Journal article
Automated, in situ measurements of dissolved CO2, CH4, and δ13C values using cavity enhanced laser absorption spectrometry: Comparing response times of air-water equilibrators
Limnology and Oceanography, Vol.14(5), pp.323-337
2016
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Source: InCites
Abstract
<p>Combining air-water equilibrators with a field deployable cavity enhanced laser absorption spectrometer (CELAS) can generate precise, high resolution, measurements of dissolved CO<sub>2</sub> and CH<sub>4</sub> concentrations and δ<sup>13</sup>C values in aquatic systems. However, equilibration response times for combined measurements of CO<sub>2</sub> and CH<sub>4</sub> isotopologues have not been assessed. We performed laboratory step experiments on six different equilibrators to constrain CO<sub>2</sub> and CH<sub>4 </sub>equilibration time constants (<em>τ</em>; high-to-low exponential decay constant). Three equilibrator types were then used in field-based step experiments to determine <em>τ</em> for the individual isotopologues <sup>12</sup>CO<sub>2</sub>, <sup>13</sup>CO<sub>2</sub>, <sup>12</sup>CH<sub>4</sub>, and <sup>13</sup>CH<sub>4</sub>. In the laboratory experiments, <em>τ</em> ranged from 34–124 s for CO<sub>2 </sub>and 117–2041 s for CH<sub>4</sub> among the six equilibrators. The ratio between response times of CO<sub>2 </sub>and CH<sub>4</sub> was substantially lower in the membrane type equilibrators (<em>τ</em>-CH<sub>4</sub> ∼5 times > <em>τ</em>-CO<sub>2</sub>) than in the showerhead and marble types (<em>τ</em>-CH<sub>4</sub> ∼15 times > <em>τ</em>-CO<sub>2</sub>). Individual isotopologue time constants under a water flow rate of ∼5.5 L min−1 ranged from 33.7–43.1 s for <sup>12</sup>CO<sub>2</sub> and <sup>13</sup>CO<sub>2</sub>, and 177–347 s for <sup>12</sup>CH<sub>4</sub>, and <sup>13</sup>CH<sub>4</sub>. The <em>τ</em> of CO<sub>2</sub> isotopologues were within 1 s while <em>τ</em> of CH<sub>4 </sub>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<sub>2</sub>, CH<sub>4</sub>, δ<sup>13</sup>C-CO<sub>2</sub>, and δ<sup>13</sup>C-CH<sub>4</sub> can be resolved in near real-time using appropriate water-air equilibration devices.</p>
Details
- Title
- Automated, in situ measurements of dissolved CO2, CH4, and δ13C values using cavity enhanced laser absorption spectrometry: Comparing response times of air-water equilibrators
- Creators
- Jackie R Webb - Southern Cross UniversityDamien T Maher - Southern Cross UniversityIsaac R Santos - Southern Cross University
- Publication Details
- Limnology and Oceanography, Vol.14(5), pp.323-337
- Identifiers
- 4178; 991012820871302368
- Academic Unit
- National Marine Science Centre; School of Environment, Science and Engineering; Science; Faculty of Science and Engineering
- Resource Type
- Journal article