Evaluating the performance of a UAV-based in situ methane sensor for quantifying point source emissions

Abstract. Methane (CH₄) is the second most important greenhouse gas, and accurate quantification of its emissions is critical for mitigating climate change. In this study, we thoroughly evaluated the performance of an in situ CH₄ sensor (Axetris) for quantifying anthropogenic CH₄ emissions when deployed on an unmanned aerial vehicle (UAV). Sensor stability was assessed through laboratory tests under controlled and varying temperature conditions. Under stable conditions, the sensor achieved a precision of 63 ppb at 2 Hz. Furthermore, the tests revealed the necessity of temperature control and provided a water vapour correction term to ensure accurate measurements. Additionally, the sensor was used to quantify whole-farm CH₄ emissions, yielding a mean flux of 4.1 ± 1.6 gCH₄/s averaged over four flights. This mean flux was comparable to the value of 4.2 ± 1.1 gCH₄/s obtained from the established AirCore technique. Finally, an uncertainty analysis based on the Ornstein-Uhlenbeck method was used to determine the influence of various sources of uncertainty. This analysis revealed that both wind-related uncertainties and background determination can significantly increase the overall uncertainty when not properly constrained. Furthermore, instrumental errors play a dominant role for smaller fluxes, while meteorological uncertainties remain significant even with repeated flights. Nevertheless, careful flight planning, e.g., ensuring extensive sampling outside of the plume and comprehensive wind monitoring, can reduce these uncertainties. Overall, our results demonstrate that a cost-effective sensor can provide reliable CH₄ flux estimates with uncertainties comparable to those of established UAV-based systems.