Evolution of the number of molecules in the IRAM 30-m beam as a function of molecule photo-destruction rate. We plot, more precisely, the average column density (number of molecules divided by the beam area) normalised to its value for infinite lifetime. We used the average beam size (θ_b = 11′′) for an average pointing offset of 2′′ at a geocentric distance of 0.08 au, which is typical for most observations of comet 46P. A change of molecular photo-destruction rate will result in a change of the column density that can be inferred from this plot. The position of the photo-destruction rate for well-known molecules is indicated.