Isotope Identification in NO as a Chemical Tracer in the Middle Atmosphere

摘要

The nitrogen isotope ratio of middle atmosphere nitrogen oxide is predicted as a function of altitude. Nitrogen oxides originate photochemically either from stratospheric nitrous oxide reacting with O(~1D) or in the mesosphere and thermosphere from direct dissociation of N_2 and ionization-initiated reactions involving O_2 and N_2. During its formation process, N_2O acquires a nitrogen isotopic composition of N isotopes different than N_2. Photodissociation within the stratosphere also modifies the proportion of isotopes. Reaction of stratospheric NO with O_3 produces NO_2, which when photodissociated yields NO depleted in ~(15)N relative to NO_2 in laboratory air. The value of δ~(15)NO in the stratosphere is -100‰. In the altitude region between 50 and 65 km, NO is transformed into NO_2 and then returned to NO by reaction of NO_2 with O and by NO_2 photodissociation. These reactions determine the isotopic makeup of NO. Above 65 km, nitric oxide is produced by local ionization processes and gas phase photochemical reactions involving N_2 and excited O_2. These processes determine the isotopic composition of NO in the upper mesosphere and thermosphere. Here δ~(15)NO is 0‰. Air transported into the mesosphere above 65 km will reflect the NO isotopic values of the region below, while mesospheric NO transported below 65 km will not be distinguishable from NO originating in the stratosphere.