| Abstract |
A new 1D coupled Radiative / Convective - Photochemical - Microphysical model for a planetary atmosphere was developed and applied to the investigation of the spatial and temporal variability of Titan's atmosphere, and in particular to photochemical haze production. The spatial variability corresponds to the vertical structure of the atmosphere and the way this is determined by the different physical, chemical and radiative processes that take place. The tem- poral variability, addresses the impact of the 11-year solar cycle on the vertical structure of the atmosphere. The model incorporates detailed radiation transfer calculations for the description of the shortwave and longwave fluxes which provide the vertical structure of the radiation field and determine the temperature profile. These are used for the generation of the photochemical and haze structure in the atmosphere, initiated by the photolysis of Titan's main constituents, nitrogen (N2) and methane (CH4). The resulting hydrocarbons and nitriles are used for the production of the haze precursors, whose evolution is described by the microphysical part of the model. The calculated aerosol and gas opacities are iteratively included in the radiation transfer calculations in order to investigate their effect on the resulting temperature profile and geometric albedo. The main purpose of this model is to help in the understanding of the missing link between the production of gaseous species and their transformation to haze particles in Titan's atmosphere. The model generates the haze structure from the gaseous species photochemistry. Model results are presented for the species vertical concentration profiles, haze formation and its radiative properties, vertical temperature/density profiles and geometric albedo. These are validated against the very recent Cassini/Huygens observations and also against other ground-based and spaceborne measurements. The model reproduces well most of the latest measurements from the Cassini/Huygens instruments for the chemical composition of Titan's atmosphere and the vertical profiles of the observed species. For the haze production, we have included pathways that are based on pure hydrocarbons, pure nitriles and hydrocarbon/nitrile copolymers. From these, the nitrile and copolymer pathways provide the stronger contribution, in agreement with the results from the ACP instrument, which support the incorporation of nitrogen in the pyrolised haze structures. The haze model reveals a new second major peak in the vertical profile of haze production rate between 500 and 900 km. This peak is produced by the copolymer family used and has important ramifications for the vertical atmospheric temperature profile and geometric albedo. In particular, the existence of this second peak determines the vertical profile of haze extinction. The solar cycle variability aects the species vertical profiles and eventually results in increase in the haze production of about 60% from solar minimum to solar maximum. This has further effects on the geometric albedo and the vertical temperature structure. The model results have been compared with the DISR retrieved haze extinction profiles and are found to be in very good agreement. Furthermore, heterogeneous chemistry on the haze particles that converts atomic hydrogen to molecular hydrogen has been incorporated in the model. The resultant H2 profile is closer to the INMS measurements, while the vertical profile of the diacetylene formed is found to be closer to that of the CIRS profile when this heterogeneous chemistry is included.
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Spatial and temporal variability in the atmosphere and surface of Titan's atmosphere: Simulation and interpretation through space and ground-based observations
