Isotopic Fractionation in Photolysis of Ozone in the Hartley and Chappuis Bands

Abstract

The isotopic enhancement of atmospheric ozone (O3) is mainly determined by two processes of formation and photolytic decomposition. The formation is known to be associated with isotopic fractionation which can be as high as 120‰ relative to the parent oxygen. Photolysis‐induced isotopic fractionation in O3, however, is not known accurately. Absorption cross sections of various ozone species have been calculated by Zero Point Energy‐based model and quantum mechanical wave packet model. But their predictions pertaining to isotopic fractionations have not been verified experimentally. In the present work, we performed irradiation experiments of O3 at several wavelengths in the Hartley and Chappuis bands and determined the dissociative isotopic fractionation factors by a Rayleigh distillation model. In laboratory conditions, photolysis of ozone produces reactive atomic and/or molecular oxygen which influence the dissociative process through secondary chemistry. The complex interplay of primary photolysis and secondary chemistry was tracked by a kinetic model, and isotopic fractionation factors associated with photolysis alone were derived within a range of uncertainties. The fractionations are found to depend on the photon wavelength and deviate significantly at some wavelengths from the predicted values. For example, at 254 nm the zero point energy‐based models predict lower photolysis rate coefficients for the lighter isotopic species; the present data show about 50‰ higher values.

Effects of Ozone Isotopologue Formation on the Clumped‐Isotope Composition of Atmospheric O2

“…Isotope effects in other reactions (e.g., during O 3 photolysis or chemical destruction) may also be important, but their cumulative effects on 16 O 18 O and 18 O 18 O‐containing species in the atmosphere are poorly known. Owing to the uncertainties in these quantities (Früchtl, Janssen, Rockmann, 2015, Früchtl, Ianssen, Taraborrelli, et al., 2015; Huang et al., 2019; Liang et al., 2006), this model only explores the relationship between the isotope effects for O( 3 P ) + O 2 isotope exchange and O 3 formation. We acknowledge that the results obtained are necessarily incomplete, although they are still informative and relevant to the global atmosphere.…”

Atmospheric Effects on the Isotopic Composition of Ozone

“…See later sections for details. To check the validity of the adopted fractionations, we compare the calculated values with recently measured numbers at several wavelengths in the Chappuis band [30][31][32].…”

“…Photolysis of ozone occurs primarily in the Hartley and Huggins bands in the UV and Chappuis bands at longer wavelengths (450-850 nm). Chappuis band photolysis dominates for altitudes <35 km and could deplete heavy ozone [5,31,32,[41][42][43]. Due to limited photolytic data for major isotopolocules of ozone in the visible band [31,32,44], we followed typical theories to estimate the fractionation.…”

Lifetimes and decay mechanisms of isotopically substituted ozone above the dissociation threshold: matching quantum and classical dynamics

“…82 It is now well established [4][5][6] that the ozone MIF is not a minor and exotic phenomenon of a limited impact: its effect is photochemically transferred to many oxygen-bearing species including atmospheric CO 2 , O 2 , and water in the early solar system. 6,91,92 There is a wide area of interdisciplinary scientific fields, including atmospheric physics, geo-and cosmochemistry or climate research, 5,[93][94][95][96][97] where the ozone isotope composition, or molecular effects of isotope fractionation, plays a major role. [98][99][100] The state-of-the art of the MIF studies and possible impacts across various branches of science have been described in recent exhaustive reviews by Thiemens and Lin, 5 Thiemens et al, 20 Dauphas and Schauble, 4 and Carlstad and Boering, 6 who have concluded that ''The origin in chemical physics of the large, robustly measured mass-independent enrichments in ozone resulting from the three-body ozone formation reaction remains elusive and represents an intriguing chemical physics puzzle that deserves further study''.…”

“…It is now well established 4–6 that the ozone MIF is not a minor and exotic phenomenon of a limited impact: its effect is photochemically transferred to many oxygen-bearing species including atmospheric CO 2 , O 2 , and water in the early solar system. 6,91,92 There is a wide area of interdisciplinary scientific fields, including atmospheric physics, geo- and cosmochemistry or climate research, 5,93–97 where the ozone isotope composition, or molecular effects of isotope fractionation, plays a major role. 98–100…”