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Published April 2012 | Published
Journal Article Open

Nitrogen Incorporation in CH_4-N_2 Photochemical Aerosol Produced by Far Ultraviolet Irradiation

Abstract

Nitrile incorporation into Titan aerosol accompanying hydrocarbon chemistry is thought to be driven by extreme UV wavelengths (λ<120 nm) or magnetospheric electrons in the outer reaches of the atmosphere. Far UV radiation (120–200 nm), which is transmitted down to the stratosphere of Titan, is expected to affect hydrocarbon chemistry only and not initiate the formation of nitrogenated species. We examined the chemical properties of photochemical aerosol produced at far UV wavelengths, using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), which allows for elemental analysis of particle-phase products. Our results show that aerosol formed from CH_4/N_2 photochemistry contains a surprising amount of nitrogen, up to 16% by mass, a result of photolysis in the far UV. The proportion of nitrogenated organics to hydrocarbon species is shown to be correlated with that of N_2 in the irradiated gas. The aerosol mass greatly decreases when N_2 is removed, which indicates that N_2 plays a major role in aerosol production. Because direct dissociation of N_2 is highly improbable given the immeasurably low cross section at the wavelengths studied, the chemical activation of N_2 must occur via another pathway. Any chemical activation of N_2 at wavelengths >120 nm is presently unaccounted for in atmospheric photochemical models. We suggest that reaction with CH radicals produced from CH_4 photolysis may provide a mechanism for incorporating N into the molecular structure of the aerosol. Further work is needed to understand the chemistry involved, as these processes may have significant implications for how we view prebiotic chemistry on early Earth and similar planets. Key Words: Titan—Photochemical aerosol—CH_4-N_2 photolysis—Far UV—Nitrogen activation.

Additional Information

© 2012 Mary Ann Liebert, Inc. Submitted 14 December 2011. Accepted 20 February 2012. M.G.T. thanks R. Lessard, C. Hasenkopf, and D. Day for help with control experiments. Y.L.Y. thanks K. Bayes, S. Sander, and W. DeMore for discussion of the kinetics of CH reactions. This work was funded by NASA grants NNX07AV55G, NNX11AD82G, and NNX08AG93G. The development of the HR-ToF-AMS and its analysis software was partially funded by NSF ATM-0449815 and NOAA NA08OAR4310565. Y.L.Y. was supported by NASA grant NX09AB72G to the California Institute of Technology. Disclosure Statement No competing financial interests exist.

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August 22, 2023
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