Column density evolution of CO₂, CO, H₂CO, and POM ices as a function of fluence. In the case of POM, a factor of 2 uncertainty was assumed for the band strength of the combination mode at 1105 cm⁻¹ (see text), as depicted by the shaded green zone. In the case of H₂CO, the band strength of the stretching mode at 1725 cm⁻¹ was also evaluated by Schutte et al. (1993) as 9.6×10⁻¹⁸ cm molecules⁻¹ at 10 K, which corresponds to the highest column density value of the shaded red zone. Otherwise, an uncertainty of 20% was assumed for the band strengths.