The role of symmetry in the mass independent isotope effect in ozone

Abstract

Understanding the internal distribution of ''anomalous'' isotope enrichments has important implications for validating theoretical postulates on the origin of these enrichments in molecules such as ozone and for understanding the transfer of these enrichments to other compounds in the atmosphere via mass transfer. Here, we present an approach, using the reaction NO 2 ؊ ؉ O3, for assessing the internal distribution of the (7) and non-mass-dependent (NOMAD) (8, 9).The most thoroughly studied MIF system is that of ozone formation. Ozone generated by electric discharge or UV photolysis can produce ⌬ 17 O values of 30-40‰ (‰ ϭ parts per thousand). There have also been numerous studies that have experimentally quantified the pressure and temperature dependence of the isotopic enrichments (10, 11). Most recently, the transfer of the ⌬ 17 O anomaly from ozone to other compounds during oxidation reactions (via atom transfer, e.g., mass balance) has been used to elucidate chemical oxidation pathways in modern atmospheres (16-21). These include ⌬ 17 O observations in atmospheric nitrate, sulfate, CO 2 , and N 2 O. ⌬ 17 O variations in atmospheric nitrate and sulfate are intriguing because they have opened the possibility of using these ⌬ 17 O variations in ice cores as a proxy for paleo-oxidation chemistry (22,23). Modeling such transfer reactions, however, requires knowledge of the internal distribution of the ⌬ 17 O anomaly, i.e., how much is contained in the central versus terminal atoms of ozone, because many atmospheric oxidations are thought to occur through terminal-only transition states.Theories as to the origin of the MIF effect have taken many turns (7,8,24,25), and some theories seem more adept at handling specific experimental cases than others. Most have symmetry as the main, albeit ad hoc, causal mechanism for producing MIF. Recent work by Marcus and coworkers (26-28) is perhaps the most thorough treatment, and they have suggested that nonstatistical RRKM effects arise because of differences in the rovibronic coupling efficiency of the asymmetric molecule when compared with the symmetric isomer. Recent NO 2 spectroscopic data appear to bear the theory out (29), but the uncertainties in that study remain significant, and no mass spectrometer isotope data on the unimolecular decay of NO 2 are available. Because the basis of the ozone MIF theory is symmetry driven, it is implied that the anomalous enrichment must be contained in the terminal atoms of ozone. To be clear, there maybe overall isotopic enrichment (␦ 18 O, ␦ 17 O) distributed throughout the molecule based on isotopologue zero-point energy differences (30), but the 17 O excess (⌬ 17 O) is expected to be only in the terminal position. Others, however, have argued against the symmetry theory (31-33) and have placed anomalous enrichments, even negative ⌬ 17 O values (34), throughout ozone in an attempt to reconcile experimental observations. Precise and accurate measurements of ozone's ⌬ 17 O internal distribution is therefore needed to test the symmet...

17O excess transfer during the NO2 + O3 → NO3 + O2 reaction

“…Generally, our experimental observations agree well with previous studies in a limited bulk ozone 17 O range 6,17,27 with respect to terminal atom only transfer and excess enrichment residing on only the terminal atom.…”

“…3,4 The 17 O signal of atmospheric ozone is a conserved quantity (i.e., it cannot be altered via mass-dependent fractionation processes), which can be partially or completely transferred to other species through oxidation reactions. [5][6][7] As a result of this 17 O excess transfer, 17 O values of oxygen-bearing atmospheric species serve as proxies for the influence of ozone a) Author to whom correspondence should be addressed. Electronic mail:…”

“…15 In particular, it is believed that the 17 O excess resides on the terminal atom alone. 6 It is also believed that during oxidation reactions, there is a preference for reaction of the terminal oxygen atom of ozone. 16 Hence, the excess enrichment on this oxygen atom will be transferred to the oxidized compound and can be used as interpretive tool to study various atmospheric reactions.…”

“…17 Presently, there is no analogous study of anomaly transfer study during the gas phase reaction of NO 2 with O 3 (R5). Michalski and Bhattacharya 6 have recently studied the aqueous phase NO 2 − reaction with ozone in order to investigate the intramolecular oxygen isotope distribution of ozone and anomaly transfer during formation of NO 3 − . This study suggests that the ozone isotopic anomaly resides entirely on terminal atoms of ozone when a limited range of 17 O values of ozone is considered (20‰-40‰).…”

NO<sub>x</sub> cycle and the tropospheric ozone isotope anomaly: an experimental investigation

“…This pattern has been observed by Tuzson and Janssen (2006) by spectroscopy, albeit with large errors. Hence, by isotope mass balance, the 17 O values of the terminal atoms should be 3/2 times higher than the 17 O value of the bulk O 3 (Michalski and Bhattacharya, 2009). This enrichment is believed to be transferred to various NO x species and to nitrate during oxidation by ozone.…”