Automated, in situ measurements of dissolved CO2, CH4, and δ13C values using cavity enhanced laser absorption spectrometry: Comparing response times of air‐water equilibrators

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AbstractCombining air‐water equilibrators with a field deployable cavity enhanced laser absorption spectrometer (CELAS) can generate precise, high resolution, measurements of dissolved CO2 and CH4 concentrations and δ13C values in aquatic systems. However, equilibration response times for combined measurements of CO2 and CH4 isotopologues have not been assessed. We performed laboratory step experiments on six different equilibrators to constrain CO2 and CH4 equilibration time constants (τ; high‐to‐low exponential decay constant). Three equilibrator types were then used in field‐based step experiments to determine τ for the individual isotopologues 12CO2, 13CO2, 12CH4, and 13CH4. In the laboratory experiments, τ ranged from 34–124 s for CO2 and 117–2041 s for CH4 among the six equilibrators. The ratio between response times of CO2 and CH4 was substantially lower in the membrane type equilibrators (τ‐CH4 ∼5 times > τ‐CO2) than in the showerhead and marble types (τ‐CH4 ∼15 times > τ‐CO2). Individual isotopologue time constants under a water flow rate of ∼5.5 L min−1 ranged from 33.7–43.1 s for 12CO2 and 13CO2, and 177–347 s for 12CH4, and 13CH4. The τ of CO2 isotopologues were within 1 s while τ of CH4 isotopologues were the same. Further investigations into water flow rate revealed an exponential decrease in equilibration time from 1.5 L min−1 to 9 L min−1 in a marble‐type equilibrator. The response time was always longer from high‐to‐low than low‐to‐high concentrations. By taking into consideration the equilibration response time, measurements of CO2, CH4, δ13C‐CO2, and δ13C‐CH4 can be resolved in near real‐time using appropriate water‐air equilibration devices.