INTRODUCTION:
The information presented in this file is applicable to all the data sets collected
on the NOAA Ship Gordon Gunter that are presented at:
www.aoml.noaa.gov/ocd/ocdweb/gunter/gunter_introduction.html.
Any temporary changes in this information will be noted in the readme files for
the individual expeditions.
Statement for use of data:
These data are made available to the public and the scientific community in the belief
that their wide dissemination will lead to greater understanding and new insights. The
availability of these data does not constitute publication of the data. We rely on the
ethics and integrity of the user to ensure that the AOML ocean carbon group receives
fair credit for its work. Please consult with us prior to use so we can ensure that the
quality and limitations of the data are accurately represented.
Platform Information:
In 2008, the Ocean Carbon Group at NOAA's Atlantic Oceanographic and Meteorological
Laboratory (AOML) installed an instrument to measure CO2 levels in surface water and
air on the NOAA Ship Gordon Gunter. The Gordon Gunter primarily operates in the Gulf
of Mexico, Atlantic and Caribbean waters.
Vessel Name: Gordon Gunter
Class of Data: Surface ocean carbon dioxide concentrations
Scientists responsible for the technical quality of this pCO2 dataset:
Rik Wanninkhof and Denis Pierrot
NOAA/AOML/Ocean Chemistry and Ecosystems Division
4301 Rickenbacker Causeway
Miami, FL 33149
Rik.Wanninkhof@noaa.gov
Denis.Pierrot@noaa.gov
Contact persons for this dataset:
Kevin Sullivan and Denis Pierrot
NOAA/AOML/Ocean Chemistry and Ecosystems Division
4301 Rickenbacker Causeway
Miami, FL 33149
Kevin.Sullivan@noaa.gov
Denis.Pierrot@noaa.gov
COMPONENT SPECIFICATIONS and ACCURACIES:
The accuracies of all components, when operating optimally, are such that the calculated
seawater fCO2 has an accuracy of 2 uatm or better and the calculated mole fraction of
CO2 (XCO2) in air has an accuracy of 0.1 uatm.
Infrared Analyzer:
LI-COR model 6262 (2008 - May 2010)
Licor_6262_Manual.pdf
CO2 resolution: 0.01 umol/m
CO2 accuracy: ±1 ppm at 350 ppm
Internal pressure transducer accuracy: ±1.2 hPa
(manufacturer specifications: ±0.1% FS, where FS = 0-1150 hPa)
LI-COR model 7000 (August 2010 - current)
Licor_7000_Manual.pdf
CO2 resolution: 0.01 umol/m
CO2 accuracy: ±1% nominal
Internal pressure transducer accuracy: ±1.2 hPa
(manufacturer specifications: ±0.1% FS, where FS = 0-1150 hPa)
Equilibrator Pressure Transducer:
Setra model 239, differential pressure (2008 - current)
http://www.setra.com/ProductDetails/model_239.htm
Resolution: 0.01 hPa
Accuracy: ±0.052 hPa
(manufacturer specifications: ±0.14% FS, where FS = ±7.5 inches WC)
The absolute pressure of the equilibrator headspace reported in data files is the sum
of the infrared analyzer pressure and the differential pressure from the pressure
transducer attached to the equilibrator.
Equilibrator Temperature:
Hart model 1521 (August 2010 - current)
http://www.testequipmentdepot.com/hart/pdfs/1521_1522.pdf
Resolution: 0.001°C
Accuracy: ±0.025°C
Atmospheric Pressure:
Druck model RPT 350 (2008 - May 2010)
http://www.ge-mcs.com/en/pressure-and-level/transducerstransmitters/rtp350.html
Resolution: 0.01 hPa
Accuracy: ±0.08 hPa
(manufacturer specifications: ±0.02% FS, where FS = 400 hPa)
RMYoung model 61201 (maintained by ship) (August 2010 - present)
http://jsinstruments.com/files/Model%2061201%20Barometric%20Pressure%20Sensor.pdf
Resolution: 0.01 hPa
Accuracy: ±0.5 hPa
Sea Surface Salinity (maintained by ship):
SeaBird model SBE-21 (2008 - 2013)
http://www.seabird.com/sbe21-seacat-thermosalinograph
Temperature Accuracy: ± 0.01 °C
Salinity Accuracy: 0.05‰
Sea Surface Salinity (maintained by ship):
SeaBird model SBE-45 (2014 - present)
http://www.seabird.com/sbe45-thermosalinograph
Temperature Accuracy: ± 0.002 °C
Salinity Accuracy: 0.005‰
Sea Surface Temperature (maintained by ship):
Furuno model T2000 (2008 - May 2013)
Furuno T2000 Manual pdf
Temperature Resolution: 0.01 °C
Temperature Accuracy: ±0.02 °C
Sea Surface Temperature (maintained by ship):
SeaBird model SBE-38 (June 2013 - present)
http://www.seabird.com/pdf_documents/manuals/38_013.pdf
Temperature Resolution: 0.00025°C
Temperature Accuracy: ±0.001°C
INSTRUMENT DESCRIPTION and CONFIGURATION:
The general principle of operation of the instrument can be found in Wanninkhof
and Thoning (1993), Ho et al. (1995), Feely et al. (1998), and Pierrot et al. (2009).
