Number density of available aluminum atoms in the dust-free extended model atmospheres M22 (top) and P20 (bottom) compared to the number density of aluminum in the dust shell. Shown is the overlapping radial range between the inner radius of the dust shell model and the maximum radius of the atmosphere model. The vertical line indicates R_Phot = 1.99 × 10¹³ cm (M22) and R_Phot = 1.74 × 10¹³ cm (P20).