Overview of minimum detectable flux
Christiansen et al. (2015) developed the Minimum Detectable Flux (MDF) metric and demonstrated its utility as a guideline for experimental design and data quality assurance for closed chamber measurements of trace gas flux. By combining the analytical accuracy of the instrument(s) being used to measure gas concentration, the chamber volume and surface area, and the total chamber closure time, the MDF produces a lower limit for flux rates that can be detected with a given methodology.To calculate MDF (?mol/m2/h) for a gas species, the equation below can be used:
where AA is the analytical accuracy of the instrument (ppm), tC is the closure time of the chamber in hours, V is the chamber volume (m3), P is the atmospheric pressure (Pa), S is the chamber surface area (m2), R is the ideal gas constant (m3 Pa K-1 mol-1) and T is the ambient temperature (K).
Once calculated, the MDF for a given chamber design and analytical methodology is easily used to iterate experimental design, data quality assurance criteria and the experimental methods as necessary to ensure measured fluxes will be above the predetermined MDF limit thereby minimizing loss of field data. Alternatively, the MDF can be used as a post-hoc quality control metric on chamber measurements, and offers a quantitative assessment tool that researchers can use to identify and discard suspect flux values.
For example, by applying the MDF to their chamber measurements, Christiansen et al. (2015) were able to show that closure times of 10 minutes or more allowed them to get good flux measurements (<5% relative error) that were well above the MDF limit for their custom 2.7 l (surface area of 0.3 m2) chamber system using the CRDS analyzer in an agricultural monitoring experiment. Contrastingly, the GC-based measurements with this 10-minute chamber closure period worked well for carbon dioxide and nitrous oxide, but underperformed for measurements of methane fluxes. Only a small portion of the GC-based chamber data could be used for flux estimates due to a combination of sample contamination and in-situ flux rates that were below the GC-based MDF limit.
Not only does this example demonstrate the use of MDF to perform a post-hoc quality control analysis of collected flux data, but it also demonstrates the utility of in-situ measurements in minimizing potential contamination issues, as well as the application of high-resolution laser based devices to yield more accurate estimates of GHG flux. The MDF metric can be further extended to show the benefits of increased measurement frequency.