A new feature in the internal heavy isotope distribution in ozone

Abstract

Ozone produced by discharge or photolysis of oxygen has unusually heavy isotopic composition ((18)O/(16)O and (17)O/(16)O ratio) which does not follow normal mass fractionation rule: δ(17)O ∼ 0.52(*)δ(18)O, expressed as an anomaly Δ(17)O = δ(17)O - 0.52(*)δ(18)O. Ozone molecule being an open isosceles triangle can have the heavy isotope located either in its apex or symmetric (s) position or the base or asymmetric (as) position. Correspondingly, one can define positional isotopic enrichment, written as δ(18)O (s) or δ(18)O (as) (and similarly for δ(17)O) as well as position dependent isotope anomaly Δ(17)O (s) and Δ(17)O (as). Marcus and co-workers have proposed a semi-empirical model based in principle on the RRKM model of uni-molecular dissociation but with slight modification (departure from statistical randomness assumption for symmetrical molecules) which explains many features of ozone isotopic enrichment. This model predicts that the bulk isotope anomaly is contained wholly in the asymmetric position and the Δ(17)O (s) is zero. Consequently, Δ(17)O (as) = 1.5 (*) Δ(17)O (bulk) (named here simply as the "1.5 rule") which has been experimentally confirmed over a range of isotopic enrichment. We now show that a critical re-analysis of the earlier experimental data demonstrates a small but significant departure from this 1.5 rule at the highest and lowest levels of enrichments. This departure provides the first experimental proof that the dynamics of ozone formation differs from a statistical model constrained only by restriction of symmetry. We speculate over some possible causes for the departure.

Global inorganic nitrate production mechanisms: comparison of a global model with nitrate isotope observations

“…Previous modeling studies showed good agreement with observations of 17 O(nitrate) when assuming that the bulk oxygen isotopic composition of ozone ( 17 O(O 3 )) is equal to 35 ‰ (Alexander et al, 2009;Michalski et al, 2003) but varied in their assumption on terminal oxygen atom versus statistical isotopic transfer from O 3 to the reactant (NO and NO 2 ). This is an important distinction because it is now known that the 17 O enrichment in O 3 is contained entirely in its terminal oxygen atoms, and it is the terminal oxygen atom that is transferred from O 3 Berhanu et al, 2012;Bhattacharya et al, 2008Bhattacharya et al, , 2014Savarino et al, 2008;Michalski and Bhattacharya, 2009), so that the 17 O value of the oxygen atom transferred from ozone to the product is 50 % larger than the bulk 17 O(O 3 ) value. Recently, much more extensive observations of 17 O(O 3 ) using a new technique (Vicars et al, 2012) consistently show 17 O(O 3 ) = 26±1 ‰ in diverse locations (Vicars et al, 2012;Ishino et al, 2017;Vicars and Savarino, 2014) and suggest that previous modeling studies are biased low in 17 O(nitrate) (e.g., Alexander et al 2009), which would occur if the model underestimated the relative role of ozone in NO x chemistry.…”