  In this study, we demonstrate the dependence of atmospheric dust size on the
near-infrared spectra of ten L dwarfs, and constrain the sizes of dust grains
in each L dwarf atmosphere. In previous studies, by comparing observed and
modeled spectra, it was suggested that the deviations of their spectral shapes
from theoretical prediction are general characteristics. Here, we focus on the
dust size in brown dwarf atmospheres to understand the observed spectra. We
confirm that changing the dust size changes the temperature-pressure structure
of the atmosphere, with the shape of the spectrum changing accordingly. At the
wavelength at which dust is the main absorber of radiation (the dust-dominated
regime), a large dust opacity combined with a medium grain size, e.g., 0.1
$\mu$m, results in a low photospheric temperature, and thus a small flux.
Conversely, for the wavelength at which gas absorption is dominant (the
gas-dominated regime), a large dust opacity modifies the temperature-pressure
structure, resulting in a high photospheric temperature, which corresponds to
large flux emissions. Taking into account the size effect, we compare the model
spectral fluxes in the wavelength range 1-5 $\mu$m with the observational ones
to constrain the main dust size in the atmosphere of each of the ten L dwarfs
observed with AKARI and SpeX or CGS4. Ultimately, we reveal that the observed
data are reproduced with higher fidelity by models based on a medium dust size
of 0.1-3.0 $\mu$m for six of these L dwarfs; therefore, we suggest that such
atmospheric dust sizes apply to the majority of L dwarfs.