Seawater flows through an equilibrator chamber where CO2 exchanges between water and
the air above it. Small changes in seawater CO2 concentration are rapidly translated
into changes in CO2 concentration in the air of the chamber (headspace). The mole
fraction of CO2 in the headspace gas is measured using a non-dispersive infrared
(NDIR) analyzer from LICOR®.
The effects of water vapor on the signal can be compensated for by the analyzer
(LiCOR model 6262) but are also kept to a minimum by removing as much water as
possible. The water is first condensed out of the gas stream by cooling to ~5 °C
using a thermoelectric device. Then water is further removed using Nafion® gas dryers
before the sample gas reaches the IR analyzer. The counterflow gas in the dryer is
pre-dried outside air. Typical water content of the gas streams are less than 3
millimoles/mole with approximately 90% of the water removed.
The infrared analyzer is calibrated regularly using four standard gases (240 - 520 ppm
CO2 in air) from Scott-Martin Inc. (Riverside, CA). Before and after use in the field,
the standards are calibrated using primary reference gases from the laboratory of
Dr. Charles D. Keeling, which are directly traceable to the WMO scale. Any value
outside the range of the standards should be considered approximate (+/-5 ppm).
While individual data points above the highest standard or below the lowest standard
may be less accurate, the general trends would be indicative of the seawater chemistry.
The exact concentrations of the standards used on a particular cruise are listed in the
individual readme file.
The system also measures the CO2 content of the outside air, which is drawn from an inlet
on the box mast approximately 15 meters above the water. Atmospheric air is constantly
being pulled (6 liters/min maximum flow) through ~60 meters of tubing (1 cm OD Dekoron)
to the analytical system located in the Hydro Chem lab. The seawater is drawn from the
ship's flowing seawater line, which also feeds a thermosalinograph and Turner fluorometer.
The three CO2 standard cylinders come from Scott-Marrin, Inc., and are calibrated with
primary standards that are directly traceable to the WMO scale. The zero gas is ultra-high
purity air. Any value outside the range of the standards should be considered approximate
(+/-5 ppm). While individual data points above the highest standard or below the lowest
standard may be less accurate, the general trends would be indicative of the seawater
chemistry. The standards used on a particular cruise are listed in the individual
readme file.
When the first analytical system was in use (2008 - May 2010), a 'Deck Box' containing
a high precision pressure transducer, a GPS and Iridium satellite modem was located outside
and several decks above the system on part of the ship's superstructure protected from
severe weather. The instrumental system via the deck box recorded the atmospheric pressure
and the position of the ship. The measured pressure is corrected for the height of the
barometer above the sea surface with the addition of dgh/u - where d is atmospheric density
(1.2 kg/m3), g is gravitational acceleration (9.8 m/sec2), h is height of the barometer above
the sea surface, and u is the conversion factor from pascals to desired pressure units. The
estimated height of 15 meters results in a change in the barometric pressure of approximately
of 1.8 mbar. When the second analytical system was is use, the barometric pressure and ship
position was obtained from the ship's sensors.
A typical sequence of continuous analyses was:
STEP TYPE REPETITIONS
1 Standards (all four) 1
2 ATM 5
3 EQU 60
With these settings, a complete set of standards and the atmospheric analyses are done every
3 hours and a full day contains about 480 analyses of the equilibrator headspace.
CALCULATIONS:
The measured xCO2 values are linearly corrected for instrument response using the standard
measurements (see Pierrot et al., 2009).
For ambient air and equilibrator headspace the fCO2a or fCO2eq is calculated assuming
100% water vapor content:
fCO2 = xCO2 P (1-pH2O) exp[(B11+2d12)P/RT]
where fCO2 is the fugacity in ambient air or equilibrator headspace, pH2O is the water
vapor pressure at the sea surface or equilibrator temperature, P is the equilibrator or
outside atmospheric pressure (in atm), T is the SST or equilibrator temperature (in K)
and R is the ideal gas constant (82.057 cm^3·atm·deg^-1·mol^-1). The exponential term
is the fugacity correction where B11 is the first virial coefficient of pure CO2
B11 = -1636.75 + 12.0408 T - 0.0327957 T^2 + 3.16528E-5 T^3
and
d12 = 57.7 - 0.118 T
is the correction for an air-CO2 mixture in units of cm^3·mol^-1 (Weiss, 1974).
The fugacity as measured in the equilibrator is corrected for any temperature difference
between sea surface temperature and equilibrator chamber using the empirical correction
outlined in Takahashi et al. (1993).
fCO2(SST) = fCO2(teq)exp[0.0423(SST-teq)]
where fCO2(SST) is the fugacity at the sea surface temperature and fCO2(teq) is the fugacity
at the equilibrator temperature. SST and teq are the sea surface and equilibrator temperatures
in degrees C, respectively.
The amount of time between the sea water passing by the SST sensor and the water flowing
through the equilibrator is estimated before assigning an SST value to each analysis.
The patterns in the temperature records for the equilibrator and for SST over time are
compared, and a time offset that optimizes the match of these patterns is determined.
