Nitrogen isotope exchange between NO and NO2 and its implications for 15N variations in tropospheric NOx and atmospheric nitrate

The nitrogen (N) isotope exchange between nitric oxide (NO) and nitrogen dioxide (NO2) has been previously suggested to influence N stable isotope compositions (N-15) of these molecules. However, there is disagreement in the magnitude of the N isotopic fractionation (NO2/NO) resulting from this exchange process between previous experimental and theoretical studies. To this end, we measured NO2/NO associated with this exchange reaction at various temperatures. Our results indicate NO2/NO to be 1.04030.0015, 1.03560.0015, and 1.03360.0014 at 278K, 297K, and 310K, respectively. These measured values are within experimental error of the values we calculated using a modified version of the Bigeleisen-Mayer equation corrected for accurate zero-point energies, indicating an agreement between experiment and theory. Modeling of this exchange reaction demonstrates that N-15-NO2 may exhibit a diurnal and seasonal profile if N isotopic equilibrium is achieved.