Desorption rates in the Herbig disc STD photosphere calculated with the Monte Carlo model for increasing cluster sizes, radii, and PAH species. Shown are thermal evaporation rates calculated from eq. (13) and eq. (23), (dash-dotted lines) and turbulent turn-over rates calculated from eq. (31), (dotted lines). For increasing PAH size and cluster size, desorption becomes slower until clusters are unlikely to desorb. PAH coagulation and adsorption on dust grains therefore lead to loss of gas-phase PAHs to the grains.