Mechanisms for the formation of benzene in the atmosphere of Titan

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are important interstellar species, and their precursor benzene (C6H6) has been detected in our solar system. In this study the possibility of benzene formation in the atmosphere of Titan is investigated. Benzene abundance in Titan's atmosphere is found to be rather highly dependent on the assumed mechanism for benzene production. Assuming reactions involved in this mechanism to proceed at the rate corresponding to 300 K, a value of 5.4 × 10−7 at 2 × 10−5 mbar is found for the mole fraction of benzene. The primary mechanism responsible for this benzene abundance involves the recombination of propargyl (C3H3) radicals. A source of benzene molecules through ion chemistry in the upper atmosphere is also investigated. The inclusion of heavy cyclic ions results in little change in the C6H6 abundance at the peak in mole fraction, where [C6H6] = 3.6 × 105 cm−3, but does produce about a factor of 2 increase in the benzene mole fraction at the 10−6 mbar level [C6H6] = 1 × 104 cm−3. This produces a negligible change in C6H6 column abundance below this microbar level. In the stratosphere, Infrared Space Observatory (ISO) measurements of the benzene abundance have been fit by our nominal profile multiplied by a factor of 3.0 ± 0.5. Taking the lower value of this factor, the ISO fit corresponds to an altitude‐dependent benzene profile with a value of 9.8 × 10−11 at 1 mbar. Benzene profiles determined in this study suggest an important path for the formation of higher‐order hydrocarbons, which may play a significant role in the formation of hazes in Titan's atmosphere.

Neutral production of hydrogen isocyanide (HNC) and hydrogen cyanide (HCN) in Titan’s upper atmosphere

“…1. Eddy diffusion profiles from various photochemical models (Yung et al 1984;Strobel et al 1992;Toublanc et al 1995;Lara et al 1996;Wilson & Atreya 2004;Vinatier et al 2007;Lavvas et al 2008;Hörst et al 2008;Krasnopolsky 2009). …”

Dissociative recombination exalts molecular growth in N2/CH4plasmas

“…Without this information, the chemical identity of the fragments is unknown, and therefore their further reactivity. This situation has for long prevented the use of a large corpus of experimental BR data in detailed chemistry models, and many modelers have instead implemented a default "H-loss" scheme, where the parent ions looses a single H atom [3][4][5][6][7][8]. The "H-loss" scheme is not supported by the experiments: many ions present major dissociation pathways involving heavy atoms bond breaking, for instance C 4 H + 8 [9] or HC 3 N + and other nitrile ions [10].…”

Formation of Polycyclic Aromatic Hydrocarbons and Nitrogen Containing Polcyclic Aromatic Compounds in Titan's Atmosphere, the Interstellar Medium and Combustion

“…Until recently, the kinetic models of the PAH formation and growth in low temperature environments such as on Titan or in the interstellar medium relied upon the models of PAH and soot formation in combustion processes and assumed that similar mechanisms are important both at combustion temperatures (1000 -2500 K) and in the extraterrestrial, low temperature environments (10 -150 K). [27][28][29][30] Wilson et al 31 suggested, for instance, that once the first aromatic ring is available, the further growth of PAHs proceeds by the hydrogen-abstraction-acetylene-addition (HACA) mechanism, 30,32 which is believed to be the most important PAH formation mechanism in combustion flames. 27 However, an apparent flaw of these models is that both hydrogen abstraction and acetylene addition reactions exhibit significant barriers and as a result are very slow at low temperatures relevant to the atmospheric conditions on Titan or in the Interstellar Medium (ISM).…”