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When studying the chemistry of PDRs, time dependence becomes important asvisual extinction increases, since certain chemical timescales are comparableto the cloud lifetime. Dust temperature is also a key factor, since itsignificantly influences gas temperature and mobility on dust grains,determining the chemistry occurring on grain surfaces. We present a study ofthe dust temperature impact and time effects on the chemistry of differentPDRs, using an updated version of the Meijerink PDR code and combining it withthe time-dependent code Nahoon. We find the largest temperature effects in theinner regions of high $G$$_{\mathrm{0}}$ PDRs, where high dust temperaturesfavour the formation of simple oxygen-bearing molecules (especially that ofO$_2$), while the formation of complex organic molecules is much more efficientat low dust temperatures. We also find that time-dependent effects stronglydepend on the PDR type, since long timescales promote the destruction ofoxygen-bearing molecules in the inner parts of low $G$$_{\mathrm{0}}$ PDRs,while favouring their formation and that of carbon-bearing molecules in high$G$$_{\mathrm{0}}$ PDRs. From the chemical evolution, we also conclude that, indense PDRs, CO$_2$ is a late-forming ice compared to water ice, and confirm alayered ice structure on dust grains, with H$_2$O in lower layers than CO$_2$.Regarding steady state, the PDR edge reaches chemical equilibrium at earlytimes ($\lesssim$10$^5$ yr). This time is even shorter ($<$10$^4$ yr) for high$G$$_{\mathrm{0}}$ PDRs. By contrast, inner regions reach equilibrium muchlater, especially low $G$$_{\mathrm{0}}$ PDRs, where steady state is reached at$\sim$10$^6$-10$^7$ yr.

Dust temperature and time-dependent effects in the chemistry of photodissociation regions