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The detection of the CO_2 absorption band at 4.2 {\mu}m in brown dwarfspectra by AKARI has made it possible to discuss CO_2 molecular abundance inbrown 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 thatthese deviations were caused by high C and O elemental abundances in theiratmospheres. To validate this hypothesis we construct a set of models of browndwarf atmospheres with various elemental abundance patterns, and investigatethe variations of the molecular composition, thermal structure and theireffects to the near-infrared spectra between 1.0 and 5.0 {\mu}m. The 4.2 {\mu}mCO_2 absorption band in some late-L and T dwarfs taken by AKARI are stronger orweaker than predictions by corresponding models with solar abundance. Bycomparing CO_2 band in the model spectra to the observed near-infrared spectra,we confirm possible elemental abundance variations among brown dwarfs. We findthat the band strength is especially sensitive to O abundance, but C is alsoneeded to reproduce the entire near-infrared spectra. This result indicatesthat both C and O abundances should increase and decrease simultaneously forbrown dwarfs. We find that a weaker CO_2 absorption band in a spectrum can alsobe explained by a model with lower C and O abundances.

AKARI Observations of Brown Dwarfs. IV. Effect of Elemental Abundances to Near-Infrared Spectra between 1.0 and 5.0 {\mu}m