------------+--------------------+--------------------+
| 25-6-02 4:04               | 1.8                | 2.1                |
+----------------------------+--------------------+--------------------+
| 25-6-02 4:14               | 0.4                | 0.5                |
+----------------------------+--------------------+--------------------+
| 25-6-02 4:49               | -0.1               | -0.1               |
+----------------------------+--------------------+--------------------+
| 28-6-02 6:34               | 1.7                | 1.8                |
+----------------------------+--------------------+--------------------+
| 28-6-02 6:47               | 0.1                | 0.2                |
+----------------------------+--------------------+--------------------+
| 30-6-02 9:32               | -0.8               | 0.0                |
+----------------------------+--------------------+--------------------+
| 1-7-02 2:39                | -1.4               | -0.6               |
+----------------------------+--------------------+--------------------+
| 1-7-02 2:48                | -0.2               | -0.2               |
+----------------------------+--------------------+--------------------+
| 2-7-02 1:24                | 0.6                | 0.6                |
+----------------------------+--------------------+--------------------+
| 3-7-02 18:40               | 0.6                | 0.3                |
+----------------------------+--------------------+--------------------+
| 3-7-02 18:49               | 0.1                | -0.1               |
+----------------------------+--------------------+--------------------+
| 6-7-02 13:48               | 0.3                | 0.2                |
+----------------------------+--------------------+--------------------+
| 6-7-02 14:07               | -0.1               | 0.0                |
+----------------------------+--------------------+--------------------+
| 8-7-02 10:33               | -0.2               | 0.3                |
+----------------------------+--------------------+--------------------+
| 9-7-02 16:37               | -0.2               | -0.3               |
+----------------------------+--------------------+--------------------+
| 11-7-02 1:29               | 0.1                | 0.4                |
+----------------------------+--------------------+--------------------+
| 11-7-02 11:41              | 0.2                | 0.1                |
+----------------------------+--------------------+--------------------+

![](media/image6.wmf){width="6.247916666666667in"
height="2.9583333333333335in"}Figure 6 shows the variation of CO~2~
fugacity (fCO~2~) during the OVIDE cruise, both in surface seawater and
the atmosphere. Showing that the Subpolar North Atlantic mainly acted as
a sink for atmospheric CO~2~ during the period.

***Figure 6**. Temporal evolution of the CO~2~ fugacity (fCO~2~) in
seawater and air along the OVIDE cruise.*

**d) Internal Consistency of Carbonic System.**

Next figure compares the CO~2~ fugacity (fCO~2~) values measured at
every station and those calculated from pH~T~ and TA with the Lueker *et
al.* (2000) dissociation constants. The agreement between both fCO~2~ is
excellent, confirming the good internal consistency of our measurements.
The average and standard deviation of the differences between both
calculated and measured fCO~2~ was -44.6 atm. To centre the fCO~2~
residuals to zero pH should be decreased in 0.004 units.

![](media/image7.wmf){width="5.677083333333333in"
height="3.9895833333333335in"}

> ***Figure 7.** Relationship between CO~2~ fugacity measured and
> calculated as a function of TA and pH measured at the surface of the
> OVIDE stations.*

Table 5 offers more detail about these comparisons, with the pH~T~ and
TA values analysed at the surface of each station, the fCO~2~ calculated
from them and that measured by the underway equipment. Note that as TA
samples were taken more unevenly than pH, surface TA was interpolated as
a function of salinity in some stations. The linear function used was:

TA = 46.89·S + 672.43, r^2^ = 0.988, n = 46

The estimated error in fCO~2~ calculated from the reproducibility of
pH~T~ (0.0014) and alkalinity (1 µmol·kg^-1^) is 3 atm. This value is
slightly lower than that estimated from the direct comparison of
measured and calculated fCO~2~, 4 µatm. The former error includes other
sources of error apart from the sampling and the analysis procedures as
those due to the oceanographic representativeness of the samples.

