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| Titel |
Estimating regional methane surface fluxes: the relative importance of surface and GOSAT mole fraction measurements |
| VerfasserIn |
A. Fraser, P. I. Palmer, L. Feng, H. Boesch, A. Cogan, R. Parker, E. J. Dlugokencky, P. J. Fraser, P. B. Krummel, R. L. Langenfelds, S. O'Doherty, R. G. Prinn, L. P. Steele, M. Schoot, R. F. Weiss |
| 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 ; 13, no. 11 ; Nr. 13, no. 11 (2013-06-13), S.5697-5713 |
| Datensatznummer |
250018699
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| Publikation (Nr.) |
 copernicus.org/acp-13-5697-2013.pdf |
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| Zusammenfassung |
| We use an ensemble Kalman filter (EnKF), together with the GEOS-Chem
chemistry transport model, to estimate regional monthly methane (CH4)
fluxes for the period June 2009–December 2010 using proxy dry-air
column-averaged mole fractions of methane (XCH4) from GOSAT (Greenhouse
gases Observing SATellite) and/or NOAA ESRL (Earth System Research
Laboratory) and CSIRO GASLAB (Global Atmospheric Sampling Laboratory) CH4
surface mole fraction measurements. Global posterior estimates using GOSAT
and/or surface measurements are between 510–516 Tg yr−1, which is
less than, though within the uncertainty of, the prior global flux of
529 ± 25 Tg yr−1. We find larger differences between regional
prior and posterior fluxes, with the largest changes in monthly emissions
(75 Tg yr−1) occurring in Temperate Eurasia. In non-boreal regions the
error reductions for inversions using the GOSAT data are at least three times
larger (up to 45%) than if only surface data are assimilated, a reflection
of the greater spatial coverage of GOSAT, with the two exceptions of
latitudes >60° associated with a data filter and over Europe
where the surface network adequately describes fluxes on our model spatial
and temporal grid. We use CarbonTracker and GEOS-Chem XCO2 model output to
investigate model error on quantifying proxy GOSAT XCH4 (involving model
XCO2) and inferring methane flux estimates from surface mole fraction data
and show similar resulting fluxes, with differences reflecting initial
differences in the proxy value. Using a series of observing system simulation
experiments (OSSEs) we characterize the posterior flux error introduced by
non-uniform atmospheric sampling by GOSAT. We show that clear-sky
measurements can theoretically reproduce fluxes within 10% of true
values, with the exception of tropical regions where, due to a large seasonal
cycle in the number of measurements because of clouds and aerosols, fluxes
are within 15% of true fluxes. We evaluate our posterior methane
fluxes by incorporating them into GEOS-Chem and sampling the model at the
location and time of surface CH4 measurements from the AGAGE (Advanced
Global Atmospheric Gases Experiment) network and column XCH4 measurements
from TCCON (Total Carbon Column Observing Network). The posterior fluxes
modestly improve the model agreement with AGAGE and TCCON data relative to
prior fluxes, with the correlation coefficients (r2) increasing by a mean
of 0.04 (range: −0.17 to 0.23) and the biases decreasing by a mean of
0.4 ppb (range: −8.9 to 8.4 ppb). |
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