Column density of NO, N₂O, NO₂, and O₃ divided by the initial column density of CO before irradiation for CO:N₂ ice mixtures. The column density ratio is plotted as a function of dose. Timescale axes (top x-axis) give an estimate of the time (years) necessary for interstellar ices to undergo the effects observed in the laboratory. Experimental data that refer to nitrogen oxides were fitted with an exponential curve y = a(1 − e ^− σD).