***Table 5.** Comparison between calculated and measured CO~2~ fugacity
on some OVIDE stations. Some alkalinity values were calculated as a
function of salinity. Total alkalinity (TA) is given in µmol·kg^-1^ and
CO~2~ fugacity (fCO~2~) in µatm. fCO~2~ is the fCO~2~ difference between
measured and calculated fCO~2~.*

  ---------- -------- --------- ---- ------- -------- ------- --------- ------ ------ -------- -------- --------
  Time       Lat      Long      St   Depth   Sal.     T (ºC)  pH~25T~   TA     TA Cal Cal.     Meas.    fCO~2~
                                                                                      fCO~2~   fCO~2~   

  15-6-02    54.170   26.491    1    8       35.138   10.67   7.901            2320   330      335      -5
  10:51                                                                                                 

  16-6-02    56.019   31.544    2    8       34.869   8.94    7.888     2302   2308   316      324      -8
  8:17                                                                                                  

  17-6-02    57.623   36.005    3    6       34.944   7.63    7.863     2308   2311   321      325      -4
  3:46                                                                                                  

  18-6-02    59.831   42.525    6    6       32.213   -0.35   7.940     2193   2183   174      179      -5
  6:15                                                                                                  

  18-6-02    59.801   42.353    7    6       33.286   0.64    7.911            2233   200      195      5
  11:06                                                                                                 

  18-6-02    59.803   42.272    8    6       34.688   5.88    7.876     2299   2299   286      293      -8
  14:46                                                                                                 

  18-6-02    59.801   42.008    9    6       34.878   6.89    7.886            2308   291      294      -3
  18:14                                                                                                 

  18-6-02    59.796   41.723    10   7       34.863   6.85    7.876     2307   2307   298      301      -3
  21:46                                                                                                 

  19-6-02    59.762   41.309    11   6       34.870   6.82    7.885            2308   291      294      -4
  1:09                                                                                                  

  19-6-02    59.759   40.905    12   6       34.845   6.69    7.874     2307   2306   298      298      0
  4:55                                                                                                  

  19-6-02    59.725   40.252    13   7       34.835   6.71    7.887            2306   287      288      0
  9:08                                                                                                  

  19-6-02    59.686   39.602    14   8       34.818   6.82    7.871     2308   2305   302      308      -5
  13:45                                                                                                 

  19-6-02    59.623   38.961    15   8       34.964   7.92    7.879            2312   311      317      -6
  18:14                                                                                                 

  19-6-02    59.560   38.319    16   8       34.911   7.90    7.866     2306   2310   321      326      -5
  22:48                                                                                                 

  20-6-02    59.495   37.678    17   7       34.919   8.16    7.887            2310   307      309      -3
  3:29                                                                                                  

  20-6-02    59.431   37.043    18   7       34.911   8.13    7.879     2306   2310   313      315      -2
  8:01                                                                                                  

  20-6-02    59.364   36.400    19   8       34.967   8.58    7.892            2312   309      313      -4
  12:43                                                                                                 

  20-6-02    59.299   35.757    20   6       34.904   8.42    7.879     2307   2309   317      320      -3
  17:20                                                                                                 

  21-6-02    59.159   34.468    23   8       34.926   8.76    7.888            2310   314      312      2
  2:06                                                                                                  

  21-6-02    59.104   33.832    24   8       34.955   8.48    7.885     2312   2312   313      316      -3
  6:11                                                                                                  

  21-6-02    58.977   32.575    26   7       35.097   9.52    7.912     2316   2318   304      312      -8
  13:52                                                                                                 

  21-6-02    58.912   31.915    27   8       35.116   9.60    7.905            2319   311      312      0
  17:39                                                                                                 

  21-6-02    58.844   31.283    28   9       35.131   9.62    7.921     2317   2320   299      315      -17
  21:09                                                                                                 

  22-6-02    58.725   30.703    29   8       35.118   10.10   7.915            2319   310      313      -3
  0:38                                                                                                  

