Application and Validation of a Novel Airborne Sampling Methodology That Uses Green's Theorem and Micrometeorological Principles to Estimate Surface Emission Rates

Abstract:

Airborne, so called top-down, estimates of greenhouse gas emissions are becoming much more prevalent with the advent of sensitive, high-rate trace gas instrumentation, and they have lead to some controversial findings when compared with bottom-up engineering estimates reported to environmental regulatory agencies. Consequently, a proper assessment of the accuracy of these airborne methods is crucial to interpreting the meaning of such discrepancies. We present a new method of sampling surface sources of methane and ethane, of spatial scales as small as about 100 m, where consecutive loops are flown around the source at many different flight altitudes. Using the principles of Reynolds decomposition for the wind and scalar concentrations, along with Green's Theorem, we show that the method accurately accounts for the smaller scale turbulent dispersion of the local plume, which is often ignored in other average "mass balance" methods. With the help of Large Eddy Simulations we further show how the sampling method can be optimized for the micrometeorological conditions encountered during any flight. Furthermore, by sampling controlled releases of methane and ethane on the ground we are able to ascertain an accuracy in the method of better than 15%, with limits of detection below 5 kg/hr for both gases. Because of the FAA mandated minimum flight safe altitude of 500 ft., placement of the plane is critical to not allowing a large portion of the plume to flow underneath the lowest sampling altitude, which is generally the leading source of uncertainty in these measurements. Finally, because the bulk of the flux is carried by rapid plume encounters, which are relatively rare, we show how the accuracy of the method is strongly dependent on the number of sampling loops, or time spent sampling the source.