Cartoon of the procedure of the first set of experiments. Step 1: water-dominated ice was deposited at 10 K, forming a highly porous amorphous ice. Three different ice compositions were chosen: (1) 200 ML H₂O, (2) mixture of 200 ML H₂O and 20 ML NH₃, and (3) mixture of 200 ML H₂O and 40 ML CO₂. Step 2: ice was warmed up to a higher temperature to anneal for 30 min. The following temperatures were chosen: 20, 40, 60, 80, 100, or 120 K. Step 3: ice was cooled down to 20 K and exposed to CO deposition. RAIRS spectra were recorded during the CO deposition, which is shown in Fig. 2.