  We investigate the effects of atmospheric circulation on the chemistry of the
hot Jupiter HD 209458b. We use a simplified dynamical model and a robust
chemical network, as opposed to previous studies which have used a three
dimensional circulation model coupled to a simple chemical kinetics scheme. The
temperature structure and distribution of the main atmospheric constituents are
calculated in the limit of an atmosphere that rotates as a solid body with an
equatorial rotation rate of 1 km/s. Such motion mimics a uniform zonal wind
which resembles the equatorial superrotation structure found by three
dimensional circulation models. The uneven heating of this tidally locked
planet causes, even in the presence of such a strong zonal wind, large
temperature contrasts between the dayside and nightside, of up to 800 K. This
would result in important longitudinal variations of some molecular abundances
if the atmosphere were at chemical equilibrium. The zonal wind, however, acts
as a powerful disequilibrium process. We identify the existence of a pressure
level of transition between two regimes, which may be located between 100 and
0.1 mbar depending on the molecule. Below this transition layer, chemical
equilibrium holds, while above it, the zonal wind tends to homogenize the
chemical composition of the atmosphere, bringing molecular abundances in the
limb and nightside regions close to chemical equilibrium values characteristic
of the dayside, i.e. producing an horizontal quenching effect in the
abundances. Reasoning based on timescales arguments indicates that horizontal
and vertical mixing are likely to compete in HD 209458b's atmosphere, producing
a complex distribution where molecular abundances are quenched horizontally to
dayside values and vertically to chemical equilibrium values characteristic of
deep layers.
