The neutral photochemistry of nitriles, amines and imines in the atmosphere of Titan

doi:10.1016/j.icarus.2014.09.039

Icarus

Volume 247, February 2015, Pages 218-247

https://doi.org/10.1016/j.icarus.2014.09.039 Get rights and content

Highlights

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We study the photochemical richness of Titan’s atmosphere.
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We review and complete the neutral chemistry of nitrogen containing compounds.
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A review of observations is performed and compared to our model.
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We identify the key chemical processes that should be studied to improve model accuracy.
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We predict that CH₃C₃N could be relatively abundant.

Abstract

The photochemistry of N₂ and CH₄ in the atmosphere of Titan leads to a very rich chemistry which is not well understood. The aim of our study is to improve our understanding of the production of nitrogen compounds and to predict the abundances of those with high molar mass with better accuracy. We have made a careful investigation of the neutral nitrogen photochemistry to improve current chemical schemes including the most abundant species and the most efficient reactions. We also studied the propagation of uncertainties on rate constants in our model and determined the key reactions from a global sensitivity analysis. Our photochemical model contains 124 species, 60 of which are nitrogen containing compounds, and 1141 reactions. Our results are in reasonable agreement with Cassini/INMS data in the higher atmosphere but our model overestimates the mole fractions of several nitriles in the lower stratosphere. New species such as CH₃C₃N and C₃H₇CN could be relatively abundant in Titan’s atmosphere. Uncertainties on some nitrogen compounds are important and further studies of the key reactions that we have identified are needed to improve the predictivity of photochemical models. Meridional transport is expected to be an efficient process to govern the abundances of several nitriles in the lower stratosphere.

Introduction

The atmosphere of Titan can be regarded as chemical reactor on the planetary scale. It produces a variety of molecules through the coupled chemistry occurring between hydrocarbons, oxygen and nitrogen containing species. The various chemical processes taking place at different atmospheric levels are so efficient that they ultimately produce aerosols, which then sink down to the surface. Several simple molecules found in Titan’s atmosphere have been followed over a period of many years, allowing us to study their spatial and temporal variations. One of the dominant processes driving the production of these chemical species is neutral photochemistry starting with the photolysis of methane (CH₄) and molecular nitrogen (N₂) in the upper atmosphere. To a lesser extent, the photochemistry of water (H₂O) resulting from the injection of oxygen into the upper atmosphere (in the form of O⁺, OH and/or H₂O) also participates to this complexification (Hörst et al., 2008, Dobrijevic et al., 2014). Many photochemical models have been developed to explain the presence of detected compounds and to predict the abundance of as yet undetected ones. Several recent studies have improved the chemical scheme of photochemical models of Titan’s atmosphere by introducing new reactions and new compounds. Vuitton et al. (2012) introduced several association reactions in their model showing that the mole fractions of certain species could change locally by as much as an order of magnitude. Hébrard et al. (2012) introduced several new reactions in their model to explain the presence of HNC, a newly detected nitrile (Moreno et al., 2011), which was not previously present in photochemical models. Hébrard et al. (2013) carefully examined the photochemistry of C₃H_p compounds in the atmosphere of Titan (including both photolysis and neutral–neutral thermal reactions), improving and updating the existing network for hydrocarbon chemistry. They demonstrated the noticeable impact that such an improvement could have on the calculated abundances of many hydrocarbons (for C₃- and C₂-hydrocarbons alike) whilst predicting that as yet undetected compounds such as the carbon trimer (C₃), cyclopropenylidene (c-C₃H₂) and the propargyl radical (C₃H₃) could be abundant enough to be contribute to Cassini/INMS data. Dobrijevic et al. (2014) improved the photochemistry of oxygen species, introducing in particular a coupling between the chemistries of hydrocarbons, oxygen and nitrogen containing species. They predicted the presence of new and as yet undetected compounds such as NO (nitric oxide), HNO (nitrosyl hydride), HNCO (isocyanic acid) and N₂O (nitrous oxide) and showed that the abundance profiles of these compounds depend on the nature and the source of oxygen compounds in the atmosphere. All of these recent models have shown that the improvement of neutral chemical schemes used in photochemical models is necessary for several reasons: it allows us to better understand which processes drive the chemistry of Titan’s atmosphere, it improves the chemistry of coupled neutral-ion models, it allows us to predict the presence of undetected compounds, and it places additional constraints on different physical parameters such as transport and external inputs. In the present paper, we extend these previous studies, updating and improving the photochemical scheme for nitrogen containing compounds with a particular focus on nitriles that have already been detected. Several nitrile compounds (HCN, HNC, HC₃N, CH₃CN and C₂N₂) have been firmly detected in Titan’s atmosphere and their altitude profiles have been determined by spectroscopic observations. Other nitrogen containing compounds have been detected by (or tentatively suggested by) the Ion and Neutral Mass Spectrometer (INMS) instrument aboard the Cassini orbiter and their abundances have been inferred from analysis of the spectra and the use of photochemical models that include ion and neutral chemistry (references are given below). Among these compounds, we can highlight certain nitriles (C₂H₃CN, C₂H₅CN, HC₅N and probably CH₃C₃N), one amine (NH₃) and one imine (CH₂NH). Three other compounds have been found to be present in INMS spectra (C₅H₅N, C₆H₃N, C₆H₇N) but many isomers are possible for these molecules. Upper limits for a few compounds have also been inferred from the INMS data (two amines: CH₃NH₂, N₂H₄ and one nitrile: C₄H₅N). C₄N₂ has been detected only as C₄N₂ ice in Titan’s North polar stratosphere.

