GTC/OSIRIS transmission spectrum of WASP-52b. Top panel: the transmission spectrum composed of 22 bins of 16.5 nm (black squares with black error bars), 3 bins of 16 Å centered on Na and K, and 1 bin of 18 Å in-between the K doublet (black squares with gray error bars). The blue line shows a 1300 K 1× solar cloud-free model (Kempton et al. 2017). The orange line shows a Rayleigh-scattering model (α = −4). The red dashed line shows a flat line at R_p/R_⋆ = 0.1608. Bottom panels: the close-up of Na, Hα, and K lines (black histogram with gray error bars) in bins of 16 Å (or 18 Å). The green line shows a 2700 K 1× solar atmosphere with the clouds at 1 mbar. The red histogram on the right shows the distribution of all the 16 + 18 Å transit depths.