Discussion of Profiles
Following is a brief day-by-day description of the major features contained in figures 12-20. We tried to identify the most important results and to point to some major trends and changes. Again we would like to emphasize that the scatter of the bottom figures is mainly an artifact of the referencing procedure. This can be readily observed in Fig. 13 (June 9, 1996): Between 08:00 and 19:00 UTC the seawater exhibits low variability resulting in very little scatter in the bottom figure. Immediately before and after this period the seawater was much more variable which translates into the high scatter of the deviation figure. The observed offsets discussed here can therefore only be identified in the trends and have to be regarded as rough approximations.
June 8, 1996 (Fig. 12)
Missing data - "A": 04:00 to 10:00 UTC, "B": before 18:30 UTC (start delayed because of sample gas leakage), "F": 03:00 to 10:30 UTC, "H": no samples measured. Agreement to within ±2 µatm - "B," "C," "D," and "E". Positive offset - "F": 3 µatm. Negative offset - "G": 6 µatm. Variable offset - "A": -3 to +3 µatm. Comment - "G" starts with a marked negative offset, which turns slowly into a positive offset during the next days and then disappears toward the end of the exercise. However, the large scatter of "G" seen in the atmospheric xCO2 readings is not visible here, which points toward problems with the handling of atmospheric air within this system (e.g., leakage in air pump, valves, or tubing).
June 9, 1996 (Fig. 13)
Missing data - "A": 10:00 to 12:00 UTC, "C": 13:00 to 24:00 UTC, "H": no samples measured. Agreement to within ±2 µatm - "B," "C," "D," and "E." Negative offset - "G": 6 µatm. Variable offset - "A": within 2 µatm (before 07:00 UTC), -5 to -8 µatm after 12:00 UTC; "F": within ±2 µatm (09:00 to 19:00 UTC), +5 to +10 µatm (before 09:00 UTC and after 19:00 UTC). Comment - "A" shows a sudden change around 12:00 UTC from good agreement to a negative offset of the order of 5 µatm. This offset remained until the end of the exercise. The scatter of profile "A" (3-min intervals) is significantly larger than in the 1-min averages of "B" as can be seen in the smooth period (13:00 to 17:00 UTC). This is contradictory to what one would expect and may be related to the rather large scatter observed in the atmospheric xCO2 readings of "A." Interestingly, the offset of the latter showed up from the beginning of the exercise (i.e., before June 9, 12:00 UTC when it suddenly appeared in seawater fCO2 readings). This is indicative of different reasons for the offsets observed in atmospheric xCO2 and seawater fCO2 readings of "A."
June 10, 1996 (Fig. 14)
Missing data - "A": after 12:00 UTC, "C": before 13:00 UTC, "G": 12:00 to 23:00 UTC. Agreement to within ±2 µatm - "C," "D," and "E." Positive offset - "B": 3 to 9 µatm, "F": 4 to 10 µatm. Negative offset - "A": 5 µatm, "G": 5 µatm. Variable offset - "H": within ±2 µatm at 13:22 and 21:09 UTC, +7 µatm at 06:12 UTC. Comments - "B" immediately started to develop a positive offset which more or less remained until about 18:00 UTC of the following day. This offset is not seen in atmospheric xCO2 measurements of "B." "F" also lost its good agreement and started to develop a positive offset which stabilized toward the end of the exercise. Interestingly these offsets of "B" and "F" show up at about the same time and with a very similar pattern over the 2-day period. Furthermore, the 06:12 UTC data point of "H" has the same positive offset as "B" and "F." Whether this is pure coincidence or an expression of something real is not known. These three systems, however, are very different in their principle of measurement and the location in the seawater supply line so that a common systematic error can be ruled out. Also their common offset seems to be inversely correlated with seawater temperature (see Fig. 7). On the basis of this observation it also has to be questioned whether a systematic offset may be present in the "reference" profiles "C," "D," and "E." This puzzle, however, cannot be solved here.
