  The detection of the CO_2 absorption band at 4.2 {\mu}m in brown dwarf
spectra by AKARI has made it possible to discuss CO_2 molecular abundance in
brown dwarf atmospheres. In our previous studies, we found an excess in the 4.2
{\mu}m CO_2 absorption band of three brown dwarf spectra, and suggested that
these deviations were caused by high C and O elemental abundances in their
atmospheres. To validate this hypothesis we construct a set of models of brown
dwarf atmospheres with various elemental abundance patterns, and investigate
the variations of the molecular composition, thermal structure and their
effects to the near-infrared spectra between 1.0 and 5.0 {\mu}m. The 4.2 {\mu}m
CO_2 absorption band in some late-L and T dwarfs taken by AKARI are stronger or
weaker than predictions by corresponding models with solar abundance. By
comparing CO_2 band in the model spectra to the observed near-infrared spectra,
we confirm possible elemental abundance variations among brown dwarfs. We find
that the band strength is especially sensitive to O abundance, but C is also
needed to reproduce the entire near-infrared spectra. This result indicates
that both C and O abundances should increase and decrease simultaneously for
brown dwarfs. We find that a weaker CO_2 absorption band in a spectrum can also
be explained by a model with lower C and O abundances.
