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

Aerosol Constraints on the Atmosphere of the Hot Saturn-mass planet WASP-49b