About the Website

Ronald H. Brown Master Readme

Introduction

The information presented in this file is applicable to all the data sets collected on the NOAA ship Ronald H. Brown that are presented on this page.

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 July of 1997, the Ocean Carbon Cycle Group (OCC) at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) installed an automated instrument in the Brown’s hydro laboratory to measure the pCO₂ concentration in surface sea water. The Ronald H. Brown is deployed for various multidisciplinary environmental research projects around the globe. While the ship conducts the research that is the focus of a cruise, the pCO₂ instrument makes regular measurements of the outside air and of the water flowing through the dedicated scientific sea water system. The pCO₂ analytical system is regularly maintained by the survey technician aboard the Brown. Occasionally, members of the OCC group will sail with the ship for the targeted research and perform more extensive maintenance or upgrades. Three versions of pCO₂ analytical system have been installed on the Brown.

Vessel Name: Ronald H. Brown

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 6251 (2000 – February 2008)
Licor_6251_Manual.pdf
CO₂ resolution: 0.01 umol/m
CO₂ accuracy: ±1ppm at 350 ppm
Pressure resolution: 0.02 hPa
Internal pressure transducer accuracy: ±1.2 hPa (1.2 hPa = 1.2 mbar)
(manufacturer specifications: ±0.01% FS, where FS = 0-1150 hPa)

LI-COR model 6262 (1997 – 1999; March 2008 – present)
Licor_6262_Manual.pdf
CO₂ resolution: 0.01 umol/m
CO₂ accuracy: ±1ppm at 350 ppm
Pressure resolution: 0.02 hPa
Internal pressure transducer accuracy: ±1.2 hPa (1.2 hPa = 1.2 mbar)
(manufacturer specifications: ±0.01% FS, where FS = 0-1150 hPa)

Setra model 270 (June 2012 – present)
http://www.setra.com/ProductDetails/270_Baro.htm
Resolution: 0.015 hPa
Accuracy: ±0.15 hPa

For the LI-COR analyzers installed with the first instrumental systems, the internal pressure transducers were used for processing the internal raw signal. After the third instrumental system was installed, an external Setra 270 pressure transducer was connected to the LI-COR 6262 for processing its internal raw signal.

Equilibrator Pressure:
Setra model 239, differential pressure (March 2008 – 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 pressure of the laboratory space was measured with LI-COR analyzers under stopped flow conditions, and this pressure was taken as the headspace pressure in the vented equilibrator. With the third instrumental system, a differential pressure sensor (Setra 239) was attached to the equilibrator. Both the LI-COR analyzer and the differential pressure sensor were vented to the laboratory space. The absolute pressure of the equilibrator headspace reported in data files for the third analytical system is the sum of the Setra 239 differential pressure and the absolute pressure measured at the analyzer exit under stopped flow condition.

Equilibrator Temperature:
Hart model 1560 (1997 – February 2008)
http://www.instrumart.com/assets/1560_manual.pdf
with Thermometrics AS125 thermister
http://www.ge-mcs.com/download/temperature/sases.pdf
Accuracy: ±0.0013°C
Resolution: 0.0001°C
The Hart 1560 was used to annually calibrate the 1000 ohm thermistor. Based on reproducibility of the annual calibrations, the equilibrator water temperatures were believed accurate to ± 0.02°C.

Hart model 1521 (March, 2008 – present)
http://www.instrumart.com/assets/1560_manual.pdf
Resolution: 0.001°C
Accuracy: ± 0.025°C

Sea Surface Temperature and Salinity:
YSI 600R Sonde (1997 – 2003)
http://science.nature.nps.gov/im/units/sean/AuxRep/FQ/FQ_YSI_6-series_sondes.pdf
Temperature resolution: 0.01°C
Temperature accuracy: ±0.15°C
Salinity resolution: 0.01‰
Salinity accuracy: ±1% of reading or 0.1‰ whichever is greater

SeaBird model SBE-45 (2004 – 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-21 (1997 – present; maintained by ship)
http://www.seabird.com/pdf_documents/manuals/21_026.pdf
Temperature resolution: 0.001°C
Temperature accuracy: ±0.01°C
Salinity resolution: 0.002‰
Salinity accuracy: ±0.05‰

Atmospheric Pressure (maintained by ship):
Vaisala model PTB330 barometer (10 meters above sea level; maintained by ship)
http://www.vaisala.com/Vaisala%20Documents/User%20Guides%20and%20Quick%20Ref%20Guides/
PTB330_User_Guide_in_English.pdf
Resolution: 0.01 hPa
Accuracy: ±0.10 hPa

