Author:

Keywords:

Atmospheres, chemistry, Atmospheres, composition, Titan, Photochemistry, Organic chemistry, Science & Technology, Physical Sciences, Astronomy & Astrophysics, POTENTIAL-ENERGY SURFACE, THERMAL RATE CONSTANTS, VOYAGER INFRARED OBSERVATIONS, HYDROGEN ISOCYANIDE HNC, EVALUATED KINETIC-DATA, ABSOLUTE RATE-CONSTANT, ETHYNYL RADICAL C2H, H-ATOM YIELDS, RATE COEFFICIENTS, TEMPERATURE-DEPENDENCE, 0201 Astronomical and Space Sciences, 0402 Geochemistry, 0404 Geophysics, 5101 Astronomical sciences, 5109 Space sciences

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.