Nitrogen Isotope Exchange Between NO and NO2 and Its Implication for 15N Variations in Tropospheric NOx

Abstract

Nitrogen oxides (NOx = NO + NO2) are trace gases that have numerous sources that include vehicles, power plants, lightning, soil emissions, agriculture, and biomass burning. It is important to estimate the budgets of these emission sources because NOx play an important role in air quality, climate, and human health. Analysis of the nitrogen stable isotope ratio (15N/14N) has been suggested as a useful method for partitioning NOx sources, as emission sources often have characteristic 15N/14N ratios (“fi ngerprints”). However, the effects of chemical and physical processes on 15N/14N ratios of NOx have not been researched in depth, which limits this isotopic technique. It has been suggested that the nitrogen isotope exchange between NO and NO2 has an infl uence on the partitioning of 15N between these two molecules. However, previous experimental and theoretical studies disagree about the value of this isotope exchange process. We have experimentally measured this isotope exchange process in which NO and NO2 were equilibrated in a vacuum system and NO2 was selectively removed from the system and measured for its 15N/14N ratio. Our experimental values agree exceptionally well with the theoretically predicted value, using corrections for anharmonicity. Using this data, we were able to model a diurnal cycle and seasonal changes in the distribution of 15N/14N ratios between NO and NO2 resulting from nitrogen isotopic exchange. Future research will measure 15N/14N ratios of NOx in ambient air to see if our predicted diurnal and seasonal change is correct.