Long-term validation of tropospheric column-averaged CH<sub>4</sub> mole fractions obtained by mid-infrared ground-based FTIR spectrometry

Abstract

Abstract. At the Izaña Atmospheric Research Center, highresolution 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 (CH 4 ) from the measured spectra. We investigate the total column-averaged dry air mole fractions of methane (totXCH 4 ) 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 CH 4 contribution and a posteriori correct the totXCH 4 data of a profile scaling retrieval accordingly (troXCH 4 post ); secondly, we directly determine the tropospheric column-averaged dry air mole fractions of methane (troXCH 4 retr ) from retrieved CH 4 profiles. Our theoretical estimation indicates that the scaling retrieval leads to totXCH 4 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 CH 4 data to Izaña's Global Atmospheric Watch (GAW) surface in-situ CH 4 data (CH 4 GAW ), which in the case of the Izaña Atmospheric Research Center high mountain observatory are very representative for the free tropospheric CH 4 amounts. Concerning totXCH 4 , the agreement between the FTIR data product and the in-situ measurement is rather poor documenting that totXCH 4 is not a valid free tropospheric CH 4 proxy, as it is significantly affected by the varying stratospheric CH 4 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 CH 4 contribution. In contrast the profile retrieval allows for a direct estimation of the tropospheric column-averaged CH 4 amounts. Results of the profile retrieval analysis correlate well with the CH 4 GAW 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 troXCH 4 retr and CH 4 GAW (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 CH 4 profile retrievals as well as the robustness of the approach for achieving total and tropospheric column-averaged XCH 4 data of high quality.

Building the COllaborative Carbon Column Observing Network (COCCON): long-term stability and ensemble performance of the EM27/SUN Fourier transform spectrometer

“…For the PROFFIT retrieval, it is suspected that part of the variability stems from insufficiencies in the utilized HITRAN 2008 H 2 O linelist. It was reported by Tallis et al (2011) that in the 8000-9200 cm −1 region, line intensities are low by up to 20 % compared to other wavenumber regions. This in return will lead to a systematic overestimation of the water column, which also affects X air .…”

Ten years of atmospheric methane from ground-based NDACC FTIR observations

“…Due to its geographic location, it is particularly valuable for the investigation of dust transport from Africa to the North Atlantic, and large-scale transport from the tropics to higher latitudes. In addition, during daytime the strong insolation generates a slight upslope flow of air originating from below the inversion layer (from a woodland that surrounds the station at a lower altitude, Sepúlveda et al, 2012). The solar absorption spectra were obtained with a Bruker IFS 120M over 1999-2004, then with a Bruker IFS 125HR thereafter (Sepúlveda et al, 2012).…”

“…However, it is worth mentioning that given the remaining inconsistencies affecting the methane spectroscopic parameters, even in the latest editions of HITRAN, the harmonization of retrieval strategies for methane for the whole infrared working group of NDACC is still ongoing. To this day, FTIR measurements are analyzed as recommended either by Rinsland et al (2006), Sussmann et al (2011), or Sepúlveda et al (2012. Table 2 presents the retrieval parameters used for each station.…”

Tropospheric CH<sub>4</sub> signals as observed by NDACC FTIR at globally distributed sites and comparison to GAW surface in-situ measurements

“…A similar -although not equivalent -independence between the tropospheric and the stratospheric retrieval results might be achieved by working with a constraint that is closer to the inverse of the actual a priori covariance than the constraint we introduce by the Tikhonov-Phillips setup. For instance, Stiller et al (1995), use a so-called "partitioning retrieval" for an a priori defined separation of the tropospheric and stratospheric amounts. The advantage of our a posteriori method is that we can globally apply a uniform retrieval setup with the same Tikhonov-Phillips constraint.…”