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 mi