Novel Remote Sensing Methods for Methane Emissions

Mitigating methane (CH4) emissions is of uttermost importance to meet the targets of the Paris Agreement as the second-most important greenhouse gas in terms of radiative forcing. The atmospheric CH4 burden is rising, and has been doing so since the industrial times predominantly due to human activities. Novel airborne and space-based instruments have the potential to address the challenges towards detecting and quantifying localised emissions. The ultimate goal of this thesis is to use these state-of-art remote sensing measurements to improve inventories currently informed by modelled and process-based data.

CH4 point source emissions require a high spatial resolution to distinguish between different sources and a high spectral resolution to characterise the CH4 component of the atmosphere. This thesis uses the hyperspectral imager AisaFENIX, with a spectral range capable of measuring CH4 in the SWIR around 2300 nm and a spatial resolution of about 2.5 m2, in the first tailored flight campaign with the BAS Twin Otter aircraft over UK CH4 point sources. Based on a fast data-driven retrieval algorithm to infer atmospheric concentrations of CH4 from spectroscopic measurements, it successfully explores the detection limits using real and simulated CH4 plumes. The detection limit suggests that assuming a constant rate of emissions throughout the year, even the highest reported UK emitter for 2018 (Drax Power Station) would not be detected. The retrieval has applications beyond the flight campaign and can be easily adapted for the next generation of hyperspectral satellites. This thesis also investigates the use of the Sentinel-5P satellite, with unprecedented daily global coverage and spatial resolution of around 7 km2, to quantify CH4 emissions over areas with limited observations. By identifying persistent CH4 enhancements, this thesis has successfully quantified emissions in South African coal mines and oil and gas facilities in Egypt. The estimates in this thesis for South Africa are in good agreement with the EDGAR v4.3.2 inventory, which underestimates emissions in Egypt by a third probably due to uncaptured fugitive

emissions.