Atmospheric chemistry and transport modeling in the outer solar system

This thesis consists of 1-D and 2-D photochemical-dynamical modeling in the upper atmospheres of outer planets. For 1-D modeling, a unified hydrocarbon photochemical model has been studied in Jupiter, Saturn, Uranus, Neptune, and Titan, by comparing with the Voyager observations, and the recent measurements of methyl radicals by ISO in Saturn and Neptune. The CH₃ observation implies a kinetically sensitive test to the measured and estimated hydrocarbon rate constants at low temperatures. We identify the key reactions that control the concentrations of CH₃ in the model, such as the three-body recombination reaction, CH₃ + CH₃ + M → C ₂H₆ + M, and the recycling reaction H + CH₃ + M → CH₄ + M. The results show reasonable agreement with ISO values. In Chapter 4, the detection of PH₃ in the lower stratosphere and upper troposphere of Jupiter has provided a photochemical-dynamical coupling model to derive the eddy diffusion coefficient in the upper troposphere of Jupiter. Using a two-layers photochemical model with updated photodissociation cross-sections and chemical rate constants for NH₃ and PH ₃, we find that the upper tropospheric eddy diffusion coefficient <10 5 cm2 sec−1, and the deeper tropospheric value >106 cm2 sec−1, are required to match the derived PH₃ vertical profile by the observation. The best-fit functional form derivation of eddy diffusion coefficient in the upper troposphere of Jupiter above 400 mbar is K = 2.0 × 104 (n/2.2 × 1019)−0.5 cm 2 sec−1. On the other hand, Chapter 5 demonstrates a dynamical-only 2-D model of C₂H₆ providing a complete test for the current 2-D transport models in Jovian lower stratosphere and upper troposphere (270 to 0.1 mbar pressure levels). Different combinations of residual advection, horizontal eddy dispersion, and vertical eddy mixing are examined at different latitudes.