The time offset is applied to the SST measurements. A linear interpolation between the
time-adjusted SST data yields the SST value assigned to each CO2 analysis and used in
the fugacity calculations.
DATA FILE STRUCTURE:
List of variables included in this dataset:
COLUMN HEADER EXPLANATION
1. EXPOCODE Expedition code, where '33GG'
is the NODC ship identifier,
and YYYYMMDD is the UTC date that
the ship starts the expedition
2. Group_Ship AOML_GordonGunter, (if present)
3. Cruise_ID dependent upon expedition's name,
(if present)
4. YD_UTC Decimal year day
5. DATE_UTC_ddmmyyyy UTC Date
6. TIME_UTC_hh:mm:ss UTC Time
7. LAT_dec_degree Latitude in decimal degrees (negative
values are in southern hemisphere)
8. LONG_ dec_degree Longitude in decimal degrees (negative
values are in western latitudes)
9. xCO2_EQU_ppm Mole fraction of CO2 in the equilibrator
headspace (dry) at equilibrator
temperature, in parts per million
10. xCO2_ATM_ppm Mole fraction of CO2 in outside air (dry),
in parts per million
11. xCO2_ATM_interpolated_ppm xCO2 in outside air associated with each
water analysis. These values are
interpolated between the bracketing
averaged good xCO2_ATM analyses, in parts
per million
12. PRES_EQU_hPa Barometric pressure in the equilibrator
headspace, in hectopascals (1 hPa = 1 millibar)
13. PRES_ATM@SSP_hPa Pressure measured by outside barometer,
corrected to sea level, in hectopascals
14. TEMP_EQU_C Water temperature in equilibrator, in
degrees centigrade
15. SST_C Sea surface temperature from the ship's
temperature sensor, in degrees centigrade
[interpolated, see note above]
16. SAL_permil Salinity from the thermosalinograph
(SBE45), on the Practical Salinity Scale
17. fCO2_SW@SST_uatm Fugacity of CO2 in sea water, in
Microatmospheres (100% humidity)
18. fCO2_ATM_interpolated_uatm Fugacity of CO2 in air corresponding to the
interpolated xCO2, in microatmospheres
(100% humidity)
19. dfCO2_uatm Sea water fCO2 minus interpolated air fCO2,
in microatmospheres
20. WOCE_QC_FLAG Quality control flag for fCO2 values
(2 = good value, 3 = questionable value)
21. QC_SUBFLAG Quality control sub flag for fCO2 values
provides explanation for atypical data,
when QC_FLAG = 3
The quality control flags are provided as an aid to the interpretation of the CO2 data.
Stringent minimum and maximum values for numerous parameters (e.g.temperature difference
between the equilibrator temperature and SST) have been established by CO2 researchers
(see Pierrot et al., 2009). These ranges were chosen so that if each parameter were
within their stringent range, the resulting CO2 data would almost certainly be good.
If a parameter is outside its range or if a parameter is estimated from surrounding good
values, the quality flag of that data record is set to 3 (questionable value). The
resulting CO2 data could be good; however, investigators should determine whether these
data are valid for their purposes.
REFERENCES:
DOE (1994). Handbook of methods for the analysis of the various parameters of the carbon
dioxide system in sea water; version 2. DOE.
Feely, R. A., R. Wanninkhof, H. B. Milburn, C. E. Cosca, M. Stapp and P. P. Murphy (1998).
A new automated underway system for making high precision pCO2 measurements onboard
research ships. Analytica Chim. Acta 377: 185-191.
Ho, D. T., R. Wanninkhof, J. Masters, R. A. Feely and C. E. Cosca (1997). Measurement of
underway fCO2 in the Eastern Equatorial Pacific on NOAA ships BALDRIGE and
DISCOVERER, NOAA data report ERL AOML-30, 52 pp., NTIS Springfield,
Pierrot, D., C. Neill, K. Sullivan, R. Castle, R. Wanninkhof, H. Luger, T. Johannessen,
A. Olsen, R. A. Feely, and C. E. Cosca (2009). Recommendations for autonomous
underway pCO2 measuring systems and data-reduction routines. Deep Sea Research II,
56: 512-522.
Wanninkhof, R. and K. Thoning (1993) Measurement of fugacity of CO2 in surface water using
continuous and discrete sampling methods. Mar. Chem. 44(2-4): 189-205.
Weiss, R. F. (1970). The solubility of nitrogen, oxygen and argon in water and seawater.
Deep-Sea Research 17: 721-735.
Weiss, R. F. (1974). Carbon dioxide in water and seawater: the solubility of a non-ideal gas.
Mar. Chem. 2: 203-215.
Weiss, R. F., R. A. Jahnke and C. D. Keeling (1982). Seasonal effects of temperature and
salinity on the partial pressure of CO2 in seawater. Nature 300: 511-513.
Takahashi, T., J. Olafsson, J. G. Goddard, D. W. Chipman, and S. C. Sutherland (1993).
Seasonal variation of CO2 and nutrients in the high-latitude surface oceans: a comparative
study, Global Biogeochem. Cycles, 7, 843-878.