  The influence of a dust grain mixture consisting of spherical dust grains
with different radii and/or chemical composition on the resulting temperature
structure and spectral energy distribution of a circumstellar shell is
investigated. The comparison with the results based on an approximation of dust
grain parameters representing the mean optical properties of the corresponding
dust grain mixture reveal that (1) the temperature dispersion of a real dust
grain mixture decreases substantially with increasing optical depth, converging
towards the temperature distribution resulting from the approximation of mean
dust grain parameters, and (2) the resulting spectral energy distributions do
not differ by more than 10% if >= 2^5 grain sizes are considered which
justifies the mean parameter approximation and the many results obtained under
its assumption so far. Nevertheless, the dust grain temperature dispersion at
the inner boundary of a dust shell may amount to >>100K and has therefore to be
considered in the correct simulation of, e.g., chemical networks.
  In order to study the additional influence of geometrical effects, a
two-dimensional configuration -- the HH30 circumstellar disk -- was considered,
using model parameters from Cotera et al. (2001) and Wood et al. (2002). A
drastic inversion of the large to small grain temperature distribution was
found within the inner approx. 1AU of the disk.