Our methodology to improve the chemical scheme, which includes new calculations as well as an extensive literature review, is presented in Section 2. The photochemical model is briefly presented in Section 3, highlighting only those parts which are different from our previous model. In Section 4, we use the recent determination of the altitude profiles of six hydrocarbons in the equatorial region (including the new detection of propene) by Nixon et al. (2013) and the water profile determined by Moreno et al. (2012) to constrain the eddy diffusion coefficient before presenting the main reaction pathways for the production of nitriles, amines and imines. A review of available observations for all the nitrogen compounds detected so far is given. For each compound, the model results with their associated uncertainties, due to uncertainties on chemical rate constants are presented. Some as yet undetected nitrogen compounds are also highlighted. In Section 5, we pinpoint the reactions that require further investigation to improve the predictivity of photochemical models. In Section 6, several important aspects of the nitrogen photochemistry in Titan’s atmosphere are discussed before presenting our conclusions in Section 7.

Section snippets

Chemical scheme

Several recent modeling studies of Titan’s atmosphere (Vuitton et al., 2012, Hébrard et al., 2012, Hébrard et al., 2013, Dobrijevic et al., 2014), have shown that current chemical schemes used in photochemical models are far from complete. Previous work (Hébrard et al., 2006, Hébrard et al., 2007, Hébrard et al., 2009, Dobrijevic et al., 2008) has also underlined the importance of uncertainties on reaction rates. Indeed, the propagation of these uncertainties in such highly non-linear systems

Photochemical model

We present in the following section the various modifications that we have performed compared to the model presented in Dobrijevic et al. (2014).

Eddy diffusion coefficient profile

Our one dimensional photochemical model computes the abundance of various compounds as a function of altitude, under daily-averaged conditions at the equator and at the equinox. Dynamical processes in the homosphere are parameterized by a vertical eddy diffusion at similar conditions. The eddy diffusion coefficient profile $K (z)$ is fitted as a function of altitude in order to reproduce the observations gathered under similar conditions. Several earlier studies (see for example Lavvas et al.,

Key reactions

From an uncertainty propagation study and a global sensitivity analysis (see Hébrard et al. (2009) for instance), we have determined a list of the key reactions for the main nitrogen species of our model. These key reactions (which can be different from reactions that give the main production rates), are the ones which are responsible for the uncertainties on the mole fraction profiles calculated from our model. They are consequently the reactions that should be studied in priority to improve

Sensitivity to Galactic Cosmic Rays

The production of nitrogen atoms in the lower stratosphere by GCR increases the production of many nitrogen compounds. We show in Fig. 16 the mole fractions of the most abundant species which are affected by this process. The contribution of GCR in the production of amines like NH₃, CH₃NH₂ and CH₃NHCH₃ or imines like CH₂NH is particularly remarkable since their mole fractions decrease by about 3 orders of magnitude when turning off the production of N atoms by GCR. Some nitriles like HNC, C₂N₂,

Conclusion

We have made a careful investigation of the neutral chemistry of nitrogen compounds in the atmosphere of Titan. Our model includes 60 species containing nitrogen atoms with the heaviest molecule reaching a molar mass of 93 amu. We have built a consistent chemical scheme based on the abundances of the main species and their reactivity. Many rate constants and their uncertainty factors have been calculated or estimated based on theoretical considerations. Based on this new scheme, we have computed

Acknowledgement

We thank the Programme National de Planétologie (PNP) of the Institut National des Sciences de l’Univers (INSU) for funding a part of this work. O.V. acknowledges support from the KU Leuven IDO Project IDO/10/2013 and from the FWO Postdoctoral Fellowship program.

