Absorption timescales are represented by the green lines for IC63 (left panel), Horsehead (middle panel), Orion Bar (right panel). Collision timescales between the most abundant aggregates (a₀ ~ 0.05 µm) are shown in red lines. The horizontal grey stripes correspond to the advection timescales (see Table 4). The vertical light green (red) stripes correspond to the region where τ_abs ~ τ_ad (τ_coll ~ τ_ad).