  22-6-02    58.402   30.117    31   8.5     35.11    10.33   7.918            2319   311      308      3
  7:26                                                                                                  

  22-6-02    57.669   28.718    33   8       35.152   10.36   7.921            2321   309      315      -7
  19:20                                                                                                 

  22-6-02    57.355   28.157    34   7       35.135   10.47   7.930     2317   2320   302      311      -9
  23:45                                                                                                 

  23-6-02    57.004   27.880    35   7       35.138   10.36   7.920            2320   309      313      -4
  4:15                                                                                                  

  23-6-02    56.619   27.523    36   9       34.952   10.03   7.928     2308   2311   297      315      -18
  9:43                                                                                                  

  23-6-02    56.242   27.281    37   7       34.885   9.94    7.907            2308   314      317      -3
  14:40                                                                                                 

  23-6-02    55.884   27.003    38   7       35.133   10.53   7.913     2316   2320   318      323      -5
  19:30                                                                                                 

  24-6-02    55.502   26.719    39   8       35.146   10.53   7.907            2321   323      324      -1
  0:37                                                                                                  

  24-6-02    55.148   26.414    40   9       35.220   11.04   7.919     2323   2324   319      321      -1
  5:34                                                                                                  

  24-6-02    54.753   26.129    41   12      35.254   11.16   7.918            2326   322      323      -1
  11:03                                                                                                 

  24-6-02    54.386   25.830    42   9       35.127   10.72   7.913     2316   2320   320      325      -5
  15:47                                                                                                 

  24-6-02    54.013   25.536    43   10      35.167   10.95   7.920            2322   317      320      -3
  20:39                                                                                                 

  25-6-02    53.632   25.238    44   6       35.270   11.28   7.927     2327   2326   316      316      0
  1:53                                                                                                  

  25-6-02    53.264   24.945    45   8       35.009   10.69   7.914            2314   318      317      1
  6:44                                                                                                  

  25-6-02    52.875   24.651    46   10      35.075   10.95   7.923     2317   2317   314      316      -2
  11:48                                                                                                 

  25-6-02    52.521   24.363    47   8       35.075   10.95   7.920            2317   316      317      -1
  16:35                                                                                                 

  25-6-02    52.146   24.071    48   11      35.110   11.42   7.910     2310   2319   333      318      15
  21:44                                                                                                 

  Time       Lat      Long      St   Depth   Sal.     T (ºC)  pH~25T~   TA     TA Cal Cal.     Meas.    fCO~2~
                                                                                      fCO~2~   fCO~2~   

  6-6-02     51.771   23.777    49   9       35.181   11.92   7.945            2322   309      307      2
  3:10                                                                                                  

  26-6-02    51.400   23.484    50   12      35.289   12.23   7.895     2327   2327   359      358      1
  8:18                                                                                                  

  26-6-02    51.028   23.190    51   12      35.599   13.76   7.960            2342   322      326      -5
  13:35                                                                                                 

  27-6-02    50.641   22.902    52   8       35.653   14.22   7.953     2346   2344   335      338      -3
  4:09                                                                                                  

  27-6-02    50.280   22.606    53   8       35.683   14.03   7.959            2346   327      328      -1
  9:39                                                                                                  

  27-6-02    49.906   22.312    54   6       35.608   14.14   7.954     2340   2342   333      333      -1
  15:01                                                                                                 

  27-6-02    49.532   22.022    55   8       35.642   14.86   7.947            2344   349      353      -3
  20:32                                                                                                 

  28-6-02    49.160   21.729    56   7       35.692   15.32   7.973     2345   2346   332      333      -1
  2:12                                                                                                  

  28-6-02    48.785   21.434    57   8       35.715   15.03   8.004            2347   300      305      -5
  7:40                                                                                                  

  28-6-02    48.410   21.139    58   8       35.709   15.49   8.023     2350   2347   290      298      -8
  13:21                                                                                                 

  28-6-02    48.039   20.849    59   7       35.716   15.37   7.999            2347   309      315      -6
  19:12                                                                                                 

