N2O2 as a function of density for the CLOUDY models described in Appendix D (triangles), colour-coded by the ionisation parameter log(U). The green solid line shows the N2O2 value corresponding to the model N/O using the calibration 3. The grey dashed, dot-dashed, and dotted lines indicate the N/O that would be inferred from the calibration at those specific N2O2. While our calibration performs well at densities ≤10³ cm⁻³, it overestimates N/O at higher gas densities, as expected.