NO x cycle and the tropospheric ozone isotope anomaly: an experimental investigation
The oxygen isotope composition of nitrogen oxides (NO x) in the atmosphere is a useful tool for understanding the oxidation of NO x into nitric acid / nitrate in the atmosphere. A set of experiments was conducted to examine change in isotopic composition of NO x due to NO x–O 2–O 3 photochemical cycling. At low NO x / O 2 mixing ratios, NO x became progressively and nearly equally enriched in 17O and 18O over time until it reached a steady state with Δ 17O values of 39.3 ± 1.9‰ and δ 18O values of 84.2 ± 4‰, relative to the isotopic composition of the initial O 2 gas. As the mixing ratios were increased, the isotopic enrichments were suppressed by isotopic exchange between O atoms, O 2, and NO x. A kinetic model was developed to simulate the observed data and it showed that the isotope effects occurring during O 3 formation play a dominant role in controlling NO x isotopes and, in addition, secondary kinetic isotope effects or isotope exchange reactions are also important during NO x cycling. The data and model were consistent with previous studies which showed that the NO + O 3 reactions occur mainly via the transfer of the terminal atoms of O 3. The model predicts that under tropospheric concentrations of NO x and O 3, the timescale of NO x–O 3 isotopic equilibrium ranges from hours (for ppbv NO x / O 2 mixing ratios) to days (for pptv mixing ratios) and yields steady state Δ 17O and δ 18O values of 45‰ and 117‰ respectively (relative to Vienna Standard Mean Ocean Water (VSMOW)) in both cases. Under atmospheric conditions when O 3 has high concentrations, the equilibrium between NO x and O 3 should occur rapidly (h) but this equilibrium cannot be reached during polar winters and/or nights if the NO x conversion to HNO 3 is faster. The experimentally derived rate coefficients can be used to model the major NO x–O 3 isotopologue reactions at various pressures and in isotope modeling of tropospheric nitrate.