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| Titel |
Three years of greenhouse gas column-averaged dry air mole fractions retrieved from satellite – Part 1: Carbon dioxide |
| VerfasserIn |
O. Schneising, M. Buchwitz, J. P. Burrows, H. Bovensmann, M. Reuter, J. Notholt, R. Macatangay, T. Warneke |
| 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 ; 8, no. 14 ; Nr. 8, no. 14 (2008-07-21), S.3827-3853 |
| Datensatznummer |
250006288
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| Publikation (Nr.) |
 copernicus.org/acp-8-3827-2008.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 XCO2 data set. The XCH4 data set is
discussed in a separate paper (Part 2). In order to assess the
quality of the retrieved XCO2 we present comparisons with Fourier
Transform Spectroscopy (FTS) XCO2 measurements at two northern
hemispheric mid-latitude ground stations. To assess the quality
globally, we present detailed comparisons with global XCO2 fields
obtained from NOAA's CO2 assimilation system CarbonTracker. For
the Northern Hemisphere we find good agreement with the reference
data for the CO2 seasonal cycle and the CO2 annual increase.
For the Southern Hemisphere, where significantly less data are
available for averaging compared to the Northern Hemisphere, the
CO2 annual increase is also in good agreement with CarbonTracker
but the amplitude and phase of the seasonal cycle show systematic
differences (up to several ppm) arising partially from the O2
normalization most likely caused by unconsidered scattering effects
due to subvisual cirrus clouds.
The retrieved XCO2 regional pattern at monthly
resolution over various regions show clear correlations with
CarbonTracker but also significant differences. Typically the
retrieved variability is about 4 ppm (1% of 380 ppm) higher but
depending on time and location differences can reach or even exceed
8 ppm. Based on the error analysis and on the comparison with the
reference data we conclude that the XCO2 data set can be
characterized by a single measurement retrieval precision (random
error) of 1–2%, a systematic low bias of about 1.5%, and by a
relative accuracy of about 1–2% for monthly averages at a spatial
resolution of about 7°×7°. When averaging the
SCIAMACHY XCO2 over all three years we find elevated CO2
over the highly populated region of western central Germany
and parts of the Netherlands ("Rhine-Main area")
reasonably well correlated with EDGAR anthropogenic CO2 emissions.
On average the regional enhancement is 2.7 ppm
including an estimated contribution of 1–1.5 ppm due to
aerosol related errors and sampling. |
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