Dataset: CO2 - Underway pCO2 - GO System

See: SO-GasEx cruise report, Section 5.4.3 pgs 21-23

Method:
The system was built by General Oceanics (GO) and is described in Pierrot, et al. (2009)

The three standard gases come from CMDL in Boulder and are directly traceable to the WMO scale.

Sampling Cycle:
The system runs on an cycle during which 3 standard gases, 5 air
samples from the bow tower and 50 surface water samples (from the
equilibrator head space) are analyzed on the following schedule:

1. Zero and span of Licor
2. Three standard gases
3. Five air samples
4. Fifty equilibrator headspace gas samples
5. Repeat steps 2 - 4 nine more times
6. Restart from step 1

Standards:
SN CA06745, 289.06 ppm;
SN CA05398, 370.90 ppm;
SN CA06352, 411.42 ppm.

Units:
All xCO2 values are reported in parts per million (ppm) and fCO2 values
are reported in microatmospheres (uatm) assuming 100 % humidity at the
equilibrator temperature.

Notes:
1. Beginning with this cruise, a new pCO2 analytical system was installed
aboard the Brown, built by General Oceanics (GO). The file format has
changed slightly and air values are now included in the file. QC flags now
apply to the fCO2 value for Equ measurements and to the xCO2 value for Atm
measurements. Most measurements with a flag of 4 (bad) are no longer
included in the data file. While the fCO2 value of Equ measurements flagged
3 are questionable, the xCO2 values should be considered good (2). For
details about the system see the master readme file.

2. Any values outside the range of the standards (289.06, 370.90, & 411.42
ppm) should be considered approximate (within 5 ppm). While individual data
points above 411 or below 289 may not be accurate, the general trends should
be indicative of the seawater chemistry.

3.Salinity readings from the ship's TSG were bad. Readings from the
Seabird Micro TSG in the Hydro Lab sink were used instead. This was
connected to the old pCO2 analytical system which was running concurrently
with the new system and thus was recorded at lower frequency than either
the data from the ship's computer system (SCS) or the new GO system (1 every
4 - 4-1/2 minutes vs. 1 per minute for SCS and ~ 1 every 2-1/2 minutes for
the GO system). The Micro TSG data were first merged into the data from
the ship's computer system (SCS) with a 4-minute offset to account for the
time it took seawater to travel from the bow intake to the Hydro Lab. This
left numerous gaps in the SCS data file which contains 1-minute averages.
Missing values in the Micro TSG data were interpolated. The Micro TSG data
was then merged into the GO system data.

4. The uncontaminated seawater system (UCS) was shut down twice while
tracers were being injected as part of the Southern Ocean Gas Exchange
Experiment. The first shutdown occurred from 3/7 at 23:49
to 3/8 at 21:27. The second was from 03:58 to 15:18 on 3/21. Good and
questionable (flags 2 & 3) air values during these periods have been
retained in the data file.

5. The system was shut down at various times. The longest of these was
from 23:51 on 4/4 to 14:30 on 4/5. Other shut downs were for 1:06 on 3/8 at
01:27, for 0:15 on 3/8 at 21:12, for 1:04 on 3/19 at 19:47, for 0:37 on 4/5
at 20:46, and for 6 minutes (0:06) on 4/8 at 16:32.

6. There are 2 short gaps in the SCS data file on 3/31 from 23:24 to
23:36 and from 23:40 to 23:57. From 23:24 to 23:57 there is no SST data.
Equ samples in this period have been retained and flagged as 4 because the
xCO2 values appear to be good although no fCO2 computations were done.

7. The GPS feed to the GO system failed on 4/5 at 20:48 and remained
off until 4/8 at 16:38. Times of measurements during this period were
derived from the PC time, which started 1 minute 49 seconds faster than
GPS and was 3 minutes 52 seconds fast when the GPS came back on. A linear
interpolation was done and 1:49 to 3:52 was added to PC time to produce a
new GPS time. Latitudes and longitudes during this period were merged
from the SCS data.

8. There were roughly 200 individual dropouts in equilibrator
temperature that were interpolated. About 95% of these were flagged as
2 (good) after examining the fCO2 values. The rest were flagged as 3
(questionable). The subflag field gives the reason for flagging points
as questionable.

9. Offset between the ship's intake and the system in the Hydro Lab
was problematic. Normally, there is an approximate 3-4 minute delay
between water arriving at the intake and water arriving in the Hydro
Lab. At the beginning of the cruise this held true but by the end, the
temperature values in the Hydro Lab were actually leading the values
for SST from the ship's intake. Consequently, no offset was applied
when merging data from the SCS.

10. The ship's fluorometer was turned off during this cruise to
reduce the drain on the UCS because many groups were drawing water
from it and one of them had their own multi-spectral fluorometer.
Therefore no fluorometer data is included in the data file.

For questions or comments contact:
Bob Castle
4301 Rickenbacker Causeway
Miami, FL 33149
305-361-4418
robert.castle@noaa.gov