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Challenges of atmospheric methane monitoring

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Methane (CH4) is an important greenhouse gas in the Earth's atmosphere. Since a few years, CH4 concentrations are increasing very fast again and our current understanding about its sources and sinks is still inadequate. To improve our understanding, the “Infrared observations” team at BIRA-IASB is involved in 3 critical activities: retrieving CH4 concentrations from the space-borne IASI instrument, using ground-based FTIR instruments to produce highly accurate reference measurements, and performing extensive satellite retrieval quality control studies.
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The increase of greenhouse gases such as carbon dioxide and methane in the Earth's atmosphere is predominantly responsible for the observed global climate change. CH4 is an important greenhouse gas (GHG) and a challenging molecule to study: although less abundant in the atmosphere than CO2, its radiative impact as a GHG is significantly larger.

The “Infrared observations” research group at BIRA-IASB contributes to the improvement of our understanding of atmospheric methane by performing measurements of its vertical abundance both from the ground and from space. Each technique has its own advantages and drawbacks.

Ground-based FTIR instruments for highly accurate reference measurements

Ground-based observations are performed by high-resolution Fourier Transform InfraRed (FTIR) spectrometers, which are well monitored, calibrated and lead to very accurate and precise measurements. The Institute is performing such observations at two stations on Réunion Island (St-Denis and Maido) [Zhou et al, 2018], which are part of the NDACC and TCCON global networks of ground-based measurements.

However, even within a network constellation, the coverage achieved by ground-based FTIR instruments remains limited and is almost absent in large areas of the globe (continental Africa, large swaths of South-America and Northern Asia).

Retrieving CH4 concentrations from the space-borne IASI instrument

Spaceborne instruments continuously measure atmospheric CH4, providing valuable information at global and regional scales not only for atmospheric monitoring but also for surface flux estimates. Researchers at BIRA-IASB used methane observations in the Thermal Infrared (TIR) region, from the MetOp/IASI instrument, to explore and consolidate the possible sources of CH4 biases in retrievals from TIR sensors in the framework of the ESA-CH4TIR project.

Extensive satellite retrieval quality control

Satellite observations have lower temporal, spatial and spectral resolution, hence they are less precise and accurate, when compared to ground-based observations. Different techniques (algorithms) can be used to derive methane abundances, either using full-physics modeling or more empirical methods, both coming with their own strengths and weaknesses.

In the framework of the EUMETSAT project VIMP (for ‘Validation of IASI Methane Products’), BIRA-IASB’s “Infrared observations” group used ground-based, aircraft and other correlative measurements to validate the CH4 abundances derived from satellite measurements using different retrieval algorithms (RAL and LMD), combining the strengths of both ground-based and satellite observations.


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Example of the evaluation of CH4 datasets. Here, you can see a comparison of CH4 column-averaged dry air mole fractions for 2 different retrieval algorithms (LMD and RAL) with the co-located CAMS (Copernicus Atmosphere Monitoring Service) reanalysis control run.
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