Transmission spectrum of MASCARA-2b atmosphere in the region of Na I D doublet. In the top panel the atmospheric transmission spectrum is presented in light gray. Black dots indicate the binned transmission spectrum by 10 pixels and the Gaussian fit to each Na I D lines is shown in red; its residuals are shown in the bottom panel. The expected wavelength position of the Na I doublet lines, in the planetary reference frame, are indicated with blue vertical lines. The uncertainties of the relative flux come from the error propagation of the photon and readout noise from the original data.