Long-term validation of tropospheric column-averaged CH₄ mole fractions obtained by mid-infrared ground-based FTIR spectrometry

At the Izaña Atmospheric Research Center, high-resolution mid-infrared solar absorption spectra have been recorded for more than 12 yr using Fourier Transform InfraRed (FTIR) spectrometers. We use the spectral fitting algorithm PROFFIT to retrieve long-term time series of methane (CH4) from the measured spectra. We investigate the total column-averaged dry air mole fractions of methane (totXCH4) obtained from a profile scaling and a profile retrieval, and apply two approaches for deriving the tropospheric column-averaged dry air mole fractions: firstly, we use the FTIR hydrogen fluoride (HF) total column amounts as an estimator for the stratospheric CH4 contribution and a posteriori correct the totXCH4 data of a profile scaling retrieval accordingly (troXCH4post); secondly, we directly determine the tropospheric column-averaged dry air mole fractions of methane (troXCH4retr) from retrieved CH4 profiles. Our theoretical estimation indicates that the scaling retrieval leads to totXCH4 amounts that are subject to a large smoothing error, which can be widely avoided by applying a profile retrieval (for the latter we estimate an overall precision of 0.41%). We compare the different FTIR CH4 data to Izaña's Global Atmospheric Watch (GAW) surface in-situ CH4 data (CH4GAW), which in the case of the Izaña Atmospheric Research Center high mountain observatory are very representative for the free tropospheric CH4 amounts. Concerning totXCH4, the agreement between the FTIR data product and the in-situ measurement is rather poor documenting that totXCH4 is not a valid free tropospheric CH4 proxy, as it is significantly affected by the varying stratospheric CH4 contribution and it rather follows the variation in the tropopause altitude. The a posteriori correction method as applied here only removes a part of this stratospheric CH4 contribution. In contrast the profile retrieval allows for a direct estimation of the tropospheric column-averaged CH4 amounts. Results of the profile retrieval analysis correlate well with the CH4GAW data (correlation coefficient of 0.60, FTIR-GAW scatter of 0.97%), and both data sets show very similar annual cycles and trend behaviour for the 2001–2010 time period. Furthermore, we find a very good absolute agreement between the troXCH4retr and CH4GAW (mid-infrared FTIR/GAW scaling factor of 0.9987) suggesting that mid-infrared FTIR data can be well combined with the surface in-situ GAW data. Our study strongly supports the value of mid-infrared ground-based FTIR CH4 profile retrievals as well as the robustness of the approach for achieving total and tropospheric column-averaged XCH4 data of high quality