Upper row: Doppler shifts for BPs (a)–(c) inside the zoomed region-of-interest, using the bisector method at a line depth, which corresponds to about Na I D₂λ₀ − 0.22 Å. The arrow in the lower left panel illustrates the depth at which the bisector was computed. Black contours mark the area of the BP. Lower panels: average quiet-Sun profile (dotted line) within the whole FOV (Fig. 1). The large dots correspond to the acquired filtergrams along the Na I D₂ line with GFPI. The green solid lines exhibit the brightest 25 profiles belonging to the green box in the upper panels. The dashed vertical line marks the wavelength at rest.