Time-dependent behaviour of COMs (lines, colour-coded by distance as in Figure 5) compared with measurements (horizontal black lines) taken from three infrared-quiet massive galactic clumps (G328.25, G335.58, and G335.78) with L = 10⁴ L_⊙. To better explain all the COM abundances reported for G335.58 (including C₂H₅OH), the average grain size is shown to be larger than the typical grain size of 0.1 μm in the diffuse ISM.