Intensity of the mean UV radiation field G₀. The black line indicates the τ_UV = 1 line, and the red line shows where the monomer desorption rate is equal to the monomer adsorption. In the photosphere the small PAH clusters quickly desorb and dissociate into monomers. In the coagulation all gas-phase PAHs coagulate and adsorb on dust grains, while in the quiet layer monomers, clusters, and PAH-carrying grains can co-exist.