Toward continuous monitoring of seawater ¹³CO₂/¹²CO₂ isotope ratio and pCO₂: Performance of cavity ringdown spectroscopy and gas matrix effects

The potential of a continuous wave cavity ringdown spectrometer for monitoring the isotope ratio 13 CO 2 / 12 CO 2 and the partial pressure p CO 2 of CO 2 dissolved in water was thoroughly analyzed by quantitative measurements. Running calibration gas standards under typical operation conditions, a relative accuracy of Δ(δ 13 C[CO 2 ]) = ±0.1‰ with 120 min averaging time has been demonstrated. Absolute uncertainties were determined to be Δ(δ 13 C[CO 2 ]) = ±0.2‰ and Δ( x CO 2 ) = ±0.5 ppmv. No principle problems were encountered when using the instrument in combination with a water‐air equilibration setup. By contrast, when performing measurements of CO 2 in gas matrices with a composition different from that of ambient air, pressure broadening linewidth effects induced significant errors in both δ 13 C(CO 2 ) and x CO 2 values. These effects, which compromise the accessible accuracy in environmental studies, can be quantitatively taken into account by using a spectroscopically based correction procedure. Relying on linewidth analysis, the instrument was shown to be capable of continuous and simultaneous measurement of δ 13 C(CO 2 ), p CO 2 , as well as water content and O 2 supersaturation, and thus holds the potential for online monitoring of these quantities aboard research vessels.