  Broad-band exoplanet transit photometry can characterize the planetary
atmosphere when observed at multiple selected filters. This observing technique
can reveal gradients in the spectra of extrasolar planets, for example the
slope of decreasing opacity from short to long optical wavelengths caused by
aerosol scattering. In this work we observed a transit of the hot Jupiter
HAT-P-32 b in the shortest wavelength possible from the ground using the Large
Binocular Telescope (LBT). The data comprise the best-quality ground-based
U-band taken so far of an exoplanet transit. Compared to broad-band
observations of intermediate and long optical wavelength published previously,
a clear scattering slope in the planetary transmission spectrum is revealed.
Most likely, the scattering particles are magnesium silicate aerosols larger
than 0.1 micrometer. We define a spectral index to compare this scattering
feature of HAT-P-32 b to published results of other exoplanets. It turns out to
be very typical in amplitude within the comparison sample. Furthermore, we
searched for correlation in this sample of the spectral index with planetary
equilibrium temperature, surface acceleration and stellar activity indicator,
but could not reveal any.
