  We use a model of aerosol microphysics to investigate the impact of
high-altitude photochemical aerosols on the transmission spectra and
atmospheric properties of close-in exoplanets, such as HD209458b and HD189733b.
The results depend strongly on the temperature profiles in the middle and upper
atmosphere that are poorly understood. Nevertheless, our model of HD189733b,
based on the most recently inferred temperature profiles, produces an aerosol
distribution that matches the observed transmission spectrum. We argue that the
hotter temperature of HD209458b inhibits the production of high-altitude
aerosols and leads to the appearance of a more clear atmosphere than on
HD189733b. The aerosol distribution also depends on the particle composition,
the photochemical production, and the atmospheric mixing. Due to degeneracies
among these inputs, current data cannot constrain the aerosol properties in
detail. Instead, our work highlights the role of different factors in
controlling the aerosol distribution that will prove useful in understanding
different observations, including those from future missions. For the
atmospheric mixing efficiency suggested by general circulation models (GCMs) we
find that aerosol particles are small ($\sim$nm) and probably spherical. We
further conclude that composition based on complex hydrocarbons (soots) is the
most likely candidate to survive the high temperatures in hot Jupiter
atmospheres. Such particles would have a significant impact on the energy
balance of HD189733b's atmosphere and should be incorporated in future studies
of atmospheric structure. We also evaluate the contribution of external sources
in the photochemical aerosol formation and find that their spectral signature
is not consistent with observations.
