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| Titel |
Long-term validation of tropospheric column-averaged CH4 mole fractions obtained by mid-infrared ground-based FTIR spectrometry |
| VerfasserIn |
E. Sepúlveda, M. Schneider, F. Hase, O. E. García, A. Gomez-Pelaez, S. Dohe, T. Blumenstock, J. C. Guerra |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 5, no. 6 ; Nr. 5, no. 6 (2012-06-26), S.1425-1441 |
| Datensatznummer |
250002975
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| Publikation (Nr.) |
 copernicus.org/amt-5-1425-2012.pdf |
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| Zusammenfassung |
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. |
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