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Title: Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model

Abstract

We introduce a one-dimensional coupled ion-neutral photochemical kinetics and diffusion model to study the atmospheric composition of Titan in light of new theoretical kinetics calculations and scientific findings from the Cassini–Huygens mission. The model extends from the surface to the exobase. The atmospheric background, boundary conditions, vertical transport and aerosol opacity are all constrained by the Cassini–Huygens observations. The chemical network includes reactions between hydrocarbons, nitrogen and oxygen bearing species. It takes into account neutrals and both positive and negative ions with masses extending up to 116 and 74 u, respectively. We incorporate high-resolution isotopic photoabsorption and photodissociation cross sections for N 2 as well as new photodissociation branching ratios for CH 4 and C 2H 2. Ab initio transition state theory calculations are performed in order to estimate the rate coefficients and products for critical reactions. Critical reactions of production and loss for neutrals and ions are quantitatively assessed and thoroughly discussed. The vertical distributions of neutrals and ions predicted by the model generally reproduce observational data, suggesting that for the small species most chemical processes in Titan’s atmosphere and ionosphere are adequately described and understood; some differences are highlighted. Notable remaining issues include (i) the total positive ionmore » density (essentially HCNH +) in the upper ionosphere, (ii) the low mass negative ion densities (CN -, C 3N -/C4H -) in the upper atmosphere, and (iii) the minor oxygen-bearing species (CO 2, H 2O) density in the stratosphere. Pathways towards complex molecules and the impact of aerosols (UV shielding, atomic and molecular hydrogen budget, nitriles heterogeneous chemistry and condensation) are evaluated in the model, along with lifetimes and solar cycle variations.« less

Authors:
ORCiD logo [1];  [2]; ORCiD logo [3]; ORCiD logo [4];  [5]
  1. Univ. of Grenoble (France)
  2. Univ. of Arizona, Tucson, AZ (United States)
  3. Argonne National Lab. (ANL), Lemont, IL (United States)
  4. Johns Hopkins Univ., Baltimore, MD (United States)
  5. Univ. Reims Champagne-Ardenne (France)
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Chemical Sciences, Geosciences & Biosciences Division; French space agency (CNES); NASA Planetary Atmospheres Program
OSTI Identifier:
1530387
Grant/Contract Number:  
AC02-06CH11357
Resource Type:
Accepted Manuscript
Journal Name:
Icarus
Additional Journal Information:
Journal Volume: 324; Journal Issue: C; Journal ID: ISSN 0019-1035
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Ab Initio Kinetics; Atmosphere; Ionosphere; Photochemistry; Titan

Citation Formats

Vuitton, V., Yelle, R. V., Klippenstein, S. J., Hörst, S. M., and Lavvas, P. Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model. United States: N. p., 2018. Web. doi:10.1016/j.icarus.2018.06.013.
Vuitton, V., Yelle, R. V., Klippenstein, S. J., Hörst, S. M., & Lavvas, P. Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model. United States. doi:10.1016/j.icarus.2018.06.013.
Vuitton, V., Yelle, R. V., Klippenstein, S. J., Hörst, S. M., and Lavvas, P. Fri . "Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model". United States. doi:10.1016/j.icarus.2018.06.013. https://www.osti.gov/servlets/purl/1530387.
@article{osti_1530387,
title = {Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model},
author = {Vuitton, V. and Yelle, R. V. and Klippenstein, S. J. and Hörst, S. M. and Lavvas, P.},
abstractNote = {We introduce a one-dimensional coupled ion-neutral photochemical kinetics and diffusion model to study the atmospheric composition of Titan in light of new theoretical kinetics calculations and scientific findings from the Cassini–Huygens mission. The model extends from the surface to the exobase. The atmospheric background, boundary conditions, vertical transport and aerosol opacity are all constrained by the Cassini–Huygens observations. The chemical network includes reactions between hydrocarbons, nitrogen and oxygen bearing species. It takes into account neutrals and both positive and negative ions with masses extending up to 116 and 74 u, respectively. We incorporate high-resolution isotopic photoabsorption and photodissociation cross sections for N2 as well as new photodissociation branching ratios for CH4 and C2H2. Ab initio transition state theory calculations are performed in order to estimate the rate coefficients and products for critical reactions. Critical reactions of production and loss for neutrals and ions are quantitatively assessed and thoroughly discussed. The vertical distributions of neutrals and ions predicted by the model generally reproduce observational data, suggesting that for the small species most chemical processes in Titan’s atmosphere and ionosphere are adequately described and understood; some differences are highlighted. Notable remaining issues include (i) the total positive ion density (essentially HCNH+) in the upper ionosphere, (ii) the low mass negative ion densities (CN-, C3N-/C4H-) in the upper atmosphere, and (iii) the minor oxygen-bearing species (CO2, H2O) density in the stratosphere. Pathways towards complex molecules and the impact of aerosols (UV shielding, atomic and molecular hydrogen budget, nitriles heterogeneous chemistry and condensation) are evaluated in the model, along with lifetimes and solar cycle variations.},
doi = {10.1016/j.icarus.2018.06.013},
journal = {Icarus},
number = C,
volume = 324,
place = {United States},
year = {2018},
month = {8}
}

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