HR: 0800h
AN: A51E-0136    [Abstracts]
TI: ¹³C¹⁸O in Earth's Atmosphere: a New Proxy for Constraining CO Budget
AU: * Guo, W
EM: wfguo@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Eiler, J M
EM: eiler@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AB: Despite its low average mixing ratio (70-100 ppbv), carbon monoxide plays an important role in atmospheric chemistry. It is the major sink of OH radicals, and thus strongly influences the oxidizing capacity of the atmosphere, and the lifetimes of many other atmospheric trace gases (e.g., methane, NHMCs and HCFCs). At present, the budget of atmospheric CO is constrained by its mixing ratio, δ¹³C, δ¹⁸O, δ¹⁷O values, inventory analysis and inverse modeling. However, the major sources of CO (CH₄-oxidation, NMHC-oxidation, biomass burning, anthropogenic emissions and the ocean) vary in flux and isotopic composition, and some overlap one another in isotopic composition. Therefore, its atmospheric budget cannot be rigorously defined by inversion of the existing isotopic and concentration records. Here we introduce measurements of the abundance anomaly of the ¹³C¹⁸O isotopologue of carbon monoxide as an additional constraint on its atmospheric budget. We define the ¹³C¹⁸O anomaly as the deviation of its actual abundance from its expected statistical abundance,Δ¹³C¹⁸O=(([¹³C¹⁸O]_actual/[¹²C¹⁶O]_actual)/([¹³C¹⁸O]_{stati stical} /[¹²C¹⁶O]_statistical)-1)×1000. Abundances of ¹³C¹⁸O are measured by quantitatively oxidizing CO to CO₂ over the Schutze reagent, and then measuring mass 47 (mainly ¹³C¹⁸O¹⁶O) in the product CO₂, which is proportional to the abundance of ¹³C¹⁸O in the starting CO. External precision of Δ¹³C¹⁸O for repeated measurements of pure CO averages 0.03‰(one standard deviation). We expect Δ¹³C¹⁸O in atmospheric carbon monoxide to be sensitive to: mixing between CO of different isotopic compositions, thermodynamic fractionations, diffusion, and kinetic isotope effects accompanying chemical reactions. We have investigated the thermodynamic fractionation of Δ¹³C¹⁸O by performing measurements on carbon monoxide samples catalytically equilibrated at high temperatures (300-1000°C). Measured Δ¹³C¹⁸O values, ranging from ~0.08‰ to ~0.47‰, vary as a function of temperature of heating and follow the trend predicted by previous thermodynamic calculations[1]. These results may be relevant for combustion and anthropogenic CO sources. Future measurements aim to determine the Δ¹³C¹⁸O signals characteristic of other sources and sinks. We predict that: mixing between CO of different isotopic compositions will generate ~0.1‰ enrichments in atmospheric Δ¹³C¹⁸O; gaseous diffusion will enrich the ``diffused'' population by up to ~1.1‰ relative to the residual gas; CO produced from high temperature processes (e.g. biomass and fuel burning) will have a relatively low Δ¹³C¹⁸O (e.g.~0.1‰ for 700°C); the Δ¹³C¹⁸O value of CO derived from oxidation of methane and NMHCs is suspected to be positive, but with a magnitude that is hard to predict at present; and reaction of CO with OH (the dominant atmospheric sink of CO), would produce Δ¹³C¹⁸O depletions in residual CO on the order of per mil to tens of per mil. Because CO doesn't undergo isotopic exchange with water at earth surface conditions, we expect these signatures to be preserved in the atmosphere and serve as an additional constraint on atmospheric CO budget. References [1]Wang, Schauble et al. (2004) GCA 68,4779-4797
DE: 0317 Chemical kinetic and photochemical properties
DE: 0322 Constituent sources and sinks
DE: 0394 Instruments and techniques
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005