SWIR absorption - global screening

SWIR absorption spectroscopy applies the same core retrieval physics across three operational scales: point-source plume imaging, regional-flux quantification, and global-screening surveys. All three exploit passive solar backscatter in shortwave infrared absorption windows, chiefly the 1.61-1.70 um methane band and the 2.2-2.4 um methane feature, along with CO2 windows near 1.6 and 2.0 um. Instruments compare spectral radiance inside and outside these absorption features; column-averaged dry-air mixing ratios (XCH4 or XCO2) are recovered via Beer-Lambert attenuation through the atmospheric column, using CO2-proxy, full-physics radiative-transfer, or Beer-Law column-enhancement retrieval approaches.^[1]

The three scales differ primarily in how the photon budget is allocated between spatial resolution and signal integration time, producing a direct sensitivity-versus-coverage trade-off.^[1]

Point-source scale (25-60 m pixels, ~10-100 km2 scenes): Fine spatial resolution resolves sub-kilometre plume structure, enabling attribution to individual super-emitting facilities such as oil and gas wellpads, landfills, and compressor stations. Because scene size is small, coverage is tasked rather than routine. Representative missions include GHGSat (WAF-P Fabry-Perot spectrometer; 1630-1675 nm; approximately 25 m resolution; 12x12 km scene; detection threshold approximately 100-1000 kg/h),^[2][3] EMIT (NASA/JPL; ISS-mounted hyperspectral 380-2500 nm; 60 m; deployed 2022), PRISMA (ASI; 30 m; 2.3 um window), EnMAP (DLR; 30 m; 2.3 um window), and Carbon Mapper (Planet/JPL; approximately 30 m; detection threshold approximately 100 kg/h).

Regional-flux scale (100 m - 7 km pixels, basin to sub-continental coverage): Wide-swath or frequent-revisit designs characterise area emissions across oil and gas basins and agricultural regions for use in atmospheric inverse modelling. Representative missions include TROPOMI on Sentinel-5P (SWIR band 2305-2385 nm; 7x5.5 km pixels; 2600 km swath; near-daily global coverage; XCH4 precision 0.6%)^[4] and MethaneSAT (EDF; 1.61-1.68 um; 100x400 m pixels; 260 km swath; launched 2024).^[5] Spatial resolution is insufficient to attribute emissions to individual facilities, so this scale relies on inverse modelling to recover lower emission values via aggregation.

Global-screening scale (10-30 km footprints, routine global coverage): Coarse footprints enable systematic multi-year records of column-averaged greenhouse gas concentrations, supporting long-term trend detection and identification of regional anomaly hotspots for follow-up by finer instruments. Coverage is cloud-dependent and sparsely sampled. Representative missions include GOSAT (JAXA/MOE/NIES; TANSO-FTS; 10.5 km circular footprint; 3-day global repeat; 1.6 and 2.0 um windows; operating since 2009),^[6] SCIAMACHY on Envisat (ESA; first satellite global CH4 measurements; 30x60 km pixels; 2002-2012), and OCO-2/OCO-3 (NASA; 1.61 and 2.06 um CO2 windows; global XCO2 screening).