A New Technique for Airborne Measurements to Quantify Methane Emissions Over a Wide Range: Implementation and Validation

Abstract. Methane (CH₄) is a powerful greenhouse gas with a global warming potential 84 times higher than carbon dioxide (CO₂) over 20 years. CH₄ is produced from many natural and anthropogenic sources which can be further classified as biogenic or thermogenic in origin. The largest biogenic sources result from anaerobic decay such as wetlands, melting permafrost, or the breakdown of organic matter in the guts of ruminant animals. Thermogenic CH₄ is generated during the breakdown of organic matter at high temperatures and pressure within the Earth's crust, a process which also produces more complex trace hydrocarbons such as ethane (C₂H₆) and propane (C₃H₈). Emissions of thermogenic CH₄ are dominated by the fossil fuel energy sector, and the presence of elevated C₂H₆ along with CH₄ can be used to distinguish oil and gas emissions from biogenic sources. This work outlines the development and deployment of an Unmanned Aerial System (UAS) outfitted with a fast (1 Hz) and sensitive (1–2 ppb s^-1) CH₄ & C₂H₆ sensor and ultrasonic anemometer. The UAV platform is a vertical-takeoff, hexarotor vehicle capable of vertical profiling to 120 m altitude and plume sampling across scales up to 1 km. This system has been used for direct quantification of point sources, as well as distributed emitters such as landfills, with source rates as low as 0.04 kg h^-1 and up to 1500 kg h^-1. Simultaneous measurements of CH₄ and C₂H₆ mixing ratios, vector winds, and positional data allows for source classification (biogenic versus thermogenic), differentiation, and emission rates without the need for modeling or a priori assumptions about winds, vertical mixing, or other environmental conditions. The UAS has been deployed throughout the Southwest United States for system validation and targeted quantification of various sources emitting at or below the detection limits of other aircraft and satellite systems. This system offers a direct, repeatable method of horizontal and vertical profiling of emission plumes at scales that provide complementary information for regional aerial surveys as well as local ground-based monitoring.