The ATLASGAL PDR survey is discussed with its high detection rates of chosen PDR tracers towards HII sources. While previous chemical modelling of specific sources shows that in a cold lower-density envelope the abundances of C2H and c-C3H2 vary little, subsequently during cloud collapse (with density increase, temperature rise, and the emergence of HII regions) from 105 yr on in the models the column density ratio increases steeply. The observed abundances of some high-column-density tracers (H13CO+ and HC15N) in the survey are almost constant over the range of H2 column densities, while others (HCO, CN, C2H and c-C3H2) fall as H2 increases. The HCO detections are confirmed as arising from clumps likely associated with PDRs, and higher HCO abundances are undoubtedly linked in the models to ongoing FUV chemistry.