Oxygen isotopic composition of carbon dioxide in the middle atmosphere
- †Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125;
- ¶Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia; and
- §Research Center for Environmental Changes, Academia Sinica, 128 Sec. 2, Academia Road, Nankang, Taipei 115, Taiwan
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Communicated by Inez Y. Fung, University of California, Berkeley, CA, November 10, 2006 (received for review January 31, 2006)
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Fig. 1.Fractionation factors of molecular oxygen in the vacuum UV calculated by using the model described in Liang et al. (25). The fractionation factor is defined by 1,000 × (σ/σ0 − 1), where σ0 and σ are the photoabsorption cross-sections of normal and isotopically substituted molecules, respectively. The absorption cross-section of normal O2 is taken from the literature (30–33).
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Fig. 2.Absorption cross-sections for O2 that lead to the production of O(3P) and O(1D) near Lyman-α. The solar profile is shown by the dotted line. All quantities are normalized. Normalization factors for the Lyman-α, cross-sections at 100, 200, and 300 K are 5.1 × 1011 photons cm−2 s−1Å−1, and 1.73 × 10−18, 1.75 × 10−18, and 1.76 × 10−18 cm2, respectively.
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Fig. 3.Vertical reaction rate profiles for the O(1D) + CO2 chemical exchange (solid line) and CO2 photolysis (dotted line). Dashed and dash-dotted lines represent O(1D) production rates for cases with and without including O2 Lyman-α photolysis, respectively.
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Fig. 4.Three-isotope plot of oxygen in CO2, from which the mean tropospheric values have been subtracted. The atmospheric measurements are from balloon measurements of Lämmerzahl et al. (8) (circles) and the full rocket data set first reported by Thiemens et al. (10) (asterisks). The solid line depicts the model, and the change in slope at A corresponds to an altitude of ≈55 km. At higher altitudes (and for fractionations greater than these fiducial values), the slope m(A-B) is ≈0.3, as expected from oxygen photolysis. Another change of slope in the calculation occurs at B for altitudes of ≈90 km and higher. Over this range, molecular diffusion dominates, and the slope becomes mass dependent, that is ≈0.5 (dash-dotted line). (Inset) Vertical profiles of δ18O(CO2) with (solid) and without (dotted) including the fractionation of O(1D) induced by O2 Lyman-α photolysis. For comparison, the vertical profile of δ18O in O2 is shown by the dash-dotted line, which can be satisfactorily explained by eddy and molecular diffusion processes for this photochemically steady-state atmosphere.
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Fig. 5.A plot of the value of the mass-independent isotopic fractionation in atmospheric CO2 vs. the nitrous oxide abundance. Rocket (asterisks) and airborne (diamonds) data are taken from Thiemens et al. (10) and Boering et al. (7), and the solid line presents the results of our standard 1D simulations using the canonical eddy diffusion coefficient of Allen et al. (19). The dotted and dashed lines represent cases for a 30% reduction and enhancement of the eddy diffusion constant of <40 km, respectively.
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Fig. 6.A three-box mixing model for CO2 in the middle atmosphere. The tropospheric δ values of CO2 have been subtracted. The symbols denote additional mixing with box 3 to varying extent. Squares, no mixing with box 3; triangles, 0.05% of air from box 3; diamonds, 0.1% of air from box 3. Asterisks are taken from Thiemens et al. (10). The solid line illustrates the fractionation expected from the interaction of CO2 and O3 only, and the dotted line presents an example of how the three isotope slope can be flattened by the mixing of air from boxes 2 (stratosphere) and 3 (mesosphere).
Footnotes
- ‡To whom correspondence should be sent at the † address. E-mail: mcl{at}gps.caltech.edu
- © 2006 by The National Academy of Sciences of the USA
