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

Abstract

At the Izaña Atmospheric Research Center high-resolution middle infrared solar absorption spectra have been recorded for more than 12 years 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 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 (troXCH4_post), and secondly, we directly determine the tropospheric column-averaged dry-air mole fractions of methane (troXCH4_retr) 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%). <br><br> We compare the different FTIR CH₄ data to Izaña's Global Atmospheric Watch (GAW) surface in-situ CH₄ data (CH_{4_GAW}), which in the case of the Izaña Atmospheric Research Center high mountain observatory are well 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 no valid free tropospheric CH₄ proxy instead 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 only removes a part of this stratospheric CH₄ contribution. On the contrary the profile retrieval allows for a direct estimation of the tropospheric column-averaged CH₄ amounts. This troXCH4_retr data correlates well with the CH_{4_GAW} data (correlation coefficient of 0.60, FTIR-GAW scatter of 0.97%) and both data show very similar annual cycles and trend behaviour (for the 2001–2010 time period). Furthermore, we find a very good absolute agreement between the troXCH4_retr and CH_{4_GAW} (middle infrared FTIR/GAW scaling factor of 0.9987) suggesting that middle infrared FTIR data can be well combined with the surface in-situ GAW data. <br><br> Our study proves the significance of middle infrared ground-based FTIR CH₄ profile retrievals as well as the robustness of the approach for achieving total and tropospheric column-averaged <i>X</i>CH₄ data of high quality

Consistency and quality assessment of the Metop-A/IASI and Metop-B/IASI operational trace gas products (O&lt;sub&gt;3&lt;/sub&gt;, CO, N&lt;sub&gt;2&lt;/sub&gt;O, CH&lt;sub&gt;4&lt;/sub&gt;, and CO&lt;sub&gt;2&lt;/sub&gt;) in the subtropical North Atlantic

“…The good quality of these long-term ground-based FTS data sets has been extensively documented by theoretical and empirical validation studies (e.g. Schneider et al, 2006Sepúlveda et al, 2012;García et al, 2012b;Sepúlveda et al, 2014;.…”

“…Table 1 lists the main FTS characteristics and highlights the main differences and similarities between IASI and these ground-based FTS instruments. For further details about the FTS instrument at IZO refer to Schneider et al (2005); Sepúlveda et al (2012); García et al (2012b, and references therein).…”

Tropospheric CH&lt;sub&gt;4&lt;/sub&gt; signals as observed by NDACC FTIR at globally distributed sites and comparison to GAW surface in situ measurements

“…The CH 4 column average is therefore strongly dependent on the tropopause altitude, which means for instance, that the seasonal cycle in column-averaged CH 4 can significantly differ from that in the free troposphere (e.g. Sepúlveda et al, 2012). The uncertainty in modelling the variations of the tropopause altitude and of stratospheric CH 4 significantly limits the usefulness of total column-averaged CH 4 observations for inverse modelling purposes.…”

“…The profiling capability is not only important for CH 4 source/sink research applications, it is also an advantage when validating column-averaged CH 4 obtained from satellites, since it allows the vertical sensitivity of the satellite data to be accounted for. In this paper, we extend the Sepúlveda et al (2012) study, which is limited to a subtropical site, to a set of nine globally distributed NDACC FTIR stations covering polar, mid-latitudinal, and subtropical regions and we focus on the quality of the lower free tropospheric CH 4 FTIR data. We document that the data are largely independent of the very local surface smallscale signals, and only weakly dependent on upper tropospheric/lower stratospheric (UTLS) CH 4 variations.…”

“…Rinsland et al (2005). Sepúlveda et al (2012) documented theoretically and empirically that the ground-based FTIR experiments operated within the NDACC (Network for the Detection of Atmospheric Composition Change, http://www.acd.ucar.edu/ irwg/, Kurylo and Zander, 2000) can provide some information on the vertical distribution of atmospheric CH 4 . They empirically documented the quality of these profile data: first, for the lower tropospheric FTIR CH 4 data there is good agreement with coincident free tropospheric GAW in situ observations and second, for the UTLS the FTIR measures CH 4 mole fraction that shows a strong anti-correlation with the stratospheric proxy HF.…”