  29-6-02    47.663   20.554    60   7       35.720   15.84   7.970     2348   2347   342      342      0
  1:05                                                                                                  

  29-6-02    46.916   19.970    62   9       35.751   16.04   8.013     2352   2349   306      314      -9
  12:13                                                                                                 

  29-6-02    46.542   19.677    63   7       35.890   16.93   8.035            2355   298      313      -15
  17:35                                                                                                 

  29-6-02    46.166   19.384    64   8       35.854   16.62   8.028     2358   2354   300      311      -11
  23:15                                                                                                 

  30-6-02    45.797   19.089    65   8       35.771   16.10   8.010            2350   309      319      -10
  4:43                                                                                                  

  30-6-02    45.421   18.796    66   8       35.770   16.15   8.020     2356   2350   301      312      -11
  10:17                                                                                                 

  30-6-02    45.050   18.504    67   8       35.875   16.84   7.990            2355   338      347      -9
  15:53                                                                                                 

  30-6-02    44.673   18.215    68   11      35.866   16.78   7.988     2357   2354   339      347      -8
  21:50                                                                                                 

  1-7-02     44.381   17.818    69   8       35.827   16.53   7.976            2352   347      346      1
  3:35                                                                                                  

  1-7-02     44.077   17.427    70   12      35.829   16.44   7.981     2355   2353   340      341      -1
  8:49                                                                                                  

  1-7-02     43.777   17.031    71   8       35.852   16.29   7.995            2354   325      328      -3
  14:06                                                                                                 

  1-7-02     43.475   16.643    72   10      35.825   16.38   7.996     2355   2352   326      332      -6
  19:34                                                                                                 

  5-7-02     43.181   -16.244   74   9       35.891   17.51   7.988     2357   2355   349      350      -1
  17:17                                                                                                 

  5-7-02     42.883   -15.854   75   8       35.903   17.59   7.991            2356   347      352      -5
  22:38                                                                                                 

  6-7-02     42.582   -15.457   76   6       35.944   17.76   7.990     2363   2358   351      354      -4
  4:29                                                                                                  

  6-7-02     42.281   -15.070   77   10      35.885   17.51   7.987            2355   350      353      -3
  10:23                                                                                                 

  6-7-02     41.983   -14.672   78   7       35.838   17.71   7.984     2353   2353   356      358      -2
  16:24                                                                                                 

  6-7-02     41.684   -14.281   79   8       35.922   17.99   7.997            2357   347      354      -7
  22:22                                                                                                 

  7-7-02     41.384   -13.890   80   7       36.031   18.22   7.996     2362   2362   352      357      -5
  4:21                                                                                                  

  7-7-02     41.082   -13.495   81   12      35.858   17.56   7.992            2354   346      353      -7
  10:18                                                                                                 

  7-7-02     40.787   -13.101   82   7       35.872   17.81   7.991     2355   2355   350      355      -5
  16:30                                                                                                 

  7-7-02     40.552   -12.646   83   8       35.957   17.91   7.994            2359   349      355      -6
  22:29                                                                                                 

  8-7-02     40.335   -12.220   84   6       35.946   17.88   7.993     2359   2358   350      355      -5
  4:20                                                                                                  

  8-7-02     40.333   -11.780   85   11      35.855   17.75   7.993            2354   348      353      -5
  9:51                                                                                                  

  8-7-02     40.333   -11.343   86   8       35.680   17.61   7.988     2344   2346   349      352      -3
  15:22                                                                                                 

  8-7-02     40.334   -10.904   87   9       35.933   18.08   7.995            2357   350      354      -4
  20:34                                                                                                 

  9-7-02     40.334   -10.574   88   8       35.890   17.94   7.994     2356   2355   350      352      -2
  1:12                                                                                                  

  9-7-02     40.333   -10.300   89   7       35.887   17.79   7.988            2355   353      350      3
  5:41                                                                                                  

  9-7-02     40.333   -10.033   90   9       35.791   17.64   7.997     2350   2351   342      346      -4
  10:22                                                                                                 

