Mass independent fractionation ozone

Thiemens, M. H. and Heidenreich, J. E. The mass independent fractionation of ozone. A novel isotope effect and its possible cosmochemical implications. Science 219, 1073 (1983). 

Thiemens MH, Heidenreich JE (1983) The mass independent fractionation of oxygen a novel isotope effect and its possible cosmochemical implications. Science 219 1073-1075 Thiemens MH, Jackson TL, Brenninkmeijer CAM (1995) Observation of a mass-independent oxygen isotopic composition in terrestrial stratospheric COj, the link to ozone chemisdy, and the possible occurrence in the Martian atmosphere. Geophys Res Lett 22 255-257 Timmes FX, Woosley SE, Weaver TA(1995) Galactic chemical evolution hydrogen through zinc. Astrophys J Suppl 98 617-658... 

A few processes in nature do not follow the above mass-dependent fractionations. Deviations from mass-dependent fractionations were first observed in meteorites (Clayton et al. 1973) and in ozone (Thiemens and Heidenreich 1983). These mass-independent fractionations (MIF) describe relationships that violate the mass-... 

A number of experimental and theoretical studies have focused on the causes of mass-independent fractionation effects, but as summarized by Thiemens (1999), the mechanism for mass-independent fractionations remains uncertain. The best studied reaction is the formation of ozone in the stratosphere. Mauersberger et al. (1999) demonstrated experimentally that it is not the symmetry of a molecule that determines the magnitude of enrichment, but it is the difference in the geometry of the molecule. Gao and Marcus (2001) presented an advanced model, which has led to a better understanding of nonmass-dependent isotope effects. 

Ozone has become one of the most important molecules in atmospheric research. In situ mass-spectrometric measurements by Mauersberger (1981, 1987) demonstrated that an equal enrichment in O and 0 of about 40% exists in the stratosphere, with a maximum at about 32 km. The rate of formation of isotopically partially substituted ozone (mass 50) is obviously faster than that of unsubstituted ozone (mass 48). Later measurements by Krankowsky et al. (2000) did not confirm the very large emichments originally reported by Mauersberger, but gave enrichments of 7-11%. Similar mass-independent fractionations have been observed in laboratory experiments by Thiemens and Heidemeich (1983) which are clearly temperature dependent. 

A chemically based, mass-independent fractionation process was first observed during ozone formation through the gas-phase recombination reaction (Thiemens Heidenreich 1983) O + O2 + M - O3 + M. The product ozone possesses equally enriched heavy-oxygen isotopes I7 IS0. by approximately lOO /oo with respect to the initial oxygen, with a slope value of unity in a three-isotope oxygen plot. This discovery led to the conclusion that a symmetry-dependent reaction can produce meteoritic isotopic anomalies (Thiemens 1999, 2006). Recently, theoretical calculations of Gao Marcus (2001) established the major role of symmetry in isotopolog-specific stabilization of vibrationally excited ozone molecules that give rise to the mass-independent compositions. 

Mauersberger (1981), utilizing in situ mass spectrometric measurements, demonstrated that ozone possesses a large 0 enrichment. The O isotopic composition was not determined and the mass-independent isotopic composition could not be detected. As reviewed by Thiemens (1999), Weston (1999), and Johnston and Thiemens (2003), there now exists an extensive literature on stratospheric ozone isotopic measurements obtained by different techniques. Measurements by Mauersberger (1987) confirmed that stratospheric ozone was mass-independently fractionated as displayed in the 1983 laboratory experiments of Thiemens and Heidenreich. Return ozone isotopic analysis by Schueler et al. (1990) demonstrated that stratospheric ozone possessed an isotopic composition entirely consistent with laboratory observations. Tropospheric ozone has also been studied for its 5 0 isotopic... 

Another important application of the CO2 isotopic measurements is their use as a measure of stratosphere-troposphere mixing. Stratospheric CO2 is mass-independently fractionated due to its coupling with ozone, while tropospheric is mass dependent because of its equilibrium with water. As originally described by Urey (1947), this is a purely mass-dependent process and has been confirmed by laboratory measurements (Thiemens et al., 1995). This feature provides an ideal marker for the two individual atmospheric reservoirs. 

