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Investigating low-temperature reaction rates for Titan atmosphere using ab initio quantum chemistry methods

Presentation #204.04 in the session Titan II: Atmospheres (Poster)

Published onOct 23, 2023
Investigating low-temperature reaction rates for Titan atmosphere using ab initio quantum chemistry methods

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 (i.e., 70–200 K) rate coefficients of “key” radical–neutral reactions. Experimental rate coefficients for such low-T 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.

We are applying high-level ab initio quantum chemical methods to calculate accurate rate coefficients, and incorporating these rate coefficients to improve the mole fraction predictions of an existing Titan photochemical model. We are using these improved mole fraction predictions to generate synthetic spectra, which will be compared against calibrated CIRS spectra to assess the overall impact of our calculated ab initio rate coefficients on the model predictions. We are using calibrated CIRS data to retrieve vertical temperature profiles and carry out spectral analyses. 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.

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