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| Titel |
Estimating regional fluxes of CO2 and CH4 using space-borne observations of XCH4: XCO2 |
| VerfasserIn |
A. Fraser, P. I. Palmer, L. Feng, H. Bösch, R. Parker, E. J. Dlugokencky, P. B. Krummel, R. L. Langenfelds |
| 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 ; 14, no. 23 ; Nr. 14, no. 23 (2014-12-08), S.12883-12895 |
| Datensatznummer |
250119214
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| Publikation (Nr.) |
 copernicus.org/acp-14-12883-2014.pdf |
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| Zusammenfassung |
| We use the GEOS-Chem global 3-D atmospheric chemistry transport
model to interpret XCH4:XCO2 column ratios
retrieved from the Japanese Greenhouse Gases Observing Satellite
(GOSAT). The advantage of these data over CO2 and
CH4 columns retrieved independently using a full physics
optimal estimation algorithm is that they are less prone to
scattering-related regional biases. We show that the model is able to
reproduce observed global and regional spatial (mean
bias =0.7%) and temporal variations (global r2=0.92) of
this ratio with a model bias < 2.5%. We also show that these
variations are driven by emissions of CO2 and CH4
that are typically 6 months out of phase, which may reduce the
sensitivity of the ratio to changes in either gas. To
simultaneously estimate fluxes of CO2 and CH4 we use
a maximum likelihood estimation approach. We use two approaches to
resolve independent flux estimates of these two gases using GOSAT
observations of XCH4:XCO2: (1) the a priori error
covariance between CO2 and CH4 describing common
source from biomass burning; and (2) also fitting independent
surface atmospheric measurements of CH4 and CO2 mole
fraction that provide additional constraints, improving the
effectiveness of the observed GOSAT ratio to constrain flux estimates. We
demonstrate the impact of these two approaches using numerical
experiments. A posteriori flux estimates inferred using only the
GOSAT ratios and taking advantage of the error covariance due to
biomass burning are not consistent with the true fluxes in our
experiments, as the inversion system cannot judge which species'
fluxes to adjust. This reflects the weak dependence of
XCH4:XCO2 on biomass burning. We find that adding
the surface data effectively provides an "anchor" to the inversion
that dramatically improves the ability
of the GOSAT ratios to infer both CH4 and CO2
fluxes. We show that the regional flux estimates inferred
from GOSAT XCH4:XCO2 ratios together with the
surface mole fraction data during 2010 are typically consistent with or
better than the corresponding values inferred from fitting
XCH4 or the full-physics XCO2 data products, as
judged by a posteriori uncertainties. We show that the fluxes inferred from
the ratio measurements perform best over regions where there is a
large seasonal cycle such as Tropical South America, for which we
report a small but significant annual source of CO2 compared
to a small annual sink inferred from the XCO2 data. We
argue that given that the ratio measurements are less compromised by
systematic error than the full physics data products, the resulting a~posteriori
estimates and uncertainties provide a more faithful description of
the truth. Based on our analysis we also argue that by using the
ratios we may be reaching the current limits on the precision of these
observed space-based data. |
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