Atlantic Oceanographic & Meteorological Laboratory Global Carbon Cycle

M/V Skogafoss Master Readme File

The information presented in this file is applicable to all the data sets collected 
on the M/V Skogafoss that are presented at:

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 insure that the AOML ocean carbon group receives 
fair credit for its work. Please consult with us prior to use so we can insure that the 
quality and limitations of the data are accurately represented.

Platform Information:

In March 2014, 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 Skogafoss.  The Bigelow operates in the North Atlantic Ocean.

Vessel Name: M/V Skogafoss

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 Division
     4301 Rickenbacker Causeway
     Miami, FL 33149

Contact persons for this dataset:

     Kevin Sullivan and Denis Pierrot
     NOAA/AOML/Ocean Chemistry Division
     4301 Rickenbacker Causeway
     Miami,  FL 33149


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 (February 2011-present)
         CO2 resolution:  0.01 umol/m
         CO2 accuracy: ± 1 ppm at 350 ppm
         Pressure resolution:  0.02 hPa
         Internal pressure transducer accuracy: ± 1.2 hPa 
            (manufacturer specifications: ±0.1% FS, where FS = 0-1150 hPa) 

Equilibrator Pressure:
   Setra model 239, differential pressure (February 2011-present) 
         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 (February 2011-present) 
         Accuracy: ± 0.025°C

Sea Surface Salinity and Temperature (maintained by ship):
   SeaBird model SBE-45 (February 2011-present)
         Temperature resolution:  0.0001°C 
         Temperature accuracy:   ± 0.002°C
         Salinity resolution:     0.0002 ‰
         Salinity accuracy:      ± 0.005 ‰

   SeaBird model SBE-38 (February 2011-present)
         Temperature resolution:  0.00025°C 
         Temperature accuracy:    ± 0.001°C

Atmospheric Pressure (maintained by ship):
   Vaisala model PTB220  barometer (February 2011 - present)
         Resolution:  0.01 hPa
         Accuracy:  ± 0.15 hPa


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 - 550 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 P. Keeling, which are directly traceable to the WMO scale.  A 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.

The system also measures the CO2 content of the atmospheric air, which is drawn from an 
inlet on the bow mast.  Atmospheric air is constantly being pulled (6 liters/min maximum 
flow) through ~100 feet of tubing (3/8" OD Dekoron) to the analytical system located in 
the dry lab.  The flushing rate of the LI-COR analyzer during ATM analyses is ~60-150 ml/min

The dedicated scientific seawater system has two inlets (3m and 5m depth), with the deeper 
inlet only being used occasionally in rougher weather.  A remote temperature sensor (SBE38) 
is located near the sea water pump in the engine room for in-situ sea surface temperature 
(SST) measurements.  A thermosalinograph (SBE45) and a fluorometer are located next to the 
CO2 instrument in the dry lab.  The seawater travels from the inlet to the CO2 instrument 
in less than one minute.

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 ~0.6 L water reservoir and a ~0.8 L gaseous headspace.  Water flow rate 
is ~1.2-1.8 L/min.  The rate that the headspace gas is recirculated through the analyzer 
during EQU analyses is ~60 - 150 ml/min. 

The ship provides serial connections for GPS locations and times and for an array of 
oceanographic and meteorological parameters (e.g. SST, salinity, atmospheric pressure).  
These supporting data are appended to the CO2 data records in real-time.  The CO2 data 
is transmitted back to land each day.

A typcial 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 hours and 
a full day contains over 500 analyses of the equilibrator headspace.


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, 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


   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 entering the ship 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.


List of variables included in this dataset:

COL  HEADER                       EXPLANATION

1.   Group_Ship                   AOML_Skogafoss

2.   Cruise_ID                    dependent upon expedition's name

3.   JD_GMT                       Decimal year day

4.   DATE_UTC_ddmmyyyy            UTC Date

5.   TIME_UTC_hh:mm:ss            UTC Time

6.   LAT_dec_degree               Latitude in decimal degrees (negative
                                  values are in southern hemisphere)

7.   LONG_ dec_degree             Longitude in decimal degrees (negative
                                  values are in western latitudes)

8.   xCO2_EQU_ppm                 Mole fraction of CO2 in the equilibrator
                                  headspace (dry) at equilibrator
                                  temperature, in parts per million

9.   xCO2_ATM_ppm                 Mole fraction of CO2 in outside air (dry), 
                                  in parts per million

10.  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

11.  PRES_EQU_hPa                 Barometric pressure in the equilibrator 
                                  headspace, in hectopascals (1 hPa = 1 millibar)

12.  PRES_ATM@SSP_hPa             Pressure measured by outside barometer,
                                  corrected to sea level, in hectopascals 

13.  TEMP_EQU_C                   Water temperature in equilibrator, in 
                                  degrees centigrade

14.  SST_C                        Sea surface temperature from the ship's 
                                  remote temperature sensor, in degrees centigrade 
                                  [interpolated, see note below]

15.  SAL_permil                   Salinity from the thermosalinograph
                                  (SBE45), on the Practical Salinity Scale

16.  fCO2_SW@SST_uatm             Fugacity of CO2 in sea water, in 
                                  Microatmospheres (100% humidity)

17.  fCO2_ATM_interpolated_uatm   Fugacity of CO2 in air corresponding to the 
                                  interpolated xCO2, in microatmospheres
                                  (100% humidity)

18.  dfCO2_uatm                   Sea water fCO2 minus interpolated air fCO2, 
                                  in microatmospheres
19.  WOCE_QC_FLAG                 Quality control flag for fCO2 values
                                  (2 = good value, 3 = questionable value)

20.  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.


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.

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