NOx cycle and the tropospheric ozone isotope anomaly: an experimental investigation

The oxygen isotope composition of nitrogen oxides (NOx) in the atmosphere is a useful tool for understanding the oxidation of NOx into nitric acid / nitrate in the atmosphere. A set of experiments was conducted to examine change in isotopic composition of NOx due to NOx–O2– O3 photochemical cycling. At low NOx / O2 mixing ratios, NOx became progressively and nearly equally enriched in 17O and 18O over time until it reached a steady state with 117O values of 39.3± 1.9 ‰ and δ18O values of 84.2± 4 ‰, relative to the isotopic composition of the initial O2 gas. As the mixing ratios were increased, the isotopic enrichments were suppressed by isotopic exchange between O atoms, O2, and NOx. A kinetic model was developed to simulate the observed data and it showed that the isotope effects occurring during O3 formation play a dominant role in controlling NOx isotopes and, in addition, secondary kinetic isotope effects or isotope exchange reactions are also important during NOx cycling. The data and model were consistent with previous studies which showed that the NO + O3 reactions occur mainly via the transfer of the terminal atoms of O3. The model predicts that under tropospheric concentrations of NOx and O3, the timescale of NOx–O3 isotopic equilibrium ranges from hours (for ppbv NOx / O2 mixing ratios) to days (for pptv mixing ratios) and yields steady state 117O and δ18O values of 45 ‰ and 117 ‰ respectively (relative to Vienna Standard Mean Ocean Water (VSMOW)) in both cases. Under atmospheric conditions when O3 has high concentrations, the equilibrium between NOx and O3 should occur rapidly (h) but this equilibrium cannot be reached during polar winters and/or nights if the NOx conversion to HNO3 is faster. The experimentally derived rate coefficients can be used to model the major NOx–O3 isotopologue reactions at various pressures and in isotope modeling of tropospheric nitrate.