INTRODUCTION:
The information presented in this file is applicable to all the data sets collected
on the M/V Equinox that are presented at:
www.aoml.noaa.gov/ocd/ocdweb/equinox/equinox_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 2015, the Ocean Carbon Group at NOAA's Atlantic Oceanographic and Meteorological Laboratory
(AOML) installed an autonomous instrument to measure CO2 levels in surface water and air
on the M/V Equinox. This installation continues a collaboration among Royal Caribbean
International (RCI), NOAA, and RSMAS that started in 2002.
Vessel Name: Equinox
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
Licor_6262_Manual.pdf
CO2 resolution: 0.01 umol/m
CO2 accuracy: ±1ppm at 350 ppm
External Pressure Transducer attached to analyzer:
Setra model 270
http://www.setra.com/ProductDetails/270_Baro.htm
Resolution: 0.015 hPa
Accuracy: ±0.15 hPa
(manufacturer specifications: ±0.05% FS, where FS = 80-110 kPa)
Differential Pressure Transducer attached to equilibrator:
Setra model 239
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 in the equilibrator.
Equilibrator Temperature:
Hart model 1523
http://www.testequipmentdepot.com/fluke-calibration/pdfs/1523-1524_data.pdf
Resolution: 0.001°C
Accuracy: ±0.021°C
Sea Surface Temperature and Salinity (maintained by other scientists):
SeaBird model SBE-45
http://www.seabird.com/pdf_documents/manuals/45_017.pdf
Temperature resolution: 0.0001°C
Temperature accuracy: ±0.002°C
Salinity resolution: 0.0002‰
Salinity accuracy: ±0.005‰
SeaBird model SBE-38
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 sample analyses are kept to a minimum by removing as
much water as possible. The water is first condensed out of the sample gas stream by
cooling to ~5 °C using a thermoelectric device. Then water is further removed using
Nafion® gas dryers before reaching the IR analyzer. The counterflow gas in the dryer
is pre-dried outside air. Typical water content of the analyzed gas is less than
3 millimoles/mole with approximately 90% of the water being removed.
The infrared analyzer is calibrated regularly using four standard gases (200 - 500 ppm
CO2 in air) from Scott-Martin Inc. (Riverside, CA) and from NOAA ESRL (Boulder, CO).
Before and after use in the field, the Scott-Martin standards are calibrated using primary
reference gases from the laboratory of Dr. Charles D. Keeling, which are directly traceable
to the WMO scale. The ESRL standards are directly traceable to the WMO scale with calibration
of each cylinder before deliver and after use. The zero gas of ultra-high purity air is
analyzed regularly. 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 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.
Sea water is drawn into the ship through a dedicated inlet in the bow very close to the
scientific instruments in the bow thruster room. A temperature sensor (SBE38) is located
between the inlet and the sea water pump, which pushes water through the instruments and
out a dedicated exit valve. The transit from the inlet to the pCO2 equilibrator takes
10 - 15 seconds and the water warms approximately 0.2 degree Celsius.
Seawater is pushed through a spray head into an equilibration chamber that includes a water
jacket for better thermal stability (optional on the General Oceanics model 8050 system).
The chamber had a 0.4 L water reservoir and a 0.6 L gaseous headspace. Water flow rate
is 1.5-2.5 L/min. The rate that the headspace gas is recirculated through the analyzer
during EQU analyses is 70 - 150 ml/min.
The system also measures the CO2 content of ambient air. During the first six cruises,
the air inlet was located at a vent, which occasionally brought outside air to the bow
thruster room. During the trans-Atlantic cruise in May, 2015, the air inlet was repositioned
to the forward-most grated opening in the starboard hull at the mooring deck. With this
new location, the results for the ATM analyses were acceptable and reported. Outside air
is constantly being pulled (6 L/min maximum flow) through ~70 m of tubing (1 cm OD Dekoron)
to the analytical system. The flushing rate of the LI-COR analyzer during ATM analyses is
70 - 150 ml/min.
The GPS position and meteorological parameters are provided to the CO2 analytical system
by the RSMAS Marine Technology Group, who also maintains the scientific sea water
infrastructure. The type and location of meteorological sensors change in accordance
with the preferences of the ship's engineers. Data is transferred within the ship and
between the ship and land via a virtual private network provided by the RCI and RSMAS.
The CO2 data is transmitted back to land each day to monitor the analytical system's
performance.
A typical sequence of continuous analyses is:
STEP TYPE REPETITIONS
1 Standards (all four) 1
2 ATM 5
3 EQU 100
The amount of time between analyses depends on whether the analyses are of the same
type of gas (e.g., STD, ATM, EQU) or not. When switching between different gases, the
connecting tubes and analyzer are flushed for an initial interval called the 'PRE-FLUSH'
time plus an interval called the 'REGULAR FLUSH' time. Between successive measurements
of the same type of gas, the system is flushed for only the 'REGULAR FLUSH' time. The gas
flow is then stopped. After the 'STOP FLOW' time interval, which is 10 seconds for all
analyses, the output of the NDIR analyzer is read. The pre-flush time is set to 180 seconds
and the regular flush time is set to 60 seconds for standard and air analyses. Both the
pre-flush and regular flush times are 120 seconds for equilibrator headspace analyses.
With these settings, a complete set of standards and the atmospheric analyses are done
every 5 hours and a full day contains about 490 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^-1mol^-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 (SBE38) 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 'MLCE'
is the NODC ship identifier,
and YYYYMMDD is the UTC date that
the ship starts the expedition
2. Group_Ship AOML_Equinox, (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
remote 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.