Instrument Description and Configuration

The general principle of instrumental design can be found in Wanninkhof and Thoning (1993), Ho et al. (1995), and Feely et al. (1998) and Pierrot et al. (2009). The concentration of CO₂ in the headspace gas was measured using the adsorption of infrared (IR) radiation by the CO₂ molecule. The LI-COR® analyzer passed IR radiation through two cells. The reference cell was constantly flushed with a gas of known concentration. For the first two instrumental systems (1997 – February, 2008), the reference gas contained CO₂ with concentration close to outside air. For the
third instrumental system from General Oceanics (March, 2008 – present), the reference gas was CO₂-free air. The sample cell was flushed with the gas of interest (standard, outside atmosphere, or headspace gas from equilibration chamber). A vacuum-sealed, heated filament was the broadband IR source. The IR radiation alternated between the two cells via a chopping shutter disc spinning at 500 Hertz. An optical filter selected an adsorption band specific for CO₂ (4.26 micron) to reach the detector. The solid state (lead selenide) detector was kept at -5 degrees C for excellent stability and low signal noise (less than 0.2 ppm).

The effects of water vapor on the analyses were kept to a minimum by removing as much water as possible before the sample gas reached the IR analyzer. Water vapor was first condensed out of the sample gas stream by cooling to approximately 6 degrees C. The first two instrumental systems used glass condensers and a circulating water bath. The third instrumental system used a thermoelectric device and milled aluminum block. Water vapor was further removed by pushing the sample gases through a canister of magnesium perchlorate in the first two systems or through NafionÆ gas dryers in the third system. The counterflow gas in the dryers was pre-dried outside air at reduced pressure. Typical water content of the analyzed gases was less than 3 millimoles/mole with approximately 90% of the water being removed.

The infrared analyzers were calibrated regularly using three or four standard gases (200 – 550 ppm) from the ESRL laboratory in Boulder, CO, 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 most of the individual readme files. The flushing rate during standard (STD) analyses was 50 – 80 ml/min.

All three instrumental systems measured CO₂ in atmospheric air. Outside air was constantly being pulled (6 liters/min maximum) from an inlet on the bow mast through approximately 75 meters of tubing (1 cm OD Dekoron) to the analytical system. The flushing rate of the LI-COR analyzer during outside air (ATM) analyses was 60 – 150 ml/min.

A dedicated scientific seawater inlet at an approximate depth of 5 meters is the source of water flowing throughout the ship. For the first (1997 – 1999) and second (2000 – February, 2008) instrumental systems, seawater was pushed through a large showerhead equilibrator (8 L of water under 16 L of headspace) at 10 L/min. For the third instrumental system (March, 2008 – present), seawater was pushed
through a smaller spray head equilibrator (0.5 L of water under 0.7 L of headspace) at 2 L/min. The initial equilibrator in the third system (March, 2008 – February, 2010) was fabricated using a filter housing (ColeParmer, U-010509-00). In March, 2010, an equilibrator with a water jacket for better thermal stability was installed and was only slightly larger (0.6 L of water under 0.8 L of headspace). During EQU analyses the headspace gas was pulled from the equilibrator chamber, pushed through the analyzer, and returned to the equilibrator at 60 – 150 ml/min.

In the first two analytical systems, the water temperature in the equilibrator as measured with a 20-cm long, 1000 Ohm thermistor that was fully submerged via a port in the bottom of the chamber. The thermistor was calibrated annually against a Hart Scientific 1560 Black Stack module with platinum resistance NIST traceable thermistor. Based on reproducibility of the annual calibrations, the temperatures were believed accurate to ± 0.02 °C. In the third analytical system, the water temperature was measured with a Hart 1521 thermometer whose probe was submerged about 5 cm.

The headspace of the equilibrator for the first two instrumental systems was vented to the surrounding laboratory space through an internal baffle and then through an external open enclosure that was constantly flushed with outside air. For the third instrumental system, the main equilibrator was vented through a smaller secondary equilibrator. Any laboratory gas drawn into the equilibrators was ‘pre-equilibrated’ with flowing sea water in the secondary equilibrator before reaching the main equilibrator.

In addition to the pCO₂ analytical system, the OCC group maintained a thermosalinograph (TSG) in the hydro laboratory. The first TSG was a YSI 600R sonde that also contained a dissolved oxygen sensor. In 2004, a Sea-Bird (SBE) model 45 TSG replaced the YSI TSG at the manifold distributing seawater in the hydro laboratory. In March, 2008, the SBE45 was relocated to immediately next to the pCO₂ analytical system. After a cruise, the salinity measured by the OCC TSG was compared with the values from the ship?s SBE21, and the salinity measurements of higher quality were used in the calculations of fCO₂. An Aanderaa oxygen sensor was added to the pCO₂ instrumental system in April 2013.

