Exploring uncertainties in the integrated mass enhancement method for remote sensing retrievals of methane emissions

Landfill methane (CH₄) emissions have increasingly become a significant concern for landfill operators and regulatory agencies. Many approaches have been applied in the past to monitor and quantify methane emissions from landfills, and remote sensing techniques based on aerial imagers have gained tremendous attention recently. However, accurately quantifying CH₄ emission rates released by sources located at ground level from aerial observations of instantaneous CH₄ plumes remains a challenging problem. In this study, we first simulated three-dimensional methane concentration fields from a 100 Kg/hr single-point source methane release, using five years of meteorological data at an hourly resolution. Then, we applied the Integrated Mass Enhancement (IME) method, a popular inverse technique for estimating point source emission rates from aerial imagers, to infer the emissions rate from the simulated methane plume. The IME method performed reasonably well during atmospheric conditions similar to the conditions under which it was developed, which only account for 0.05 % of all simulated 5-year period. However, the performance of IME method varied under other atmospheric conditions. Stability classes and wind speeds can have significantly impact the performance of IME method. The largest overestimation (3.2 times of true rates) occurs during very stable conditions with low wind speeds. Underestimation occurs at high wind speed combined with very unstable conditions. The results of this study are significant to the design of future inverse techniques for aircraft/satellite-based remote sensing approaches. Future work is needed to better understand the performance of the IME approach under different climatological conditions using more sophisticated models.