About the Selfoss

In November 2018, the Ocean Carbon Cycle Group (OCC) at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) installed an automated system to measure surface pCO2 on the M/V Selfoss (OZ2171). The M/V Selfoss is the fourth Volunteer Observing Ship from Eimskip’s fleet of vessels that AOML’s OCC scientists have used to gather data in the North Atlantic Ocean. The previous three ships were the M/V Skogafoss (V2XM) from 2003 to 2007, M/V Reykjafoss (V2FB6) from 2011 to 2014 and M/V Skogafoss (V2EF3) from 2014 to 2018. Originally, the M/V Selfoss would transit between Portland, ME and Reykjavik, Iceland, with occasional extensions to Norway. Unfortunately, the vessel changed route before the installation could be completed, and then the 2020 pandemic prevented us to finalize it up until early 2022. The vessel is now collecting data which, combined with the previous three ships, will provide insights on monthly, seasonal, and longer time scale patterns in a region that is very active hydrologically and thus important for climate change. The installation was greatly facilitated by the cooperation of the ship’s officers and crew, especially the engineers. The data are organized with each 3-week cruise beginning and ending in Portland, Maine. When the ship is underway, the pCO2 instrument takes 5 air and 100 water measurements every 4.5 hours. Data files are sent to AOML via Iridium satellite every day so that the system operation can be monitored. The final data are processed after a cruise is completed and then posted to international databases and to this web site.

Selfoss Master Readme

Introduction

The information presented in this file is applicable to all the data sets collected on the Selfoss that are presented on this webpage. 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 2019, the Ocean Carbon Group at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) installed an autonomous instrument to measure CO₂ levels in surface water aboard the Celebrity Flora, a ship specifically designed for tours in the Galapagos Islands. This installation continues a collaboration among Royal Caribbean International (RCI), NOAA, and
RSMAS that started in 2002.

Vessel Name: Celebrity Flora

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
NOAA/AOML/Ocean Chemistry and Ecosystems Division
4301 Rickenbacker Causeway
Miami, FL 33149
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 (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 ‰

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</span?

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:

References

DOE (1994). OE (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.