Abstract. The Total
Carbon Column Observing Network (TCCON) column-averaged dry air mole fraction
of CH4 (XCH4) 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 CH4 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 CH4
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 XCH4 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
XCH4 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 XCH4 and a
4.0 ± 2.0 % underestimation in the SFIT4NIR stratospheric
XCH4, which are within the systematic uncertainties of
SFIT4NIR-retrieved partial columns in the troposphere and stratosphere
respectively.
Data processing and performance evaluation of a tempo-spatially mixed modulation imaging Fourier transform spectrometer based on stepped micro-mirror