  We investigate dust growth due to settling in a 1D vertical column of a
protoplanetary disk. It is known from the observed 10 micron feature in disk
SEDs, that small micron-sized grains are present at the disk atmosphere
throughout the lifetime of the disk. We hope to explain such questions as what
process can keep the disk atmospheres dusty for the lifetime of the disk and
how does the particle properties change as a function of height above the
midplane. We use a Monte Carlo code to follow the mass and porosity evolution
of the particles in time. The used collision model is based on laboratory
experiments performed on dust aggregates. As the experiments cannot cover all
possible collision scenarios, the largest uncertainty of our model is the
necessary extrapolations we had to perform. We simultaneously solve for the
particle growth and motion. Particles can move vertically due to settling and
turbulent mixing. We assume that the vertical profile of the gas density is
fixed in time and only the solid component evolves. We find that the used
collision model strongly influences the masses and sizes of the particles. The
laboratory experiment based collision model greatly reduces the particle sizes
compared to models that assume sticking at all collision velocities. We find
that a turbulence parameter of alpha = 10^-2 is needed to keep the dust
atmospheres dusty, but such strong turbulence can produce only small particles
at the midplane which is not favorable for planetesimal formation models. We
also see that the particles are larger at the midplane and smaller at the upper
layers of the disk. At 3-4 pressure scale heights micron-sized particles are
produced. These particle sizes are needed to explain the 10 micron feature of
disk SEDs. Turbulence may therefore help to keep small dust particles in the
disk atmosphere.
