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This document describes the quality for two satellite-derived atmospheric carbon dioxide (CO2) and methane (CH4) C3S data products, v10.2 MTCO2_OBS4MIPS and v10.1 MTCH4_OBS4MIPS. These two products are mid-tropospheric-averaged air mixing ratios (mole fractions) of CO2 andCH4 products from observations made by Infrared Atmospheric Sounding Interferometer (IASI) and Advanced Microwave Sounding Unit-A (AMSU-A) instruments onboard the European Metop-A (July 2006-August 2021), Metop-B (since February 2013) and Metop-C (since May 2019) platforms (for more details, see MTGHG ATBD, 2024). The IASI hyperspectral observations in the thermal infrared at 7.7 µm for CH4 and at 15 µm for CO2 are sensitive to both temperature and gas concentrations of CH4 / CO2. These are used in conjunction with microwave observations form the AMSU-A instrument, which is only sensitive to temperature. These AMSU-A observations are used to decorrelate temperature variations from CO2/CH4 variations in the infrared radiance detected by IASI (Crevoisier et al., 2009a, 2009b, 2013).
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For the comparison, all L3 MT-CH4 products falling in a 5°x5° grid cell centered on each AirCore profile for the same day are averaged. For that, the averaging kernel of each selected L3 MT-CH4 is applied on the CH4 profiles provided by the AirCore to obtain an AirCore IASI-like MT-CH4. The averaging kernels are defined on the Thermodynamic Initial Guess Retrieval (TIGR) pressure grid (provided in the L3 MTGHG_OBS4MIPS netcdf files, see MTGHG PUGS, 2024 for more details). The CH4 profiles provided by the AirCores are linearly interpolated as a function of pressure altitude on the TIGR pressure grid used in the retrieval. Thus, the CH4 profiles of the AirCores and the averaging kernels are defined on the same pressure grid. To obtain the IASI-like MT-CH4 from the AirCore measurements, we apply the following formula (Crevoisier et al., 2009b):
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PUGS MTGHG, 2024: Crevoisier, C., Meilhac, N., C3S Greenhouse Gas (GHG: MTCO2 v10.1 & MTCH4 v10.2): Product User Guide and Specification (PUGS)
Sawa et al. 2015:Sawa, Y., Machida, T., Matsueda, H., Niwa, Y., Tsuboi, K., Murayama, S., Morimoto, S., and Aoki, S.: Seasonal changes of CO2, CH4, N2O, and SF6 in the upper troposphere/lower stratosphere over the Eurasian continent observed by commercial airliner, Geophys. Res. Lett., 42, 2001–2008, https://doi.org/10.1002/2014GL062734, 2015.
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