Supplementary material to "A versatile spaceborne architecture for immediate monitoring of the global methane pledge"

Abstract

Methods for uncertainty analysis(1) Uncertainties in the PRISMA-based methane retrievals. The PRISMA-based methane retrievals can present systematic and random errors. We thus evaluated their performance using end-to-end simulations, as shown in previous studies. First, the ideal plumes were prescribed via the large-eddy-driven Weather and Research Forecasting Model (the WRF-Chem-LES model) [1]. The key configuration included common wind fields (i.e., 3.5 m/s), high resolution (i.e., 30 ×30 m 2 ), and constant emission rates (e.g., 500, 1000, and 1500 kg/h). On this basis, uncorrelated noises with random increments were then added.They represented the expected instrument precision presenting normal distributions with zero mean biases and standard deviations of 1 ~ 5 %. Second, the enhancements of volume mixing ratios were converted into two-way spectral atmospheric transmittance. The calculation basis included air mass factors based on the real observation zenith angles, vertical profiles of dry air column densities, and methane absorption cross-section data. These three data came from the satellite instrument records, ERA5 reanalysis dataset, and the HIgh-resolution TRANSmission molecular absorption (HITRAN2016) database. Third, the subsequent transmittance spectra were convolved with the PRISMA-based spectral responses and then multiplied by the original PRISMA top-of-atmosphere radiance spectrums. To prevent across-track variations in spectral calibration, we performed such processes on a per-column basis. Finally, the resulting PRISMA-based top-of-atmosphere radiance images were processed with the same matched-filter algorithm over the cases explored in this work. Methane retrieval results via the WRF-Chem-LES simulations and associated biases were presented in Table S3. Therefore, we did not observe systematic errors in the PRISMA-based methane retrievals. Detailed evaluations are shown in Supplementary Information.(2) Uncertainties in the TROPOMI-based methane emission estimates. We provided independent emission estimates for the TROPOMI-based methane hotspots using the WRF-Chem model. On this basis, the differences between the WRF-Chembased and IME-based results reflected the intrinsic uncertainties in the IME method.The WRF simulation was nudged to National Centers for Environmental Prediction final analysis data at 0.25° × 0.25° spatial resolution and six-hour temporal resolution. For each hotspot, the model was performed at 5 × 5 km 2 horizontal resolution over a 50 × 50 km 2 domain. The boundary condition was obtained from the CAMS reanalysis dataset. Note that the inner domain did not feedback with the outer domain (i.e., so-called one-way nested simulations). The grid-specific methane emissions were originally taken from the bottom-up emission inventories (EDGARv6.0). Other general configurations could be found in our previous studies [2,3].Methane emissions were estimated via the Bayesian inverse solution which optimized a single state vector 𝐱 as: 𝐱 ̂= 𝐱 𝐀 + 𝐒 𝐀 𝐊 𝑻 (𝐊𝐒 𝐀 𝐊 𝑻 + 𝐒 𝐂 ) −𝟏 (𝐲 − 𝐊𝐱 𝐀 )...