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Bimolecular diffusion coefficients are important parameters used byatmospheric models to calculate altitude profiles of minor constituents in anatmosphere. Unfortunately, laboratory measurements of these coefficients werenever conducted at temperature conditions relevant to the atmosphere of Titan.Here we conduct a detailed uncertainty analysis of the bimolecular diffusioncoefficient parameters as applied to Titan's upper atmosphere to provide abetter understanding of the impact of uncertainty for this parameter on models.Because temperature and pressure conditions are much lower than the laboratoryconditions in which bimolecular diffusion parameters were measured, we apply aBayesian framework, a problem-agnostic framework, to determine parameterestimates and associated uncertainties. We solve the Bayesian calibrationproblem using the open-source QUESO library which also performs a propagationof uncertainties in the calibrated parameters to temperature and pressureconditions observed in Titan's upper atmosphere. Our results show that, afterpropagating uncertainty through the Massman model, the uncertainty in moleculardiffusion is highly correlated to temperature and we observe no noticeablecorrelation with pressure. We propagate the calibrated molecular diffusionestimate and associated uncertainty to obtain an estimate with uncertainty dueto bimolecular diffusion for the methane molar fraction as a function ofaltitude. Results show that the uncertainty in methane abundance due tomolecular diffusion is in general small compared to eddy diffusion and thechemical kinetics description. However, methane abundance is most sensitive touncertainty in molecular diffusion above 1200 km where the errors arenontrivial and could have important implications for scientific research basedon diffusion models in this altitude range.

Uncertainty for calculating transport on Titan: a probabilistic description of bimolecular diffusion parameters