Mixing ratio profiles of O₃, H₂O, CH₄, and N₂O; all potential biosignature species. Each row represents results for model atmospheres orbiting different stellar hosts as indicated on the right-hand vertical axis. Each plot shows the mixing ratio profiles for O₂ abundances of 100%, 10%, 1% and 0.1% PAL. O₂ on modern Earth is a significant UV shield, allowing model atmospheres with decreasing O₂ values to have photolysis occur at consistently lower altitudes. This is demonstrated well with the bulk of O₃ forming at lower altitudes with less O₂, as well as increased upper atmospheric depletion for H₂O, CH₄, and N₂O via photolysis. The downward shift of O₃ and upper atmospheric depletion of other species is shown to decrease for model atmospheres around cooler stars with lower incident UV flux, and hence lower photolysis rates. Full details of this atmospheric chemistry is shown in Sects. 4.1 and 4.3.