Retrieval of atmospheric CH4 vertical information from ground-based FTS near-infrared spectra

• Published in: Atmospheric Measurement Techniques
• Main Authors: Zhou, Minqiang, Langerock, Bavo, Sha, Mahesh Kumar, Kumps, Nicolas, Hermans, Christian, Petri, Christof, Warneke, Thorsten, Chen, Huilin, Metzger, Jean-Marc, Kivi, Rigel, Heikkinen, Pauli, Ramonet, Michel, Mazière, Martine
• Format: Text
• Language: English
• Published: 2019
• Subjects: Ny-Ålesund; Sodankylä; Ny Ålesund
• Online Access: https://doi.org/10.5194/amt-12-6125-2019; https://amt.copernicus.org/articles/12/6125/2019/

The Total Carbon Column Observing Network (TCCON) column-averaged dry air mole fraction of CH 4 ( <math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>X</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="970988a62a75b445a35b4961e8449a4f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-6125-2019-ie00001.svg" width="24pt" height="14pt" src="amt-12-6125-2019-ie00001.png"/></svg:svg> ) 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 CH 4 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 CH 4 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 <math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>X</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="16034bc4bd3a9c25739a652a6809287a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-6125-2019-ie00002.svg" width="24pt" height="14pt" src="amt-12-6125-2019-ie00002.png"/></svg:svg> 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 <math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>X</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="0e660ea5f9703e988416fe4b3f1c14cd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-6125-2019-ie00003.svg" width="24pt" height="14pt" src="amt-12-6125-2019-ie00003.png"/></svg:svg> from SFIT4NIR retrievals are assessed by comparison with AirCore and aircraft measurements, and there is a 1.0 ± 0.3 % overestimation in the SFIT4NIR tropospheric <math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>X</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="92ddc872d984a732d25dbdf219dd194d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-6125-2019-ie00004.svg" width="24pt" height="14pt" src="amt-12-6125-2019-ie00004.png"/></svg:svg> and a 4.0 ± 2.0 % underestimation in the SFIT4NIR stratospheric <math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>X</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="a302efc37f65e72e8fb91bc32f887b10"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-6125-2019-ie00005.svg" width="24pt" height="14pt" src="amt-12-6125-2019-ie00005.png"/></svg:svg> , which are within the systematic uncertainties of SFIT4NIR-retrieved partial columns in the troposphere and stratosphere respectively.