Physical (approximately vertical) structure of d203-506. Upper panel: model profiles of gas density, gas temperature, and a^−3.5 grain-size distribution, showing the highest and lowest dust temperatures (corresponding to a_min and a_max grain radii, respectively) as a function of depth into the PDR. The green curve shows the density of ${\text{H}}_2^{*}\left(v\ge 4\right)$. Lower panels: abundance profiles of selected species. Bottom sketch: simplified sketch illustrating the PDR component of an externally irradiated protoplanetary disk. The different colored regions correspond to the chemical zones discussed in Sect. 4.