Interpreting Cassini CIRS Data with a Photochemical Model using Improved ab initio Reaction Rate Coefficients

Abstract

<p>Spectra taken by the Cassini Composite Infrared Spectrometer (CIRS) can be used to retrieve vertical temperature profiles and mole fractions of trace gases in the 100 to 500 km region of Titan’s atmosphere. The retrieved mole fractions, along with the mole fraction predictions from a photochemical model, can be used to infer dynamics in Titan’s atmosphere if it can be shown that the transport life-time is shorter than the photochemical life-time. In addition, the chemical evolution of Titan’s atmosphere, haze formation rate, etc. can be deduced by comparing the retrieved mole fractions with the photochemical model predictions. However, previous studies and global sensitivity analyses have shown that large errors exist in Titan photochemical model predictions. These model errors originate mainly from the uncertainties in the low-temperature (<em>i.e.</em>, 70–200 K) rate coefficients of “key” radical–neutral reactions. Experimental rate coefficients for such low-<em>T</em> reactions are difficult, if not impossible, to measure, and the lab data are often associated with large errors due to the uncertainties in determining the absolute concentrations of the radical species. Therefore, we propose application of high-level <em>ab initio</em> quantum chemical methods to calculate accurate rate coefficients, and then incorporate these rate coefficients to improve the mole fraction predictions of an existing Titan photochemical model. We will use calibrated CIRS data to retrieve vertical temperature profiles and carry out spectral analyses, which will enable us to assess the impact of our calculated <em>ab initio</em> rate coefficients on our photochemical model predictions. With our improved photochemical model, we will advance our understanding of Titan’s fascinating atmospheric chemical factory.</p> <p>We will use high-level quantum chemical methods to calculate accurate rate coefficients for reactions that have not yet been studied in the lab under Titan conditions. We will perform our rate-coefficient calculations employing <em>ab initio</em> electron correlation methods such as CASPT2, MRCI, CCSD(T), etc. Next, we will retrieve vertical temperature profiles from calibrated CIRS data using the radiative transfer program NEMESIS (<strong>N</strong>on-linear Optimal <strong>E</strong>stimator for <strong>M</strong>ultivariat<strong>E</strong> Spectral analy<strong>SIS</strong>). Then we will use the <em>ab initio</em> rate coefficients and the retrieved vertical temperatures to improve the mole fraction predictions in an existing Titan photochemical model. Finally, we will use the improved mole fraction predictions to generate synthetic spectra, which will be compared against calibrated CIRS spectra to assess the overall impact of our calculated <em>ab initio</em> rate coefficients on the model predictions. Direct comparison between our generated synthetic spectra and CIRS spectra is an essential step as small mole fraction and/or temperature changes can have a dramatic change in the spectra.</p>