Underway pCO₂

PIs

• Simone Alin (NOAA/PMEL)

Technicians

• Julian Herndon (UW/NOAA/PMEL)

• Andrew Collins (UW/NOAA/PMEL)

The partial pressure of carbon dioxide (pCO₂) in the surface ocean was measured throughout the cruise track of this cruise with a General Oceanics 8050 pCO₂ Measuring System. Uncontaminated seawater was continuously passed (~1.7-2.1 L/min) through a chamber where the seawater concentration of dissolved CO₂ was equilibrated with an overlaying headspace gas. The CO₂ mole fraction of this headspace gas (xCO₂) was measured every two minutes via a non-dispersive infrared analyzer (LiCor 7000) for 60 consecutive measurements. At the end of these 60 discrete measurements, a set of five standard gases was analyzed; four of these standards have known CO₂ concentrations certified by the NOAA Earth Science Research Laboratory (ESRL) ranging from ~300 to ~900 ppm CO₂ (see Table 1). The fifth standard is a tank of 99.9995% ultra-high purity nitrogen gas, used as a baseline 0% CO₂. Following the measurements of standard gases, six consecutive measurements of atmospheric xCO₂ were made of air supplied through tubing fastened to the ships forward jack staff. Twice a day, the infrared analyzer was zeroed and spanned using the nitrogen gas and the highest concentration CO₂ standard (911.41 ppm). In addition to measurements of seawater xCO₂, atmospheric xCO₂, and standard gases, other variables were monitored to evaluate system performance (e.g. gas and water flow rates, pump speeds, equilibrator pressures, etc.). For more detail on the general design and operation of this underway pCO₂ system, see [Pierrot2009].

Throughout the duration of the research cruise, the system performed well. Initial problems with non-ideal water flow (i.e. <1.5 L/min) add some uncertainty to some of these measurements and will need to be carefully evaluated before determining their suitability for inclusion in the dataset (see details regarding water flow below). Of approximately 18,730 sea surface xCO₂ measurements, 243 have been flagged questionable (quality flag 3) and 51 have been flagged bad (quality flag 4) during this preliminary round of data quality control and assurance.

Notes on seawater source and data:

For details regarding the source of seawater to the pCO2 system, see Julian Herndon’s report from Leg One of this research cruise. During the first several days of this leg, substantial effort was made by research technicians and the chief engineer to establish sufficient water flow to the system. Initial efforts included trying to balance the water flow to meet the needs of the various (i.e. biological, chemical, shipboard) users onboard. However, insufficient flow persisted to the pCO₂ system. Flow was increased by closing down a forward overflow drain, but quickly dropped below 1.5 L/min. After exhausting nearly every possible avenue by balancing flows, adjusting pump speeds and pressures, it was determined that an obstruction must be preventing sufficient flow to the system. John Ballard and Andrew Collins disassembled the primary equilibrator in the pCO₂ system to learn that it was indeed obstructed. After cleaning, the flow to the pCO₂ system was greatly enhanced and remained sufficient for the remainder of the cruise. No other major problems were encountered during the cruise that affected data quality. One issue that persists relates to occasional dropouts from the MET system, likely due to a timing mismatch in the communications setup.

The Resident Technicians onboard used bleach to clean/sanitize the scientific seawater system after we departed Hawaii and prior to the pCO₂ system being turned on. Model and serial numbers for the pCO₂ instrument components and ancillary instruments have been recorded in a separate Excel file and will reported as part of the metadata that will accompany the final/processed pCO₂ data submission. This pCO₂ system does not have a Druck or other external barometer installed in the dry box to measure the pressure in the LiCor cell. The primary equilibrator in this pCO₂ system is an older, non-jacketed equilibrator built using a clear plastic filter housing.

To facilitate data processing and future troubleshooting of the Revelle pCO₂ system, the column headings for data in the pCO₂ files sourced from the ship are identified in Table #2. Serial numbers and additional details for the instruments in table #2 are in a separate excel file and will be reported as part of the metadata for pCO₂ data submitted from this cruise.

Table 1: Standard gases for P02 2022 cruise UW pCO₂ system.

Standard	Concentration (ppm)	Tank Serial Numbers
1	0.0	Praxair 5.0 Ultra High Purity N2
2	283.42	LL55868
3	399.51	LL127199
4	539.97	LL127204
5	911.41	LL127176

Table 2: pCO₂ system ship supplied data column headers for P02 2022 cruise, Leg 1. MWL = mean water level.

Column Header	Instrument	System	Location
TSGF1	SW flow meter	3	Bow
TSGT2	TSG45 temperature	2 and 3	Hydrolab
TSGS2	TSG45 salinity	2 and 3	Hydrolab
TSGF2	SW flow meter to TSG45	2 and 3	Hydrolab
PCO2F	SW flow meter to pCO2	2 and 3	Hydrolab
SST	SBE 38 temperature	3	Bow
AT	RM Young temperature	MET	56’ above MWL*
BP	RM Young barometer	MET	56’ above MWL*
HDG	Konsberg GPS
SOG	Speed over ground

While the raw data is not reported here, it has been collected and will be analyzed using MATLAB® routines developed by Dr. Denis Pierrot of the Atlantic Oceanographic and Meteorological Lab in Miami, FL. Measurements of gas standards were generally within 1% of their certified value throughout the duration of the cruise.

The data from the pCO₂ system that is included as part of the ships data supplied by the onboard Resident Technicians is xCO₂ (not pCO₂) that has not been processed or evaluated for QA/QC and as such should be considered preliminary.

Pierrot2009

Pierrot, D., Neill, C., Sullivan, K., Castle, R., Wanninkof, R.W., Lüger, H., Johannessen, T., Olsen, A., Feely, R.A., Cosca, C.E.; 2009. Recommendations for autonomous underway pCO2 measuring systems and data-reduction routines. Deep-Sea Research II 56 (2009) 512–522