  Carbon monoxide and ammonia have been detected in the spectrum of Gl 229B at
abundances that differ substantially from those obtained from chemical
equilibrium. Vertical mixing in the atmosphere is a mechanism that can drive
slowly reacting species out of chemical equilibrium. We explore the effects of
vertical mixing as a function of mixing efficiency and effective temperature on
the chemical abundances in the atmospheres of brown dwarfs and on their
spectra. The models compare favorably with the observational evidence and
indicate that vertical mixing plays an important role in brown dwarf
atmospheres.
