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| Titel |
A consistent simulation of oxygen isotope mass-independent fractionation (MIF) in CO and O3 using AC-GCM EMAC |
| VerfasserIn |
Sergey Gromov, Patrick Jöckel, Carl A. M. Brenninkmeijer |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250106939
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| Publikation (Nr.) |
 EGU/EGU2015-6622.pdf |
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| Zusammenfassung |
| We present the most consistent estimate of the atmospheric distribution of oxygen
mass-independent fractionation (MIF) of carbon monoxide (δ17O(CO)Â=
(δ17O(CO)+1)/(δ18O(CO)+1)β-1, βÂ= 0.528, V-SMOW scale) inferred using the
ECHAM/MESSy Atmospheric Chemistry (EMAC) model (JöckelÂetÂal.,Â2010). Although
MIF of CO is largely determined by its removal reaction with OH, implementing a
comprehensive chemistry scheme and detailed surface emissions in EMAC allows to single
out the lesser inputs of MIF due to oxygen from ozone and other atmospheric oxygen
reservoirs. The model shows that less than 2% of CO molecules inherit their oxygen atoms
from O3 (mostly via ozonolysis reactions) which translates into an additional +0.60‰ in the
average tropospheric δ17O(CO) value. The remaining non-MIF oxygen (from water and
atmospheric O2) outbalances this input by -0.24o respectively. The chemical
kinetics of alkene ozonolysis (viz.Âyield of CO per reacted O3 and O atoms transfer
to CO) simulated in EMAC is in good agreement with the laboratory studies of
RöckmannÂetÂal.Â(1998a). This also pertains to the inferred (OH) sink-induced effective
tropospheric MIF of +(4.3±0.2)‰ in comparison to +(4.1±0.3)‰ reckoned by
RöckmannÂetÂal.Â(1998b).
The explicitly simulated tropospheric δ17O(O3) value in EMAC averages at
+30.4‰ and has small variation, which is consistent with that expected from the laboratory
data. Instead, the most recent observations of ozone tropospheric MIF (VicarsÂand
Savarino,Â2014) suggest a value of +25‰ being the most representative, which renders
the simulated MIF input from O3 in CO potentially overestimated by ~20%. The
EMAC-simulated δ18O(O3), however, agrees well with observational data, whilst sensitivity
studies confirm non-negligible increase in atmospheric δ18O(CO) due to input of O3 oxygen
to CO. A pronounced CO enrichment in heavy oxygen is expected in the stratosphere via the
reactions of methane and O(1D), provided that the latter inherits the isotope composition of
O3. Despite slightly underestimated variation, the simulated δ17O(CO) surface seasonal
cycles are in very good agreement with the observations in the NH. For the SH, where
observations of CO MIF are not available to date, the model predicts a substantially
higher average and smaller variation of δ17O(CO). Finally, EMAC ascertains that
boundary layer 13C and 18O sink effective enrichments of CO tightly correlate with
the δ17O(CO) signal, indicating that the latter can be used as a measure of CO
chemical age, i.e.Âexposure to OH. Moreover, the MIF of CO constitutes a tool for
inferring the actual (i.e.Ânot modified by sink fractionation) isotope composition of its
sources.
References:
Jöckel,ÂP., Kerkweg,ÂA., Pozzer,ÂA., Sander,ÂR., Tost,ÂH., Riede,ÂH., Baumgaertner,ÂA.,
Gromov,ÂS.,Âand Kern,ÂB.: Development cycle 2 of the Modular Earth Submodel
System (MESSy2), Geosci.ÂModelÂDev., 3, 717-752, doi:Â10.5194/gmd-3-717-2010,
2010.
Röckmann,ÂT., Brenninkmeijer,ÂC.ÂA.ÂM., Neeb,ÂP.,Âand Crutzen,ÂP.ÂJ.: Ozonolysis of
nonmethane hydrocarbons as a source of the observed mass independent oxygen
isotope enrichment in tropospheric CO, J.ÂGeophys.ÂRes.ÂAtm., 103, 1463-1470,
doi:Â10.1029/97JD02929, 1998a.
Röckmann,ÂT., Brenninkmeijer,ÂC.ÂA.ÂM., Saueressig,ÂG., Bergamaschi,ÂP.,
Crowley,ÂJ.ÂN., Fischer,ÂH.,Âand Crutzen,ÂP.ÂJ.: Mass-independent oxygen isotope
fractionation in atmospheric CO as a result of the reaction CO+OH, Science, 281, 544-546,
doi:Â10.1126/science.281.5376.544, 1998b.
Vicars,ÂW.ÂC.Âand Savarino,ÂJ.: Quantitative constraints on the 17O-excess
(δ17O) signature of surface ozone: Ambient measurements from 50Ë N to 50Ë S
using the nitrite-coated filter technique, Geochim.ÂCosmochim.ÂActa, 135, 270-287,
doi:Â10.1016/j.gca.2014.03.023, 2014. |
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