An Explanation for the Nitrous Oxide Layer Observed in the Mesopause Region

Recent satellite measurements of a layer of enhanced nitrous oxide (N₂O) in the mesosphere-lower thermosphere (MLT) from the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer have suggested an unexpected, minor high-altitude production source. Here we report the development of a mechanism and the first model simulations, which can explain the formation of this MLT N₂O layer. N₂O production occurs primarily via a reaction route involving the excitation of N₂ from secondary electrons. Simulations using the Whole Atmosphere Community Climate Model, with external forcing from the Global Airglow model, quantitatively reproduce the observed vertical, latitudinal, and seasonal N₂O variations. Sensitivity results indicate that photoelectrons are far more important than previously predicted, causing approximately two thirds of global N₂O production in the MLT. Energetic electron precipitation over high latitudes provides the remaining contribution. Solar cycle analysis reveals N₂O enhancements of up to ×2 at solar maximum compared to solar minimum.