Small grain fraction carrying MIR features (q_AF) versus average interstellar radiation field intensity (⟨U⟩). Each filled circle represents a pixel of 8″ ∼ 560 pc, while the angular resolution is 25″ ∼ 1.75 kpc. Pearson correlation coefficients are listed in the top right. Left panel: Pixels colour-coded by total gas surface density (${\mathrm{\Sigma }}_{\mathrm{gas}}^{\mathrm{tot}}$), a proxy for gas density. For reference, filled stars represent the disc, spiral arms, and centre of M 99, and the red diamond indicates the integrated values. Grey circles mark low S/N pixels excluded from analysis (Appendix E). Right panel: Pixels colour-coded by gas-phase metallicity. Grey circles lie outside the metallicity (see Fig. B.5). The green cross indicates the MW (q_AF = 0.17).