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| Titel |
Three years of greenhouse gas column-averaged dry air mole fractions retrieved from satellite – Part 2: Methane |
| VerfasserIn |
O. Schneising, M. Buchwitz, J. P. Burrows, H. Bovensmann, P. Bergamaschi, W. Peters |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 9, no. 2 ; Nr. 9, no. 2 (2009-01-19), S.443-465 |
| Datensatznummer |
250006735
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| Publikation (Nr.) |
 copernicus.org/acp-9-443-2009.pdf |
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| Zusammenfassung |
| Carbon dioxide (CO2) and methane (CH4) are the two most
important anthropogenic greenhouse gases. SCIAMACHY on ENVISAT is
the first satellite instrument whose measurements are sensitive to
concentration changes of the two gases at all altitude levels down
to the Earth's surface where the source/sink signals are largest. We
have processed three years (2003–2005) of SCIAMACHY near-infrared
nadir measurements to simultaneously retrieve vertical columns of
CO2 (from the 1.58 μm absorption band), CH4
(1.66 μm) and oxygen (O2 A-band at 0.76 μm) using the
scientific retrieval algorithm WFM-DOAS. We show that the latest
version of WFM-DOAS, version 1.0, which is used for this study, has
been significantly improved with respect to its accuracy compared to
the previous versions while essentially maintaining its high
processing speed (~1 min per orbit, corresponding to
~6000 single measurements, and per gas on a standard PC). The
greenhouse gas columns are converted to dry air column-averaged mole
fractions, denoted XCO2 (in ppm) and XCH4 (in ppb), by
dividing the greenhouse gas columns by simultaneously retrieved dry
air columns. For XCO2 dry air columns are obtained from the
retrieved O2 columns. For XCH4 dry air columns are obtained
from the retrieved CO2 columns because of better cancellation of
light path related errors compared to using O2 columns retrieved
from the spectrally distant O2 A-band. Here we focus on a
discussion of the XCH4 data set. The XCO2 data set is
discussed in a separate paper (Part 1). For 2003 we present detailed
comparisons with the TM5 model which has been optimally matched to
highly accurate but sparse methane surface observations. After
accounting for a systematic low bias of ~2% agreement with TM5
is typically within 1–2%. We investigated to what extent the
SCIAMACHY XCH4 is influenced by the variability of atmospheric
CO2 using global CO2 fields from NOAA's CO2 assimilation
system CarbonTracker. We show that the CO2 corrected and
uncorrected XCH4 spatio-temporal pattern are very similar but
that agreement with TM5 is better for the CarbonTracker CO2
corrected XCH4. In line with previous studies (e.g., Frankenberg
et al., 2005b) we find higher methane over the tropics
compared to the model. We show that tropical methane is also higher
when normalizing the CH4 columns with retrieved O2 columns
instead of CO2. In consistency with recent results of Frankenberg et al.
(2008b) it is shown that the magnitude of the
retrieved tropical methane is sensitive to the
choice of the spectroscopic line parameters of water vapour.
Concerning inter-annual variability we
find similar methane spatio-temporal pattern for 2003 and 2004. For
2005 the retrieved methane shows significantly higher variability
compared to the two previous years, most likely due to somewhat
larger noise of the spectral measurements. |
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