Underway Surface pCO₂ Measurements on P16S_2005

The NOAA/PMEL group (Dr. Christopher Sabine, PI) was responsible for underway surface pCO₂ measurements during the CLIVAR/GO-SHIP Repeat Hydrography Section P16S_2005. Dr. Sabine operated the system during the cruise.

The automated underway LICOR 6262 pCO₂ sensor with showerhead equilibrator was used during the cruise. The measurement method based on infrared absorption of dried gas and described in Feely et al. (1998) and Wanninkhof and Thoning (1993). Equilibrator volume was ~0.5 L with a headspace of ~0.8 L. During the cruise resolution/uncertainty was 0.3 µatm for equilibrator measurements, 0.2 µtam for atmospheric measurements.

Standard gases were supplied by NOAA's Climate Monitoring Diagnostics Laboratory in Boulder, Colorado, and were directly traceable to the WMO scale. Any value outside the range of the standards should be considered approximate, although the general trends should be indicative of the seawater chemistry.

Serial numbers and CO₂ concentrations for the cylinders used on this cruise:

The system ran a full cycle in approximately 112 min. The cycle started with 4 standard gases, then measured 10 atmospheric samples followed by 60 surface water samples. Each new gas was flushed through the LICOR analyzer for 4 min prior to a 10 second reading from the analyzer during which the sample cell was open to the atmosphere. Subsequent samples of the same gas are flushed through the LICOR Analyzer for 30 s prior to a stop-flow measurement.

All xCO₂ values are reported in parts per million by volume (ppmv) and fCO₂ values are reported in microatmospheres (µatm) assuming 100 % humidity at the equilibrator temperature.

The mixing ratios of ambient air and equilibrated headspace air were calculated by fitting a second-order polynomial through the hourly averaged response of the detector versus mixing ratios of the standards. Mixing ratios of dried equilibrated headspace and air are converted to fugacity of CO₂ in surface seawater and water saturated air to determine the fCO₂. For ambient air and equilibrator headspace the fCO₂a, or fCO₂eq is calculated assuming 100% water vapor content:

fCO₂a/eq = xCO₂a/eq(P - pH₂O) exp(B11 + 2d12)P/RT

where fCO₂a/eq is the fugacity in ambient air or equilibrator, pH₂O is the water vapor pressure at the sea surface temperature, P is the atmospheric pressure (in atm), T is the sea surface temperature (SST) or equilibrator temperature (in K) and R is the ideal gas constant (82.057 cm³ atm deg^-1 mol^-1). The exponential term is the fugacity correction where B11 is the second virial coefficient of pure CO₂:

B11 = -1636.75 + 12.0408T - 0.032795T² + 3.16528E^-5T³ and d12 = 57.7 - 0.118T

is the correction for an air-CO₂ mixture in units of cm³ mol^-1 (Weiss 1974). The calculation for the fugacity at SST involves a temperature correction term for the increase of fCO₂ due to heating of the water from passing through the pump and through 5 cm inside diameter (ID) PVC tubing within the ship. The water in the equilibrator is typically 0.2 °C warmer than sea surface temperature. The empirical temperature correction from equilibrator temperature to SST is outlined in Weiss et al. (1982):

Δln(fCO₂) = (T_eq - SST) * [0.0317 - 2.7851E^-4T_eq - 1.839E^-3 ln(fCO₂eq)]

where Δln(fCO₂) is the difference between the natural logarithm of the fugacity at T_eq and SST, and T_eq is the equilibrator temperature in °C.