  We have developed 1-D photochemical and thermochemical kinetics and diffusion
models for the transiting exoplanets HD 189733b and HD 209458b to study the
effects of disequilibrium chemistry on the atmospheric composition of "hot
Jupiters." Here we investigate the coupled chemistry of neutral carbon,
hydrogen, oxygen, and nitrogen species, and we compare the model results with
existing transit and eclipse observations. We find that the vertical profiles
of molecular constituents are significantly affected by transport-induced
quenching and photochemistry, particularly on cooler HD 189733b; however, the
warmer stratospheric temperatures on HD 209458b can help maintain
thermochemical equilibrium and reduce the effects of disequilibrium chemistry.
For both planets, the methane and ammonia mole fractions are found to be
enhanced over their equilibrium values at pressures of a few bar to less than a
mbar due to transport-induced quenching, but CH4 and NH3 are photochemically
removed at higher altitudes. Atomic species, unsaturated hydrocarbons
(particularly C2H2), some nitriles (particularly HCN), and radicals like OH,
CH3, and NH2 are enhanced overequilibrium predictions because of quenching and
photochemistry. In contrast, CO, H2O, N2, and CO2 more closely follow their
equilibrium profiles, except at pressures < 1 microbar, where CO, H2O, and N2
are photochemically destroyed and CO2 is produced before its eventual
high-altitude destruction. The enhanced abundances of HCN, CH4, and NH3 in
particular are expected to affect the spectral signatures and thermal profiles
HD 189733b and other, relatively cool, close-in transiting exoplanets. We
examine the sensitivity of our results to the assumed temperature structure and
eddy diffusion coefficientss and discuss further observational consequences of
these models.
