Authors– Asfa Siddiqui, Poonam S. Tiwari and Raghavendra Pratap Singh

Greenhouse gases, especially CO₂ and CH₄ are amongst the primary contributors of anthropogenic climate change. Methane (CH₄), a potent greenhouse gas, has a global warming potential over 28 times and 84 times greater than CO₂ over a 100-year and 20-year period, respectively (Saunois et al. 2024). In the wake of rising global emissions and escalating climate impacts, monitoring GHGs is vital for effective climate action, policy development, and accountability.

Satellite based methods of GHG retrieval methods rely on shortwave infrared (SWIR) and thermal infrared (TIR) bands to detect atmospheric methane through absorption spectroscopy. CH₄ shows a strong absorption around 1650-1700 nm and 2300-2350 nm, whereas, CO₂ shows strong absorption around 1600 nm and 2000 nm (Thompson et al. 2015). These absorption features are critical in designing sensors and retrieval algorithms in greenhouse gas monitoring missions (Borger et al. 2025).

Ground based monitoring of greenhouse gases was performed using Total Carbon Column Observing Network (TCCON), Network for the Detection of Atmospheric Composition Change (NDACC) and Global Atmospheric Watch (GAW) program of World Meteorological Organisation (WMO) networks and eddy covariance towers providing columnar flux measurements. Additionally, Cavity Ring-Down Spectroscopy (CRDS) instruments and flux chambers were also used for quantifying localized measurements of methane.

The first ever satellite to attempt for space based retrieval of methane using SWIR spectrometry was SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) onboard European Space Agency’s Envisat mission (2002-2012). The first dedicated GHG mission was Greenhouse Gases Observing Satellite (GOSAT) capturing SWIR and TIR spectra. Other area-flux mappers providing long-term datasets include The Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua satellite, Sentinel-5 Precursor TROPospheric Monitoring Instrument (TROPOMI), progressing in retrieval algorithms and spatial resolution. Some of the point-source imagers include GHGSat (Constellation of GHG satellites), PRISMA (Hyperspectral Precursor of the Application Mission), EnMap (Environmental Mapping and Analysis Program), AVIRIS-NG (Airborne Visible/Infrared Imaging Spectrometer), Carbon Mapper, EMIT (Earth Surface Mineral Dust Source Investigation), etc.

Figure 1: Plume concentration (in ppm-m) of Methane over Ghazipur landfill, Delhi

Retrieval of greenhouse gases like matched filter, differential optical absorption spectroscopy (DOAS), iterative Maximum A Posteriou (IMAP)-DOAS, Weighting Function Modified (WFM)-DOAS, full-physics retrieval algorithm like RemoTec, etc. convert measured radiances to mole fractions and emission fluxes using inversion models (Foote at al. 2020).

In the current study, EMIT satellite (aboard International Space Station, spatial resolution of 60 m) enhancement data of October 10, 2023 (Thorpe at al. 2023; Siddiqui et al. 2024) was used to retrieve the methane hotspot over Ghazipur landfill site in Delhi (refer Fig. 1). The plume extraction and flux estimation of methane was performed using Integrated Mass Enhancement (IME) technique. The maximum concentration of CH₄ within the plume was 2795 ppm-m and the total flux estimated was 1949 kg/hr. In the long-run, point source imagers can be immensely useful in continuous monitoring of greenhouse gas hotspots and facilitate policymakers towards more transparent, data-driven, and sustainable future.

References:

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