Left: evolution of the m/z = 28 QMS signal corresponding to the photodesorption of CO during UV irradiation of a pure CO ice. The ion current in the y-axis corresponds, approximately, to the partial pressure (mbar) in the main chamber. Dotted lines represent the baselines used to calculate A(28) in every irradiation period. Note that the y-axis is on a logarithmic scale. Middle: evolution of the C=O stretching mode of CO at 2139 cm^-1 during UV irradiation of pure CO ice. Differences in the column densities after every period of irradiation correspond to N(CO) in Eq. (A.3). Right: relation between A(28) and N(CO). The solid line is a linear fit.