A new class of oxygen isotopic fractionation in photodissociation of carbon dioxide: Potential Implications for atmospheres of Mars and Earth

Bhattacharya, S. K.; Savarino, Joël; Thiemens, M. H.

doi:10.1029/1999gl010793

Cited by 72 publications

(53 citation statements)

References 23 publications

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“…Discrepancies between laboratory and stratospheric measurements (6 -8) have prompted questions about whether the O( 1 D)ϩCO 2 isotope exchange reaction acts alone on stratospheric CO 2 or whether other photochemical (9) or dynamical (10) processes significantly affect the stable isotopologue distribution in CO 2 . Stratospheric oxygen isotope covariations in CO 2 (8,11,12) consistently differ from those found in laboratory experiments simulating stratospheric photochemistry (6,(13)(14)(15)(16) Table 1).…”

mentioning

confidence: 99%

Large and unexpected enrichment in stratospheric ¹⁶ O ¹³ C ¹⁸ O and its meridional variation

Yeung¹,

Affek

Hoag³

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

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The stratospheric CO 2 oxygen isotope budget is thought to be governed primarily by the O( 1 D)+CO 2 isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare CO 2 isotopologue 16 O 13 C 18 O, in concert with 18 O and 17 O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric 16 O 13 C 18 O, observed as proportions in the polar vortex that are higher than in any naturally derived CO 2 sample to date. We show, through photochemical experiments, that lower 16 O 13 C 18 O proportions observed in the midlatitudes are determined primarily by the O( 1 D)+CO 2 isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher 16 O 13 C 18 O proportions in the polar vortex show correlations with long-lived stratospheric tracer and bulk isotope abundances opposite to those observed at midlatitudes and, thus, opposite to those easily explained by O( 1 D)+CO 2 . We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO 2 or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric 16 O 13 C 18 O enrichments may impose additional isotopic constraints on biosphere–atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change.

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confidence: 99%

Large and unexpected enrichment in stratospheric ¹⁶ O ¹³ C ¹⁸ O and its meridional variation

Yeung¹,

Affek

Hoag³

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

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“…This light is below the singlet channel dissociation threshold at 167 nm, and dissociation can only proceed via coupling to the triplet states. The experiments (32,33) found that product CO and O 2 are significantly enriched in 17 O, although being neither enriched nor depleted in 18 O compared with the initial CO 2 . This observed mass-independent fractionation was interpreted as evidence of a predissociative mechanism dominated by hyperfine interaction, particularly coupling between singlet and triplet electronic states dependent on both the electronic and nuclear spin ( 16 O and 18 O are spin 0, whereas 17 O is spin 5=2).…”

Section: Isotope Effectsmentioning

confidence: 99%

“…Our assertation, that hyperfine interactions are not causing the observed fractionation pattern, is further supported by the observed photolysis quantum yield of unity at 184.9 nm (35) for CO 2 , which leaves no room for a preferential predissociation of 17 O-substituted CO 2 . The experimental results (32)(33)(34) can be explained if the width of the Hg emission is narrower than indicated, with structure on a subnanometer scale. The experimental observations may arise from resonance between the narrowly structured emission lines of the lamp and the isotopologue-dependent fine structure of the CO 2 cross-section.…”

Section: Isotope Effectsmentioning

confidence: 99%

Carbon dioxide photolysis from 150 to 210 nm: Singlet and triplet channel dynamics, UV-spectrum, and isotope effects

Schmidt

Schinke

2013

Proc. Natl. Acad. Sci. U.S.A.