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The reactions of atomic nitrogen in its first electronically excited state, N(²D), with acetonitrile (CH₃CN) and cyanoacetylene (HC₃N) have been investigated by performing electronic structure calculations of the doublet potential energy surfaces and RRKM estimates of the product branching fractions. According to our results, the insertion of N(²D) into one of the sigma CH bonds of acetonitrile leading to the formation of cyanomethanimine (also known as iminoacetonitrile) and the N(²D) addition to the CC triple bond of cyanoacetylene leading to the formation of dicyanocarbene are the main reaction pathways under the conditions typical of the upper atmosphere of Titan, the massive moon of Saturn. Other molecular products originating from other reaction pathways only give a minor contribution.
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The neutral photochemistry of nitriles, amines and imines in the atmosphere of Titan

Highlights

Abstract

Introduction

Section snippets

Chemical scheme

Photochemical model

Eddy diffusion coefficient profile

Key reactions

Sensitivity to Galactic Cosmic Rays

Conclusion

Acknowledgement

Icarus

Energy

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Planet. Space Sci.

Icarus

Planet. Space Sci.

J. Photochem. Photobiol. A

J. Photochem. Photobiol. C – Photochem. Rev.

Planet. Space Sci.

Chem. Phys. Lett.

Chem. Phys.

Icarus

Spectrochim. Acta A

Icarus

Icarus

Icarus

Planet. Space Sci.

Chem. Phys. Lett.

Chem. Phys. Lett.

Planet. Space Sci.

Planet. Space Sci.

Icarus

Icarus

Icarus

Reaction of cyanomethylene with nitric oxide and oxygen at 298 K: HCCN + NO, O2

J. Phys. Chem. A

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I – Gas phase reactions of Ox, HOx, NOx and SOx species

Atmos. Chem. Phys.

Observation of nitrogen-bearing organic molecules from reactions of nitrogen atoms with hydrocarbons: A crossed beam study of N(2D) + ethylene

J. Phys. Chem. A

Combined crossed molecular beam and theoretical studies of the N(2D) + CH4 reaction and implications for atmospheric models of Titan

J. Chem. Phys. A

Formation of nitriles and imines in the atmosphere of Titan: Combined crossed-beam and theoretical studies on the reaction dynamics of excited nitrogen atoms N(2D) with ethane

Faraday Discuss.

Radiative relaxation and isomeric branching of highly excited H/C/N: The importance of delocalized vibrational states

Astrophys. J.

DFT study of HCN and N≡C-C=N reactions with hydrogen species

Int. J. Quantum Chem.

Evaluated kinetic data for combustion modelling: Supplement I

J. Phys. Chem. Ref. Data

Evaluated kinetic data for combustion modeling: Supplement II

J. Phys. Chem. Ref. Data

Photodissociation of acrylonitrile at 193 nm: A photofragment translational spectroscopy study using synchrotron radiation for product photoionization

J. Chem. Phys.

Rate constants and branching ratios for the reaction of CH radicals with NH3: A combined experimental and theoretical study

J. Phys. Chem. A

A direct study of the reactions of methylene(X∼3B1) radicals with hydrogen and deuterium atoms

J. Phys. Chem.

Product distributions of the C2H2 + H and HCCO (ketenyl) + H reactions. Rate constant of CH(X∼3B1) + H

J. Phys. Chem.

Bimolecular rate constant and product branching ratio measurements for the reaction of C2H with ethene and propene at 79 K

J. Phys. Chem. A

Gas phase equilibrium between acetaldehyde and paraldehyde, thermodynamic values for the trimerisation of acetaldehyde and the polymerisability of paraldehyde

J. Chem. Soc., Faraday Trans. 1: Phys.l Chem. Cond. Phases

Recurring chains following addition of atomic hydrogen to acetylene

J. Phys. Chem.

Reaction of cyanomethylene with nitric oxide and oxygen at 298 K: HCCN + NO, O₂

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I – Gas phase reactions of $O_{x}$ , ${HO}_{x}$ , ${NO}_{x}$ and ${SO}_{x}$ species

Observation of nitrogen-bearing organic molecules from reactions of nitrogen atoms with hydrocarbons: A crossed beam study of N(²D) + ethylene

Combined crossed molecular beam and theoretical studies of the N(²D) + CH₄ reaction and implications for atmospheric models of Titan

Formation of nitriles and imines in the atmosphere of Titan: Combined crossed-beam and theoretical studies on the reaction dynamics of excited nitrogen atoms N(²D) with ethane

DFT study of HCN and $N \equiv C - C = N$ reactions with hydrogen species

Rate constants and branching ratios for the reaction of CH radicals with NH₃: A combined experimental and theoretical study

A direct study of the reactions of methylene( ${\tilde{X}}^{3}$ B₁) radicals with hydrogen and deuterium atoms

Product distributions of the C₂H₂ + H and HCCO (ketenyl) + H reactions. Rate constant of CH( ${\tilde{X}}^{3}$ B₁) + H

Bimolecular rate constant and product branching ratio measurements for the reaction of C₂H with ethene and propene at 79 K