Automated, in situ measurements of dissolved CO2, CH4, and delta C-13 values using cavity enhanced laser absorption spectrometry: Comparing response times of air-water equilibrators

作者：Webb, Jackie R.^*; Maher, Damien T.; Santos, Isaac R.

来源：Limnology and Oceanography: Methods , 2016, 14(5): 323-337.

DOI：10.1002/lom3.10092

• RING-DOWN SPECTROSCOPY
• CARBON-DIOXIDE
• ISOTOPIC FRACTIONATION
• METHANE EMISSIONS
• SURFACE OCEAN
• NITROUS-OXIDE
• INLAND WATERS
• BALTIC SEA
• SEAWATER
• SYSTEMS

Combining 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 delta C-13 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 (tau; high-to-low exponential decay constant). Three equilibrator types were then used in field-based step experiments to determine s for the individual isotopologues (CO2)-C-12, (CO2)-C-13, (CH4)-C-12, and (CH4)-C-13. In the laboratory experiments, tau 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 (tau-CH4 similar to 5 times> tau-CO2) than in the showerhead and marble types (tau-CH4 similar to 1 5 times > tau-CO2). Individual isotopologue time constants under a water flow rate of similar to 5.5 L min 21 ranged from 33.7-43.1 s for (CO2)-C-12 and (CO2)-C-13, and 177-347 s for (CH4)-C-12, and (CH4)-C-13. The tau of CO2 isotopologues were within 1 s while tau 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, delta C-13-CO2, and delta C-13-CH4 can be resolved in near real-time using appropriate water-air equilibration devices.

• 出版日期2016-5