109

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We present a first principles study of the carbon dioxide (CO 2 ) photodissociation process in the 150-to 210-nm wavelength range, with emphasis on photolysis below the carbon monoxide + O( 1 D) singlet channel threshold at ∼167 nm. The calculations reproduce experimental absorption cross-sections at a resolution of ∼0.5 nm without scaling the intensity. The observed structure in the 150-to 210-nm range is caused by excitation of bending motion supported by the deep wells at bent geometries in the 2 1 A′ and 1 1 A″ potential energy surfaces. Predissociation below the singlet channel threshold occurs via spin-orbit coupling to nearby repulsive triplet states. Carbon monoxide vibrational and rotational state distributions in the singlet channel as well as the triplet channel for excitation at 157 nm satisfactorily reproduce experimental data. O) are calculated, demonstrating that strong isotopic fractionation will occur as a function of wavelength. The calculations provide accurate, detailed insight into CO 2 photoabsorption and dissociation dynamics, and greatly extend knowledge of the temperature dependence of the cross-section to cover the range from 0 to 400 K that is useful for calculations of propagation of stellar light in planetary atmospheres. The model is also relevant for the interpretation of laboratory experiments on massindependent isotopic fractionation. Finally, the model shows that the mass-independent fractionation observed in a series of Hg lamp experiments is not a result of hyperfine interactions making predissociation of 17 O containing CO 2 more efficient.mass-independent fractionation | photodissociation dynamics | fine interaction | magnetic isotope effect | Mars C arbon dioxide (CO 2 ) is the main component of the atmospheres of Mars, Venus, and the Hadean Earth (1). Its photoabsorption screens solar UV light, determining altitude-dependent photolysis rates, and its concentrations and infrared absorptions make it a powerful greenhouse gas. CO 2 photodissociation is the basis of these atmospheres' photochemistry and is the primary source of carbon monoxide (CO) and O 2 . Although the initially high concentration of CO 2 during Earth's Hadean era decreased as carbonate rocks accumulated, CO 2 continued to be a prominent atmospheric gas, enhancing surface temperature and attenuating UV light, with a partial pressure >10 mbar in the Archean (2) and >1 mbar in the Proterozoic (3). Its influence on the radiative properties and chemical composition of Earth's atmosphere has continued to the present day; one example is that CO 2 photolysis is the main source of mesospheric CO (4). Variations in the abundances of naturally occurring stable isotopes, including reaction mechanisms exhibiting mass-independent fractionation, are central to efforts to interpret environmental records ranging from sedimentary rocks to oceanic carbonates to glacial ice (5). Thus, CO 2 photolysis is both directly and indirectly linked to isotopic variations found in the environment.The UV absorption band of CO 2 ð120 nm < ...

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“…Thus, there can then be unusually high-isotopic fractionations. 19,20 Also, in a threeisotope plot a slope of opposite sign compared to that for the regular mass-dependent processes also becomes possible. The result of the present work is not applicable to such spinforbidden cases.…”

Section: General Commentsmentioning

confidence: 99%

“…The rates of such reactions for isotopes of odd atomic masses with a nonzero nuclear spin and even atomic masses with a zero nuclear spin are different. 19,20 In such spin-forbidden photodissociation reactions, the isotopic fractionation de-pends on spin as well as mass. Thus, there can then be unusually high-isotopic fractionations.…”

Section: General Commentsmentioning

confidence: 99%

Three-isotope plot of fractionation in photolysis: A perturbation theoretical expression

Prakash

Marcus

2005

The Journal of Chemical Physics

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The slope of the three-isotope plot for the isotopomer fractionation by direct or nearly direct photodissociation is obtained using a perturbation theoretical analysis. This result, correct to first order in the mass difference, is the same as that for equilibrium chemical exchange reactions, a similarity unexpected a priori. A comparison is made with computational results for N 2 O photodissociation. This theoretical slope for mass-dependent photolytic fractionation can be used to analyze the data for isotopic anomalies in spin-allowed photodissociation reactions. Earlier work on chemical equilibria is extended by avoiding a high-temperature approximation.

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A new class of oxygen isotopic fractionation in photodissociation of carbon dioxide: Potential Implications for atmospheres of Mars and Earth

Cited by 72 publications

References 23 publications

Large and unexpected enrichment in stratospheric ¹⁶ O ¹³ C ¹⁸ O and its meridional variation

Large and unexpected enrichment in stratospheric ¹⁶ O ¹³ C ¹⁸ O and its meridional variation

Carbon dioxide photolysis from 150 to 210 nm: Singlet and triplet channel dynamics, UV-spectrum, and isotope effects

Three-isotope plot of fractionation in photolysis: A perturbation theoretical expression

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A new class of oxygen isotopic fractionation in photodissociation of carbon dioxide: Potential Implications for atmospheres of Mars and Earth

Cited by 72 publications

References 23 publications

Large and unexpected enrichment in stratospheric 16 O 13 C 18 O and its meridional variation

Large and unexpected enrichment in stratospheric 16 O 13 C 18 O and its meridional variation

Carbon dioxide photolysis from 150 to 210 nm: Singlet and triplet channel dynamics, UV-spectrum, and isotope effects

Three-isotope plot of fractionation in photolysis: A perturbation theoretical expression

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Large and unexpected enrichment in stratospheric ¹⁶ O ¹³ C ¹⁸ O and its meridional variation

Large and unexpected enrichment in stratospheric ¹⁶ O ¹³ C ¹⁸ O and its meridional variation