Spectra computed with SOC for different dust abundance values M_a-C/M_H and Horsehead PDR layer thickness l_PDR, highlighting the degeneracy between dust abundance and l_PDR. The models shown correspond to a strongly depleted abundance (2.6 × 10⁻⁴, blue), a diffuse ISM-like abundance (1.7 × 10⁻³, green), and a high abundance (6.0 × 10⁻³, orange). An increase in M_a-C/M_H is counterbalanced by a decrease in l_PDR. The dashed vertical line marks the longest observed wavelength of 25.5 μm, which is thus most sensitive to the emission from large grains.