  We present a method to include the evolution of the grain size and grain
opacity $\kappa_\mathrm{gr}$ in the equations describing the structure of
protoplanetary atmospheres. The key assumption of this method is that a single
grain size dominates the grain size distribution at any height $r$. In addition
to following grain growth, the method accounts for mass deposition by
planetesimals and grain porosity. We illustrate this method by computation of a
simplified atmosphere structure model. In agreement with previous works, grain
coagulation is seen to be very efficient. The opacity drops to values much
below the often-used `ISM-opacities' ($\sim$$1\ \mathrm{cm^2\ g}^{-1}$) and the
atmosphere structure profiles for temperature and density resemble that of the
grain-free case. Deposition of planetesimals in the radiative part of the
atmosphere hardly influences this outcome as the added surface is quickly
coagulated away. We observe a modest dependence on the internal structure
(porosity), but show that filling factors cannot become too large because of
compression by gas drag.
