Automated, in situ measurements of dissolved CO 2 , CH 4 , and δ 13 C values using cavity enhanced laser absorption spectrometry: Comparing response times of air‐water equilibrators

Combining air‐water equilibrators with a field deployable cavity enhanced laser absorption spectrometer (CELAS) can generate precise, high resolution, measurements of dissolved CO 2 and CH 4 concentrations and δ 13 C values in aquatic systems. However, equilibration response times for combined measurements of CO 2 and CH 4 isotopologues have not been assessed. We performed laboratory step experiments on six different equilibrators to constrain CO 2 and CH 4 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 12 CO 2 , 13 CO 2 , 12 CH 4 , and 13 CH 4 . In the laboratory experiments, τ ranged from 34–124 s for CO 2 and 117–2041 s for CH 4 among the six equilibrators. The ratio between response times of CO 2 and CH 4 was substantially lower in the membrane type equilibrators ( τ ‐CH 4 ∼5 times >  τ ‐CO 2 ) than in the showerhead and marble types ( τ ‐CH 4 ∼15 times >  τ ‐CO 2 ). Individual isotopologue time constants under a water flow rate of ∼5.5 L min −1 ranged from 33.7–43.1 s for 12 CO 2 and 13 CO 2 , and 177–347 s for 12 CH 4 , and 13 CH 4 . The τ of CO 2 isotopologues were within 1 s while τ of CH 4 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 CO 2 , CH 4 , δ 13 C‐CO 2 , and δ 13 C‐CH 4 can be resolved in near real‐time using appropriate water‐air equilibration devices.