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Finally, the remote instrument was also deployed at the ZERT field site as depicted in Figure 3.10. The instrument was kept in a Pelican case to protect it from the weather as shown in the bottom of the picture in Figure 3.10. The laser was directed to a corner cube placed 30 m away from the laser source that would direct the laser back to the FMS instrument. The remote FMS instrument would autonomously collect FMS spectra similar to those depicted in Figure 3.7.

Figure 3.8 A demonstration of the FMS accuracy and sensitivity where calibrated concentrations of (a) ¹²CO₂ and (b) ¹³CO₂ were probed in the White cell.

Figure 3.9 A comparison of the FMS stable isotope concentrations measured in the field against IRMS concentrations from collected samples. The different colors represent different years between 2008 and 2012. The shapes represent collections above, around, and away from the source. The black line is the 1:1 line (perfect agreement), while the red and blue dashed lines represent ± 10% and ± 20% relative agreement, respectively.

Figure 3.10 The remote FMS instrument was deployed to the ZERT controlled‐release field site. The instrument remained in the weatherproof case and the laser was directed to the corner cube identified by the orange arrow.

Unlike the in situ instrument that would analyze a point source of atmosphere, the remote FMS instrument measured a column average of CO₂ in the atmosphere. As one might expect, the CO₂ stable isotope ratio changes with both the flux of CO₂ that seeped to the surface along the observed column as well as due to changes in weather. This instrument was deployed in 2010 and 2011 and monitored δ¹³C ~ ‐9% to ‐28% and δ¹³C ~ ‐6% to ‐28%, respectively. Recall that the atmosphere has a natural CO₂ stable isotope ratio of ‐7% and these results were in good agreement with background levels when seepage did not reach the surface. The instrument also recorded concentrations up to ‐28%_, about half of the maximum value observed by the in situ instrument and well below the ‐15% that would have clearly indicated seepage. While the remote instrument is capable of identifying the presence of a leak, it is not capable of determining the exact position or positions of the leak as a point source detector could.