June 11, 1996 (Fig. 15)
Missing data - "A": before 05:30 UTC, "B": 17:00 to 19:00 UTC, "C": 12:00 to 16:00 UTC, "F": 20:30 to 23:30 UTC. Agreement to within ±2 µatm - "C," "D," "E," and "H." Positive offset - "F": 2 to 8 µatm, "G": 2 to 9 µatm. Negative offset - "A" 3 to 8 µatm. Variable offset - "B": within ±2 µatm (after 18:00 UTC), +3 to +9 µatm (00:00 to 17:00 UTC). Comment - Positive offsets of "B," "F," and "G" are essentially parallel throughout the day (see also comment of the previous day). Between 12:00 and 19:00 UTC the positive offset of "B" slowly disappears while at the same time a negative offset in the atmospheric xCO2 measurements of "B" develops.
June 12, 1996 (Fig. 16)
Missing data - "C": before 16:00 UTC. Agreement to within ±2 µatm - "C," "D," and "E." Positive offset - "F": 3 to 9 µatm. Negative offset - "A": 3 to 6 µatm. Variable offset - "B": 0 to +3 µatm, "G" -2 to +8 µatm, "H": +1 to -6 µatm. Comments - The top figure shows nice parallel patterns of all fCO2 profiles even in this strongly variable environment. The bottom figure heavily suffers from the artificial scatter but nevertheless reveals the general offsets and trends. The negative offset of "H" at 17:29 UTC is likely due to this artifact because "H" is in very good agreement with "B" which does not show a general offset here.
June 13, 1996 (Fig. 17)
Missing data - "A": after 18:00 UTC, "B": after 22:00 UTC, "F": 16:00 to 20:00 UTC, "G": 03:00 to 05:00 UTC. Agreement to within ±2 µatm - "B," "C," "D," "E," and "G." Positive offset - "F": 6 to 9 µatm. Negative offset - "A": 5 to 6 µatm. Variable offset - "H": -2 and -5 µatm. Comment - This is about the beginning of the "smooth regime" with comparatively low variability in surface water which persisted for the rest of the exercise. The kind of agreement seen in this figure continues to exist in the following figures with very little alteration. In contrast to the highly variable situation encountered earlier this cruise, this situation is probably more representative of typical oceanic conditions in underway fCO2 field work. Again, the negative offset of "H" at 00:31 UTC is likely an artifact as it follows the profile of "B" which itself is in good agreement with "C," "D," and "E." System "A" had to quit the exercise at about 18:00 UTC because of a technical problem associated with the NDIR instrument, and no more data from this system are available beyond this point.
June 14, 1996 (Fig. 18)
Missing data - "A": no data available, "B": 17:00 to 22:00 UTC, "G": before 09:00 UTC. Agreement to within ±2 µatm - "B," "C," "D," "E," "G," and "H." Positive offset - "F": 7 to 10 µatm. Comment - Whereas the general agreement of all profiles except "F" is rather good, even among them slight trends toward positive ("H") or negative ("B" and "G") offsets can be identified that persist for the rest of the exercise.
June 15, 1996 (Fig. 19)
Missing data - "A": no data available. Agreement to within ±2 µatm - "C," "D," and "E." Positive offset - "F": 5 to 10 µatm. Negative offset - "B": 3 µatm, "G": 4 µatm. Variable offset - "H": +1.5 to +4.5 µatm. Comment - See comment for previous day.
June 16, 1996 (Fig. 20)
Missing data - "A": no data available, "F": 17:00 to 21:00 UTC. Agreement to within ±2 µatm - "C," "D," "E," and "H." Positive offset - "F": 5 to 8 µatm. Negative offset - "B": 2 to 3 µatm, "G": 2 to 4 µatm. Comment - As the hydrographic conditions have become much less variable, the overall picture of agreement among the various systems is very consistent for the last three days of the exercise.
