Exoplanets with relatively clear atmospheres are prime targets for detailed studies of chemical compositions and abundances. Alkali metals have long been suggested to exhibit broad wings due to pressure broadening, but most of the alkali detections only show very narrow absorption cores, which probably hints the presence of clouds. We have been conducting an optical transmission spectroscopy survey using the low-resolution OSIRIS spectrograph at the 10.4 m GTC. Our survey has revealed very diverse alien worlds, from cloudy or hazy atmospheres to partly cloudy atmospheres, and to clear skies. We will present a comparative study of low-density exoplanets with similar equilibrium temperatures but different masses. We detected alkali metals in these planets, which show distinct absorption profiles. In particular, we detected Li in one of them, which, if confirmed, could play a critical role in understanding the planet formation histories. When pressure-broadening is resolved, we performed spectral retrieval to constrain the chemical abundances and cloud properties. The absorption profiles of alkali metals, when acquired at low- and high-resolution, can probe a wide range of atmospheric layers. This poses a bright future for us to comprehensively understand the atmospheric structures, from troposphere to thermosphere, as well as the underlying physics, chemistry and dynamics.