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We study whether polycyclic aromatic hydrocarbons (PAHs) can be a weightysource of small hydrocarbons in photo-dissociation regions (PDRs). We modeledthe evolution of 20 specific PAH molecules in terms of dehydrogenation anddestruction of the carbon skeleton under the physical conditions of twowell-studied PDRs, the Orion Bar and the Horsehead nebula which representprototypical examples of PDRs irradiated by "high" and "low" ultravioletradiation field. PAHs are described as microcanonical systems. The acetylenemolecule is considered as the main carbonaceous fragment of the PAHdissociation as it follows from laboratory experiments and theory. We estimatedthe rates of acetylene production in gas phase chemical reactions and comparedthem with the rates of the acetylene production through the PAH dissociation.It is found that the latter rates can be higher than the former rates in theOrion Bar at $A_{\rm V}<1$ and also at $A_{\rm V}>3.5$. In the Horseheadnebula, the chemical reactions provide more acetylene than the PAHdissociation. The produced acetylene participate in the reactions of theformation of small hydrocarbons (C$_2$H, C$_3$H, C$_3$H$^{+}$, C$_3$H$_2$,C$_4$H). Acetylene production via the PAH destruction may increase theabundances of small hydrocarbons produced in gas phase chemical reactions inthe Orion Bar only at $A_{\rm V}>3.5$. In the Horsehead nebula, thecontribution of PAHs to the abundances of the small hydrocarbons is negligible.We conclude that the PAHs are not a major source of small hydrocarbons in bothPDRs except some locations in the Orion Bar.

Impact of PAH photodissociation on the formation of small hydrocarbons in the Orion Bar and the Horsehead PDRs