In all known aqueous, solid-state and UV-shielded atmospheric processes = 0.5155 " S and = 1.905 S the fractionation is thus said to be mass dependent. Recently (Farquhar et al, 2000a, 2002 Pavlov and Kasting, 2002), mass-independent fractionation of sulfur, an atmospheric process that occurs only in the presence of a high UV-flux, has been used as a tool to help resolve the debate concerning the rise of stratospheric ozone and atmospheric oxygen. 

Isotope Effects in Unimolecular Processes Mass Independent Isotope Fractionation and the Ozone Problem... 

Theory of Mass Independent Isotope Fractionation of Ozone... 

Mass-independent isotopic fractionations are widespread in the earth s atmosphere and have been observed in O3, CO2, N2O, and CO, which are all linked to reactions involving stratospheric ozone (Thiemens 1999). For oxygen, this is a characteristic marker in the atmosphere (see Sect. 3.9). These processes probably also play a role in the atmosphere of Mars and in the pre-solar nebula (Thiemens 1999). Oxygen isotope measurements in meteorites demonstrate that the effect is of significant importance in the formation of the solar system (Clayton et al. 1973a) (Sect. 3.1). 

Ozone is not the only molecule that shows a symmetry-based mass-independent effect the reaction CO + O —> CO2 (Bhattacharya Thiemens 1989 Pandey Bhattacharya 2006) also shows similar behavior and emphasizes the role of symmetry as being of general nature in isotopic fractionation processes. This is particularly significant for the nebula as the oxidative reactions leading to solid symmetric silicates undoubtedly occurred there, e.g. SiO + O —OSiO. 

The principal focus of the present article is on the "mass-independent isotope fractional effect" (MIF) found in atmospheric and laboratory produced ozone. When this MIF occurs, a plot of the positive or negative "enrichment" in samples versus that of in those same samples has a slope of approximately unity, rather than its typical value of about 0.52. The 0.52 is the value expected using conventional transition state theory when nuclear tunneling effects are absent. For an isotope Q, 5Q is defined in per mil as 1000 [(Q/0)/(Q/0)std - 1]/ where Q/O is the ratio of Q to in the sample and std refers to its value in some standard sample, standard mean ocean water. An example of a three-isotope plot showing a slope of 0.52 is given in Figure 2.1. 

Understanding the mass-independent isotope fractionation effect for ozone in the laboratory [5] and stratosphere [9] poses interesting challenges. These chal-... 

The mass-independent isotope fractionation in the gas-phase synthesis of O3 from O2 produces a slope-1 line on a three-isotope plot, with ozone being depleted in and residual oxygen... 

When the mass-independent isotopic fractionation chemical process was first discovered by Thiemens and Heidenreich (1983), there existed no physical-chemical mechanism that accounted for the ozone observations. In this paper, a mechanism based upon optical self-shielding was proposed. Although this mechanism may not account for the experimental results, there are potential cosmochemical environments where self-shielding may be operative, as discussed in this paper. These potential applications will be discussed in detail in a later section of this chapter. 

This chapter focuses upon some recent observations of mass-independent isotopic processes in nature. As discussed by Thiemens et al. (2001) and Thiemens (2002), there exist other mass-independent isotope effects in nature that derive from non-ozone reactions. For example, CO2 photolysis produces a large mass-independent isotope effect that, in part, may account for observations in the SNC (martian) meteorites and the synthesis of their secondary minerals. UV photolysis of SO2 produces new isotopic fractional effect. An accompanying mass-independent isotopic composition determines the evolution of oxygen in the Earth s earliest atmosphere. 

Laboratory studies have shown that the most likely explanation for mass-independent sulfur isotope fractionations within the Earth system is through reactions which take place within the gas phase and thereby provide an important geochemical fingerprint of atmospheric processes (Farquhar et al., 2000, 2002). A particularly important reaction is the photochemical oxidation of sulfur in the atmosphere. Today this reaction is prevented by the presence of ozone and oxygen in the atmosphere which shield the lower atmosphere from the ultraviolet radiation required for this reaction. Experimental studies show that only tiny amounts of atmospheric oxygen are needed to prevent the photochemical oxidation of sulfur, indicating that photochemical oxidation can only take place in an atmosphere with very low levels of oxygen. 

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