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| Titel |
Tropospheric methane in the tropics – first year from IASI hyperspectral infrared observations |
| VerfasserIn |
C. Crevoisier, D. Nobileau, A. M. Fiore, R. Armante, A. Chédin, N. A. Scott |
| 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. 17 ; Nr. 9, no. 17 (2009-09-02), S.6337-6350 |
| Datensatznummer |
250007603
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| Publikation (Nr.) |
 copernicus.org/acp-9-6337-2009.pdf |
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| Zusammenfassung |
| Simultaneous observations from the Infrared Atmospheric Sounding
Interferometer (IASI) and from the Advanced Microwave Sounding Unit (AMSU),
launched together onboard the European MetOp platform in October 2006, are
used to retrieve a mid-to-upper tropospheric content of methane (CH4)
in clear-sky conditions, in the tropics, over sea, for the first 16 months of
operation of MetOp (July 2007–October 2008). With its high spectral
resolution, IASI provides nine channels in the 7.7 μm band highly
sensitive to CH4 with reduced sensitivities to other atmospheric
variables. These channels, sensitive to both CH4 and temperature, are
used in conjunction with AMSU channels, only sensitive to temperature, to
decorrelate both signals through a non-linear inference scheme based on
neural networks. A key point of this approach is that no use is made of prior
information in terms of methane seasonality, trend, or geographical patterns.
The precision of the retrieval is estimated to be about 16 ppbv
(~0.9%). Features of the retrieved methane space-time distribution
include: (1) a strong seasonal cycle of 30 ppbv in the northern tropics with
a maximum in January–March and a minimum in July–September, and a flat
seasonal cycle in the southern tropics, in agreement with in-situ
measurements; (2) a latitudinal decrease of 30 ppbv from 20° N to
20° S, in boreal spring and summer, lower than what is observed at the
surface but in excellent agreement with tropospheric aircraft measurements;
(3) geographical patterns in good agreement with simulations from the
atmospheric transport and chemistry model MOZART-2, but with a higher
variability and a higher concentration in boreal winter; (4) signatures of
CH4 emissions transported to the middle troposphere such as a large
plume of elevated tropospheric methane south of the Asian continent, which
might be due to Asian emissions from rice paddies uplifted by deep convection
during the monsoon period and then transported towards Indonesia. In addition
to bringing a greatly improved view of methane distribution, these results
from IASI should provide a means to observe and understand atmospheric
transport pathways of methane from the surface to the upper troposphere. |
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