Dataset: Underway CO2

The following fields have been QC’ed by the UNH Coastal Ocean Observing Center:
year
day_of_year
secDay
lat
lon
speed
Water_temp
Salinity
xCO2_w
fCO2_w
xCO2_a
fCO2_a

The following fields came at least partly from National Buoy Data Center data, and cannot be verified:
pressure_atm
air_temp

A shipboard flow-through system was used to continuously measure physical and chemical properties of surface water. Seawater temperature and salinity were measured using a Sea-Bird SBE 45 thermosalinograph, pumped at 1.5 L/min. Tests using a handheld temperature and salinity sensor (YSI 85, YSI Instruments Yellow Springs OH), aboard the R/V Gulf Challenger indicate temperature differences are less than 0.01 degree Celsius between equilibrator outflow and intake sea surface water. Therefore, no temperature correction is applied to data taken aboard the RV Gulf Challenger. A temperature offset was observed between the sea surface temperature measured by the continuous-flow SBE-45 and that measured at the water surface by a SBE-37 thermosalinograph deployed as part of a profiling package. For each estuary survey, the average temperature offset between the continuous-flow and profiler sea-surface temperature was removed, to bring the continuous-flow temperature into agreement with in-situ sea surface temperature. The SBE-45 thermosalinograph sensor received annual manufacturer calibrations, but was not calibrated in the field against discrete measurements.

Flow to the shipboard flow-through system was also pumped to an equilibrator, similar to that described by Wanninkhof and Thoning (1993), but consisting of three Plexiglas chambers instead of a single chamber. Equilibrated air was drawn out of the third chamber, while ambient air was drawn into the first chamber and passed through the second and third chambers, equilibrating with the pumped water supply at each step. Equilibrated air was drawn at 100 mL/min through tubing containing a Nafion selectively permeable membrane (Perma Pure, Toms River NJ) with a counter-flowing stream of dry nitrogen, which dried the sample gas stream of water vapor. Due to the short run of tubing between the water source for both the continuous-flow system and the gas equilibrator, no water temperature difference was observed between that measured by the continuous-flow SBE-45 and the outflow from the equilibrator (measured with a handheld meter- YSI Yellow Springs, Ohio- manufacturer accuracy +/- 0.2 degrees C). Temperature from the continuous-flow SBE-45 was used in sea-surface temperature corrections during the calculation of pCO2. After drying, the sample was pumped to a non-dispersive infrared gas analyzer (Li-cor, LI-6262 or LI-840), which measured the molar fraction of carbon dioxide (xCO2) of the sample stream. The Li-cor was calibrated several times each survey with pure nitrogen (0 ppm CO2 molar fraction) and one span tank.  Over the study period we employed a succession of span tanks containing a gas mixture with CO2 molar fraction between 819 and 851 ppm (Scott-Marin, Riverside, CA).  Corrections of the data for water vapor pressure and sea surface temperature and conversion from xCO2 to the partial pressure of carbon dioxide (pCO2) were carried out according to standard methods (Dickson et al. 2007).  Atmospheric pCO2 was periodically measured as well while the ship was underway.  Ambient air was drawn from the ship’s bow through a length of tubing and pumped into the non-dispersive infrared gas analyzer described above.  The estimated uncertainty of pCO2 measurements is +/- 3 uatm. All pCO2 data have been banked with the Carbon Dioxide Information Analysis Center (http://cdiac.ornl.gov/oceans/Coastal/unh_ts.html).

CO2 Analyzer: Licor 6262 or Licor 840 infrared (IR) analyzer (Li-cor Biosciences Inc, Lincoln NE).

Drying method:  Nafion selectively permeable membrane drying loop (Perma Pure, Toms River NJ) with counter-flowing nitrogen stream.

Equilibrator type:  Three-chamber fast-rate equilibrator similar to that of Wanninkhof and Thoning (1993, reference below), flow rate 1.5 L/min.

CO2 calibration standards:  Two-point calibration using ultrapure nitrogen and a mixture of CO2 in nitrogen, ranging from 832.3 to 856.1 ppm.  Mixtures of CO2 in nitrogen, analyzed to within +/- 1%, were obtained from Scott-Marrin Inc (Riverside, CA).  These standards are then adjusted based on comparison to a series of standards ranging from 378-515 ppmv obtained from NOAA/ESRL (http://www.esrl.noaa.gov/).

CO2 data calculation: Corrections of the data for water vapor pressure and sea surface temperature and conversion from pCO2 to the fugacity of carbon dioxide (fCO2) were carried out according to the DOE handbook (1994).

CO2 data quality: all data, excepting those from 20040721, have been verified to an accuracy of +/- 2 mmol/mol or uatm.

References:
Dickson, A.G., Sabine, C.L. and J.R. Christian (Eds.). 2007. Guide to best practices for Ocean CO2 Measurements.  PICES Special Publication 3, 191 pp. http://cdiac.ornl.gov/oceans/Handbook_2007.html

DOE (2007) Guide to Best Practices for Ocean CO2 Measurements, A.G. Dickson, C.L. Sabine and J.R. Christian, eds.  PICES Special Publication, 191 pp.

Sea-bird Electronics. 2013. SBE 43 Dissolved Oxygen Sensor- Background Information, Deployment Recommendations, and Cleaning and Storage. (http://www.seabird.com/application_notes/AN64.htm). Accessed 13 September 2013.

Wanninkhof, R. and K. Thoning. 1993. Measurement of fugacity of CO2 in surface water using continuous and discrete sampling methods. Marine Chemistry 44: 189-204. doi:10.1016/0304-4203(93)90202-Y