  9-7-02     40.334   -9.943    91   9       35.738   17.19   7.996            2348   336      344      -8
  13:47                                                                                                 

  9-7-02     40.334   -9.877    92   7       35.679   16.78   7.992     2347   2346   334      337      -3
  16:38                                                                                                 

  9-7-02     40.335   -9.803    93   11      35.668   16.75   7.993     2348   2345   332      337      -5
  19:15                                                                                                 

  9-7-02     40.333   -9.766    94   8       35.672   16.66   7.993     2344   2345   332      339      -7
  21:01                                                                                                 

  9-7-02     40.335   -9.641    95   5       35.612   16.35   7.977            2342   342      351      -9
  22:47                                                                                                 

  10-7-02    40.334   -9.454    96   7       35.615   15.63   7.969     2345   2343   339      349      -10
  0:31                                                                                                  
  ---------- -------- --------- ---- ------- -------- ------- --------- ------ ------ -------- -------- --------

One of the final aims of measuring TA and pH during the OVIDE cruise is
the estimation of the total inorganic carbon (C~T~) concentration. The
error incurred in TA ( 1 µmol·kg^-1^) and pH (0.0014) causes a maximum
error in C~T~ of 2 µmol·kg^-1^. Decreasing the pH values in 0.004 pH
units to adjust the pH values to the fCO~2~ values measured on the
surface, would suppose a mean increase of the final C~T~ values of 1.8
µmol·kg^-1^. The latter negative bias in the C~T~ calculations is within
the range of 2 µmol·kg^-1^ due to uncertainties in the determination of
pH or TA. A recent synthesis of the Pacific Ocean CO~2~ data from
twenty-five WOCE/JGOFS/OACES cruises showed that the best data coverage
was for coulometric C~T~ measurements which had an estimated overall
accuracy of 3 µmol·kg^-1^ (Lamb *et al.*, 2002). Accordingly, the second
most common carbon parameter analysed, TA, had an estiamted overall
accuracy of 5 µmol·kg^-1^. Intercomparisons of CO~2~ system variables in
deep waters of the North Atlantic from different cruises will be done to
check the consistency of the measurements. Multilinear regressions of
CO~2~ variables as a function of temperature, salinity, oxygen and
nutrients will help us to discern if applying any final adjustment over
the pH measurements during the OVIDE cruise.

**REFERENCES.**

Byrne R. H. (1987). Standardization of standard buffers by visible
spectrometry. *Analytical Chemistry*, 59, 1479-1481.

Clayton, T.D. and R.H. Byrne (1993). Spectrophotometric seawater pH
measurements: total hydrogen ion concentration scale concentration scale
calibration of m-cresol purple and at-sea results. *Deep-Sea Res. I*,
Vol. 40, 10, 2115-2129.

DelValls, T.A and A.G. Dickson (1998). The pH of buffers based on
2-amino-2-hydroxymethyl-1,3-propanediol ('tris') in synthetic sea water.
*Deep-Sea Res. I*, Vol. 45, 1541-1554.

DOE, (1994). Handbook of the Methods for the Analysis of the Various
Parameters of the Carbon Dioxide System in Sea Water, Version 2,
ORNL/CDIAC-74. A.G. Dickson and C. Goyet (Editors).

Lamb, M.F, C. L. Sabine, R. A. Feely, R. Wanninkhof, R. M. Key, G. C.
Johnson, F. J. Millero, K. Lee, T. -H. Peng, A. Kozyr, J. L. Bullister,
D. Greeley, R. H. Byrne, D. W. Chipman, A. G. Dickson, C. Goyet, P. R.
Guenther, M. Ishii, K. M. Johnson, C. D. Keeling, T. Ono, K. Shitashima,
B. Tilbrook, T. Takahashi, D. W. R. Wallace, Y. W. Watanabe, C. Winn and
C. S. Wong. (2002). Consistency and synthesis of Pacific Ocean CO~2~
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