  We investigate variations in the strengths of three molecular bands, CH4 at
3.3 um, CO at 4.6 um and CO2 at 4.2 um, in 16 brown dwarf spectra obtained by
AKARI. Spectral features are examined along the sequence of source classes from
L1 to T8. We find that the CH4 3.3 um band is present in the spectra of brown
dwarfs later than L5, and the CO 4.6 um band appears in all spectral types. The
CO2 absorption band at 4.2 um is detected in late-L and T type dwarfs. To
better understand brown dwarf atmospheres, we analyze the observed spectra
using the Unified Cloudy Model (UCM). The physical parameters of the AKARI
sample, i.e., atmospheric effective temperature Teff, surface gravity logg and
critical temperature Tcr are derived. We also model IRTF/SpeX and UKIRT/CGS4
spectra in addition to the AKARI data in order to derive the most probable
physical parameters. Correlations between the spectral type and the modeled
parameters are examined. We confirm that the spectral type sequence of late-L
dwarfs is not related to Teff, but instead originates as a result of the effect
of dust.
