Theoretical models also shown in Fig. 6, with the same colour-coding (orange: BF-HST, blue: AF, green: 10Na), but before wavelength normalization. The HTT model is not shown for clarity. Aerosols do not affect the cores of the sodium lines of the BF-HST model, which overlaps with the aerosol-free model in the yellow band used to compute the merit functions. However, they mask the wings in the reference bands (grey background). This example shows that high-resolution data are sensitive to the presence of aerosols. The 10Na model has a higher absorption in the sodium region than the aerosol-free model because the global optical depth is higher. However, HARPS alone cannot distinguish between the two cases (Fig. 6) because of the double differential technique used to obtain transmission spectra.