These additional sensors along with various sensors maintained by the ship (e.g. GPS, SST, fluorometer, wind speed and direction) were logged in real time with the CO₂ measurements. These data were used in processing the CO₂ data, especially during the quality assurance procedures. Not all of the logged data is presented in the posted data files. Requests for the ancillary data may be submitted to Mr. Bob Castle, whose contact information is listed above.

The first two instrumental systems ran an hourly cycle comprised of the three STD gases, four EQU analyses, three ATM analyses, and then four EQU analyses. The infrared analyzer was flushed with the chosen gas at least four minutes before the flow was stopped. After waiting 10 seconds for the analyzer to reach ambient pressure, most of the raw data was recorded. This data included the infrared analyzer, the numerous sensors internal to the CO₂ instrumental systems, and the ship?s sensors chosen for inclusion in the data feed to the instrumental system. The timing of the hourly cycle was based on the computer time internal to the instrumental system. The processed data is associated and posted with the UTC time logged from the ship’s GPS.

The third instrumental system ran a sequence comprised of the four STD gases, five ATM analyses, and then at least fifty EQU analyses. When data acquisition was initiated, the zero and span of the analyzer were set with CO₂-free air and the highest standard gas, respectively. At least six complete sequences (4 STD, 5 ATM, 50-65 EQU) were done before the zero and span of the analyzer was set again. For the first analysis of any type, the analyzer was flushed for four minutes before the 10 seconds of stopped flow and data recording. For repeated analyses of sample gas (i.e. ATM or EQU), the analyzer was flushed 2 – 2.5 minutes. The analytical sequence was configured to calibrate the analyzer every 2.5 – 3.3 hours and to set the zero and span at least once a day.

Calculations

For data collected with the first two instrumental systems (1997 – February, 2008), the xCO₂ values of ambient air and equilibrated headspace gas are calculated by fitting a second-order polynomial through the hourly averaged response of the detector versus xCO₂ values of the standards. For data collected with the third instrumental system (March, 2008 – present), 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 calculation for the fugacity at SST involves a temperature correction term for the changes in fCO₂ due to changes in water temperature while the water passes through the
pump and connecting pipe within the ship. The water in the equilibrator was typically 0.2 °C warmer than SST. For all data before 2006, the empirical temperature correction from equilibrator temperature to SST is outlined in Weiss et al. (1982):

dln(fCO₂) = (Teq-SST)(0.0317 – 2.7851E-4 Teq – 1.839E-3 ln(fCO₂eq))

where dln(fCO₂) is the difference between the natural logarithm of the fugacity at Teq and at SST. Teq and SST are the equilibrator and sea surface temperatures in degrees C,
respectively.

Starting in 2006, the fugacity as measured in the equilibrator is corrected for the temperature difference between equilibrator chamber and SST 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(eq) is the fugacity at the equilibrator temperature.

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 (1997-2003)

List of variables included in this dataset:

Data File Structure (2004 – February 2008)

Starting in 2004, the appearance of the web page and the data file structure were changed. The current charts for the individual cruises contain color-coded cruise tracks representing the fCO₂ in surface water. The new data files include auxiliary meteorological, oceanographic, and ship’s navigation data; plus the ‘Date’ field has a new format. These changes follow the recommendations of the International Ocean Carbon Coordination Project (http://ioc.unesco.org/ioccp). The purpose of this project is to standardize measurement techniques and QA/QC procedures, coordinate international ocean carbon observations, and improve accessibility to carbon data sets in order to better meet the needs of the research community. Most of the additional data is taken from the ship’s computer system “as is” with no quality control. Quality controlled meteorological data for the Ron Brown can be obtained from the Center for Ocean-Atmosphere Prediction Studies:
https://www.coaps.fsu.edu//a>.

List of variables included in this dataset:

Data File Structure (2008 – 2009)

When the third instrumental system was installed, the format of the final data file was also changed. The most significant changes were related to ATM analyses. All acceptable ATM analyses are included in the final data file and columns for interpolated ATM xCO₂ and fCO₂ were added.

Extra explanations of the quality of questionable analyses (QC flag =3) are placed in the new QC_SUBFLAG column. The QC flags apply to the fCO₂ results for EQU analyses and to the xCO₂ result for ATM analyses and 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 and a text explanation could be added in the QC_SUBFLAG column. The resulting CO₂ data could be good; however, investigators should determine whether these data are valid for their purposes.

List of variables included in this dataset:

Data File Structure (2010 – present)

Starting with cruise RB1001, several columns of data not directly used in computations of fCO₂ or xCO₂ were removed from the final data file format. Starting in 2013, a column for ExpoCode was added.

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.