Comparison between collision timescale of monomers (filled contours) and dissociation timescale of dimers (black contours, same units) for the Herbig model (top row) and the T Tauri model (bottom row). Left: coronene, middle: circumcoronene, and right: circumcircumcoronene. Estimated τ_UV = 1 surfaces are indicated by coloured lines. Monomer collisions occur in all cases on disc-relevant timescales within 100 AU below the τ = 1 surfaces.