# Retrieval of atmospheric CH 4 vertical information from ground-based FTS near-infrared spectra

The Total Carbon Column Observing Network (TCCON) column-averaged dry air mole fraction of inline-formulaCH4 (inline-formula $M4inlinescrollmathml{X}_{chem{\mathrm{normal CH}}_{normal 4}}$ 24pt14ptsvg-formulamathimg970988a62a75b445a35b4961e8449a4f amt-12-6125-2019-ie00001.svg24pt14ptamt-12-6125-2019-ie00001.png ) measurements have been widely used to validate satellite observations and to estimate model simulations. The GGG2014 code is the standard TCCON retrieval software used in performing a profile scaling retrieval. In order to obtain several vertical pieces of information in addition to the total column, in this study, the SFIT4 retrieval code is applied to retrieve the inline-formulaCH4 mole fraction vertical profile from the Fourier transform spectrometer (FTS) spectrum at six sites (Ny-Ålesund, Sodankylä, Bialystok, Bremen, Orléans and St Denis) during the time period of 2016–2017. The retrieval strategy of the inline-formulaCH4 profile retrieval from ground-based FTS near-infrared (NIR) spectra using the SFIT4 code (SFIT4NIR) is investigated. The degree of freedom for signal (DOFS) of the SFIT4NIR retrieval is about 2.4, with two distinct pieces of information in the troposphere and in the stratosphere. The averaging kernel and error budget of the SFIT4NIR retrieval are presented. The data accuracy and precision of the SFIT4NIR retrievals, including the total column and two partial columns (in the troposphere and stratosphere), are estimated by TCCON standard retrievals, ground-based in situ measurements, Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS) satellite observations, TCCON proxy data and AirCore and aircraft measurements. By comparison against TCCON standard retrievals, it is found that the retrieval uncertainty of SFIT4NIR inline-formula $M7inlinescrollmathml{X}_{chem{\mathrm{normal CH}}_{normal 4}}$ 24pt14ptsvg-formulamathimg16034bc4bd3a9c25739a652a6809287a amt-12-6125-2019-ie00002.svg24pt14ptamt-12-6125-2019-ie00002.png is similar to that of TCCON standard retrievals with systematic uncertainty within 0.35 % and random uncertainty of about 0.5 %. The tropospheric and stratospheric inline-formula $M8inlinescrollmathml{X}_{chem{\mathrm{normal CH}}_{normal 4}}$ 24pt14ptsvg-formulamathimg0e660ea5f9703e988416fe4b3f1c14cd amt-12-6125-2019-ie00003.svg24pt14ptamt-12-6125-2019-ie00003.png from SFIT4NIR retrievals are assessed by comparison with AirCore and aircraft measurements, and there is a 1.0 inline-formula± 0.3 % overestimation in the SFIT4NIR tropospheric inline-formula $M10inlinescrollmathml{X}_{chem{\mathrm{normal CH}}_{normal 4}}$ 24pt14ptsvg-formulamathimg92ddc872d984a732d25dbdf219dd194d amt-12-6125-2019-ie00004.svg24pt14ptamt-12-6125-2019-ie00004.png and a 4.0 inline-formula± 2.0 % underestimation in the SFIT4NIR stratospheric inline-formula $M12inlinescrollmathml{X}_{chem{\mathrm{normal CH}}_{normal 4}}$ 24pt14ptsvg-formulamathimga302efc37f65e72e8fb91bc32f887b10 amt-12-6125-2019-ie00005.svg24pt14ptamt-12-6125-2019-ie00005.png , which are within the systematic uncertainties of SFIT4NIR-retrieved partial columns in the troposphere and stratosphere respectively.

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Zhou, Minqiang / Langerock, Bavo / Sha, Mahesh Kumar / et al: Retrieval of atmospheric CH4 vertical information from ground-based FTS near-infrared spectra. 2019. Copernicus Publications.

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