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| Titel |
Simulation of atmospheric N2O with GEOS-Chem and its adjoint: evaluation of observational constraints |
| VerfasserIn |
K. C. Wells, D. B. Millet, N. Bousserez, D. K. Henze, S. Chaliyakunnel, T. J. Griffis, Y. Luan, E. J. Dlugokencky, R. G. Prinn, S. O'Doherty, R. F. Weiss, G. S. Dutton, J. W. Elkins, P. B. Krummel, R. Langenfelds, L. P. Steele, E. A. Kort, S. C. Wofsy, T. Umezawa |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1991-959X
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| Digitales Dokument |
URL |
| Erschienen |
In: Geoscientific Model Development ; 8, no. 10 ; Nr. 8, no. 10 (2015-10-08), S.3179-3198 |
| Datensatznummer |
250116601
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| Publikation (Nr.) |
 copernicus.org/gmd-8-3179-2015.pdf |
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| Zusammenfassung |
| We describe a new 4D-Var inversion framework for nitrous oxide (N2O) based on the
GEOS-Chem chemical transport model and its adjoint, and apply it in a series
of observing system simulation experiments to assess how well N2O
sources and sinks can be constrained by the current global observing
network. The employed measurement ensemble includes approximately weekly and
quasi-continuous N2O measurements (hourly averages used) from several
long-term monitoring networks, N2O measurements collected from discrete
air samples onboard a commercial aircraft (Civil Aircraft for the Regular Investigation of the
atmosphere Based on an Instrument Container; CARIBIC), and quasi-continuous
measurements from the airborne HIAPER Pole-to-Pole Observations (HIPPO) campaigns. For a
2-year inversion, we find that the surface and HIPPO observations can
accurately resolve a uniform bias in emissions during the first year;
CARIBIC data provide a somewhat weaker constraint. Variable emission errors
are much more difficult to resolve given the long lifetime of N2O, and
major parts of the world lack significant constraints on the seasonal cycle
of fluxes. Current observations can largely correct a global bias in the
stratospheric sink of N2O if emissions are known, but do not provide
information on the temporal and spatial distribution of the sink. However,
for the more realistic scenario where source and sink are both uncertain, we
find that simultaneously optimizing both would require unrealistically small
errors in model transport. Regardless, a bias in the magnitude of the
N2O sink would not affect the a posteriori N2O emissions for the
2-year timescale used here, given realistic initial conditions, due to the
timescale required for stratosphere–troposphere exchange (STE). The same
does not apply to model errors in the rate of STE itself, which we show
exerts a larger influence on the tropospheric burden of N2O than does
the chemical loss rate over short (< 3 year) timescales. We use a
stochastic estimate of the inverse Hessian for the inversion to evaluate the
spatial resolution of emission constraints provided by the observations, and
find that significant, spatially explicit constraints can be achieved in
locations near and immediately upwind of surface measurements and the HIPPO
flight tracks; however, these are mostly confined to North America, Europe,
and Australia. None of the current observing networks are able to provide
significant spatial information on tropical N2O emissions. There,
averaging kernels (describing the sensitivity of the inversion to emissions
in each grid square) are highly smeared spatially and extend even to the
midlatitudes, so that tropical emissions risk being conflated with those
elsewhere. For global inversions, therefore, the current lack of constraints
on the tropics also places an important limit on our ability to understand
extratropical emissions. Based on the error reduction statistics from the
inverse Hessian, we characterize the atmospheric distribution of
unconstrained N2O, and identify regions in and downwind of South
America, central Africa, and Southeast Asia where new surface or profile
measurements would have the most value for reducing present uncertainty in
the global N2O budget. |
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