Intramolecular isotope distribution in heavy ozone (¹⁶O¹⁸O¹⁶O and ¹⁶O¹⁶O¹⁸O)

The ratio of asymmetric vs. symmetric molecules of heavy ozone, r 50 = [ 16 O 16 O 18 O]/[ 16 O 18 O 16 O], or similarly r 49 for 17 O‐containing ozone, is an essential feature of the unusual oxygen isotope effect in ozone formation. Whereas information on r 49 is sparse and of limited accuracy, precision in laboratory measurements of r 50 has recently been improved. The two most recent experiments, however, disagree on whether there is a preference for the formation of asymmetric over symmetric molecules or not ( r 50 > 2 versus r 50 = 2). To throw light on this discrepancy, the temperature dependence of r 50 has been derived from a reanalysis of existing measurements: r 50 is found to decrease from 2.13 at 360 K to about 2.00 at 170 K. Kinetic model calculations show that the discrepancy between the two measurements disappears when temperature conditions for ozone formation are accounted for. The reanalysis confirms that the isotope enrichment in atmospheric 16 O 2 18 O predominantly but not exclusively resides in the asymmetric molecule, with the preference for asymmetric molecules becoming smaller at low‐temperature conditions. The enrichment of 16 O 16 O 18 O which likely characterizes the source strength of ozone to transfer heavy oxygen ( 18 O) into other atmospheric molecules is quantified as a function of temperature and in relation to the overall enrichment of heavy ozone. This information can now be used in atmospheric models that simulate isotope transfer effects.