  The strong, nearly wavelength-independent absorption cross section of
aerosols produces featureless exoplanet transmission spectra, limiting our
ability to characterize their atmospheres. Here we show that even in the
presence of featureless spectra, we can still characterize certain atmospheric
properties. Specifically, we constrain the upper and lower pressure boundaries
of aerosol layers, and present plausible composition candidates. We study the
case of the bloated Saturn-mass planet WASP-49b, where near-infrared
observations reveal a flat transmission spectrum between 0.7 and 1.0
{\microns}. First, we use a hydrodynamic upper-atmosphere code to estimate the
pressure reached by the ionizing stellar high-energy photons at $10^{-8}$ bar,
setting the upper pressure boundary where aerosols could exist. Then, we
combine HELIOS and Pyrat Bay radiative-transfer models to constrain the
temperature and photospheric pressure of atmospheric aerosols, in a Bayesian
framework. For WASP-49b, we constrain the transmission photosphere (hence, the
aerosol deck boundaries) to pressures above $10^{-5}$ bar (100$\times$ solar
metallicity), $10^{-4}$ bar (solar), and $10^{-3}$ bar (0.1$\times$ solar) as
lower boundary, and below $10^{-7}$ bar as upper boundary. Lastly, we compare
condensation curves of aerosol compounds with the planet's pressure-temperature
profile to identify plausible condensates responsible for the absorption. Under
these circumstances, we find as candidates: Na$_{2}$S (at 100$\times$ solar
metallicity); Cr and MnS (at solar and 0.1$\times$ solar); and forsterite,
enstatite, and alabandite (at 0.1$\times$ solar).
