Direct comparison of optical depth effects derived from model and observations. The column density (blue dots) obtained by fitting the synthetic RT spectra is plotted as a function of the effective column density . The values plotted as red dots are obtained by considering that the radiative transfer model underestimates the opacity effects by a factor f = 2.3 (see text). The dashed line shows the two-line fit of values from dataset S1 (see text). The plain line shows optically thin conditions.