  We study ten molecular transitions obtained from an unbiased 3 mm molecular
line survey using the IRAM 30 m telescope toward 409 compact dust clumps
identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) to
understand photodissociation regions (PDRs) associated with the clumps. The
main goal of this study is to investigate whether the abundances of the
selected molecules show any variations resulting from the PDR chemistry in
different clump environments. We selected HCO, HOC$^+$, C$_{2}$H,
c-C$_{3}$H$_{2}$, CN, H$^{13}$CN, HC$^{15}$N, and HN$^{13}$C as PDR tracers,
and H$^{13}$CO$^{+}$ and C$^{18}$O as dense gas tracers. We find that the
abundances of HCO, CN, C$_{2}$H and c-C$_{3}$H$_{2}$ decrease as the H$_{2}$
column density increase, indicating high visual extinction, while those of high
density tracers (i.e., H$^{13}$CO$^{+}$ and HC$^{15}$N) are constant. In
addition, $N$(HCO)/$N$(H$^{13}$CO$^{+}$) ratios significantly decrease as
H$_{2}$ column density increase, and in particular, 82 clumps have $X$(HCO)
$\gtrsim 10^{-10}$ and $N$(HCO)/$N$(H$^{13}$CO$^{+}$) $\gtrsim 1$, which are
the indication of far-ultraviolet (FUV) chemistry. This suggests the observed
HCO abundances are likely associated with FUV radiation illuminating the PDRs.
We also find that high $N$(c-C$_{3}$H$_{2}$)/$N$(C$_{2}$H) ratios found for HII
regions having high HCO abundances ($\gtrsim\,10^{-10}$) are associated with
more evolved clumps with high $L_{\rm bol}$/$M_{\rm clump}$. This trend might
be associated with gain-surface processes, which determine initial abundances
of these molecules, and time-dependent effects in the clumps corresponding to
the envelopes around dense PDRs and HII regions. In addition, some fraction of
the measured abundances of the small hydrocarbons of the HII sources can be the
result of the photodissociation of PAH molecules.
