Sketch of PAH processing cycle in the protoplanetary disc driven by vertical mixing. PAH monomers will eventually mix into the UV depleted coagulation layer where PAH collisions lead to sticking. The cluster growth continues until the PAH clusters freeze out on dust grains. If the cluster carrying grains are mixed into the UV-rich photosphere, PAH monomers and small clusters can desorb while large clusters remain on the grain. Every cycle of coagulation, adsorption, desorption, and dissociation leads to a depletion of gas-phase PAHs as with every cycle large clusters form and remain on the grains.