INTRODUCTION:
The information presented in this file is applicable to all the data sets collected
on the M/V Selfoss that are presented at:
www.aoml.noaa.gov/ocd/ocdweb/selfoss/selfoss_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 November 2018, 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 M/V Selfoss. The ship operates in the North Atlantic Ocean.
Vessel Name: M/V Selfoss
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:
Denis Pierrot and Kevin Sullivan
NOAA/AOML/Ocean Chemistry and Ecosystems Division
4301 Rickenbacker Causeway
Miami, FL 33149
Denis.Pierrot@noaa.gov
Kevin.Sullivan@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 (November 2018-present)
ftp://ftp.licor.com/perm/env/LI-6262/Manual/LI-6262_Manual.pdf
CO2 resolution: 0.01 umol/m
CO2 accuracy: ± 1 ppm at 350 ppm
Equilibrator Pressure:
Setra model 270, absolute pressure at analyzer exit (November 2018-present)
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)
Setra model 239, differential pressure at equilibrator headspace (November 2018-present)
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 differential pressure from the pressure transducer attached to the equilibrator
and the absolute pressure measured at the analyzer exit to the space surrounding
the instrument.
Equilibrator Temperature:
Hart model 1523 (November 2018-present)
http://www.testequipmentdepot.com/fluke-calibration/pdfs/1523-1524_data.pdf
Accuracy: ± 0.021°C
Resolution: 0.001°C
Sea Surface Salinity and Temperature:
SeaBird model SBE-45 (November 2018-present)
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 (November 2018-present)
http://www.seabird.com/pdf_documents/manuals/38_013.pdf
Temperature resolution: 0.00025°C
Temperature accuracy: ± 0.001°C
Atmospheric Pressure:
Druck model RPT 350 (November 2018-present)
Resolution: 0.01 hPa
Accuracy: ± 0.08 hPa
(manufacturer specifications: ± 0.02% FS, where FS = 400 hPa)
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 (300 - 600 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-Marrin standards are calibrated using
primary reference gases from the laboratory of Dr. Charles P. 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 (+/-100 ppm)should be considered approximate (+/-5 ppm). While
individual data points above the higheststandard or below thelowest 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 from a sea chest under the engine room using a pump
supplied by AOML. Sometimes the port on the sea chest is not flooded, which causes
inconsistent seawater flow. An alternate source of sea water is being investigated.
A remote temperature sensor (SBE38) is located between the sea chest and the pump,
at about 2 m away from the intake. The seawater travels through 15 m of insulated
stainless steel pipe before reaching the pCO2 instrument, a thermosalinograph (SBE45),
and other sensors located one deck above the pump. The water warms 0.2 - 0.5 degree C
during its transit between the sea chest and the pCO2 equilibrator.
Seawater is pushed through a spray head into an equilibration chamber that includes
a water jacket for better thermal stability. The chamber has a 0.6 L water reservoir
and a 0.8 L gaseous headspace. Water flow rate is 1.5 – 3.0 L/min. The rate that
the headspace gas is recirculated through the analyzer during EQU analyses is
60 - 150 ml/min.
The system also measures the CO2 content of ambient air, which is drawn from an inlet on
a mast on the starboard rail above the bridge. Outside air is constantly being pulled
(6 L/min maximum flow) through ~70 m of tubing (1 cm OD Synflex 1300) to the analytical
system located in the engine room. The flushing rate of the LI-COR analyzer during ATM
analyses is 60 - 150 ml/min.
A 'Deck Box' containing a high precision barometer, a GPS, and Iridium satellite modem is
located on a radar mast above the bridge. The instrumental system communicates with the
deck box and records 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 (m),
and u is the conversion factor from pascals to desired pressure units. The estimated
height of 25 meters resulted in a change in the barometric pressure of approximately
3.0 mbar. The CO2 data is transmitted back to land via Iridium satellite each day.
A typical sequence of continuous analyses is:
STEP TYPE REPETITIONS
1 Standards (all four) 1
2 ATM 5
3 EQU 100
A complete set of standards and the atmospheric analyses are done every 4.5 hours and
a full day contains over 500 analyses of the equilibrator headspace. The zero and span
of the analyzer is about once a day.
CALCULATIONS:
The measured xCO2 values are linearly corrected for instrument response using
the standard measurements (see Pierrot et al., 2009).
For the equilibrator headspace the 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 (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 (pending)
is the ICES ship identifier,
and YYYYMMDD is the UTC date that
the ship starts the expedition
2. Group_Ship AOML_Selfoss
3. Cruise_ID dependent upon the UTC date that the ship
starts the expedition
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 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.
