Left panel: χ² values comparing model and data for different α laws as a function of bulk diffusion energy. The value α₀ corresponds to the exact experimental values of unreacted species. Right panel: comparison between model (lines) and experimental data (dots). Experimental data are obtained by integrating IR peaks of NO and NO₂ as a function of O-atom fluence for five different thickness of water ices (0, 2, 3, 6, and 10 mL) held at 40 K. Simulated curves are obtained by using three laws for the α parameter (α₁, α₂, and α₃) and measured experimental values (α₀) for the best case corresponding to the minimum χ² of left panel. For α₀ any choice of diffusion parameter greater than 950 K gives similar results.