The role of benzene photolysis in Titan haze formation

Abstract During the Cassini mission to the saturnian system, benzene (C 6 H 6 ) was observed throughout Titan’s atmosphere. Although present in trace amounts, benzene has been proposed to be an important precursor for polycyclic aromatic hydrocarbon formation, which could eventually lead to haze production. In this work, we simulate the effect of benzene in Titan’s atmosphere in the laboratory by using a deuterium lamp (115–400 nm) to irradiate CH 4 /N 2 gas mixtures containing ppm-levels of C 6 H 6 . Proton-transfer ion-trap mass spectrometry is used to detect gas-phase products in situ . HCN and CH 3 CN are identified as two major gases formed from the photolysis of 2% CH 4 in N 2 , both with and without 1 ppmv C 6 H 6 added. Inclusion of benzene significantly increases the total amount of gas-phase products formed and the aromaticity of the resultant gases, as shown by delta analysis of the mass spectra. The condensed phase products (or tholins) are measured in situ using high-resolution time-of-flight aerosol mass spectrometry. As reported previously by Trainer et al. (Trainer, M.G., Sebree, J.A., Yoon, Y.H., Tolbert, M.A. [2013]. Astrophys. J. 766, L4), the addition of C 6 H 6 is shown to increase aerosol mass, but decrease the nitrogen incorporation in the organic aerosol. The pressure dependence of aerosol formation for the C 6 H 6 /CH 4 /N 2 gas mixture is also explored. As the pressure decreases, the %N by mass in the aerosol products decreases.