Quantifying methane point sources from fine-scale satellite observations of atmospheric methane plumes

Anthropogenic methane emissions originate from a large number of relatively small point sources. The planned GHGSat satellite fleet aims to quantify emissions from individual point sources by measuring methane column plumes over selected inline-formula $M1inlinescrollmathml\sim \mathrm{normal\; 10}\times \mathrm{normal\; 10}$ 48pt10ptsvg-formulamathimg411ec8e836610e8a62315ec50e28041a amt-11-5673-2018-ie00001.svg48pt10ptamt-11-5673-2018-ie00001.png  kminline-formula² domains with inline-formula $M3inlinescrollmathml\le \mathrm{normal\; 50}\times \mathrm{normal\; 50}$ 48pt11ptsvg-formulamathimg26d711ddb4b4ae58f898ead201f945a2 amt-11-5673-2018-ie00002.svg48pt11ptamt-11-5673-2018-ie00002.png  minline-formula² pixel resolution and 1 %–5 % measurement precision. Here we develop algorithms for retrieving point source rates from such measurements. We simulate a large ensemble of instantaneous methane column plumes at inline-formula50×50 minline-formula² pixel resolution for a range of atmospheric conditions using the Weather Research and Forecasting model (WRF) in large eddy simulation (LES) mode and adding instrument noise. We show that standard methods to infer source rates by Gaussian plume inversion or source pixel mass balance are prone to large errors because the turbulence cannot be properly parameterized on the small scale of instantaneous methane plumes. The integrated mass enhancement (IME) method, which relates total plume mass to source rate, and the cross-sectional flux method, which infers source rate from fluxes across plume transects, are better adapted to the problem. We show that the inline-formulaIME method with local measurements of the 10 m wind speed can infer source rates with an error of 0.07–0.17 t hinline-formula $M8inlinescrollmathml{}^{-\mathrm{normal\; 1}}+\mathrm{normal\; 5}$ 30pt13ptsvg-formulamathimgf784f7b5f9231748f77e747dc1dc99f2 amt-11-5673-2018-ie00003.svg30pt13ptamt-11-5673-2018-ie00003.png  %–12 % depending on instrument precision (1 %–5 %). The cross-sectional flux method has slightly larger errors (0.07–0.26 t hinline-formula $M9inlinescrollmathml{}^{-\mathrm{normal\; 1}}+\mathrm{normal\; 8}$ 30pt13ptsvg-formulamathimg59a80a28ff28c1f76b9ab769b952aba7 amt-11-5673-2018-ie00004.svg30pt13ptamt-11-5673-2018-ie00004.png  %–12 %) but a simpler physical basis. For comparison, point sources larger than 0.3 t hinline-formula⁻¹ contribute more than 75 % of methane emissions reported to the US Greenhouse Gas Reporting Program. Additional error applies if local wind speed measurements are not available and may dominate the overall error at low wind speeds. Low winds are beneficial for source detection but detrimental for source quantification.