O₂–O₃ relationship for Earth-like planets orbiting different host stars (indicated in legend) with varying amounts of O₂ mixing ratios. Molecular oxygen levels compared to Earth’s present atmospheric level (PAL) are shown as dashed vertical lines for reference. The left-hand figure shows the relationship in terms of total integrated O₃ column density, and the right-hand figure in terms of total O₃ normalized at the amount produced in the 100% PAL O₂ case for each stellar host. The nonlinearity in these relationships is primarily due to the pressure dependence of Reaction (2) which forms O₃. The main takeaway is that the O₂–O₃ relationship is significantly different for hotter host stars (GOV, Sun, K2V) versus cooler host stars (K5V, M5V), with hotter hosts experiencing peak O₃ formation occurring at O₂ levels under 100% PAL. See Sect. 4.1 for a detailed explanation.