The ratios of both intensity of column density are shown. The ¹²CO/¹³CO column density ratio derived from RADEX is shown in (a) while the line intensity ratio ¹²CO/¹³CO for the J = 1–0 transition from the ALCHEMI observations is shown in (b). The regions unaffected by the absorption effects observed near GMC-5 in NGC 253 are shown in full colour, whereas those regions that are affected are shaded.