Vertical distributions of the most abundant atmospheric constituents, after H₂ and He, at four longitudes: substellar point (0°), evening limb (+ 90°), antistellar point (± 180°), and morning limb (−90°) in the atmosphere of HD 209458b. We show the mole fractions calculated by the pseudo two-dimensional chemical model (solid lines), by a model that neglects vertical mixing (horizontal transport case; dashed-dotted lines), by a one-dimensional vertical model that neglects horizontal transport (vertical mixing case; dashed lines), and by local thermochemical equilibrium (dotted lines). Photochemistry is taken into account in all cases but the last.