Photochemically simulated CO (a, b), NO and NO₂ (c, d) mixing ratios in the oxic atmosphere (4.6% CO₂, 21% O₂) of an Earth-sized planet orbiting the Sun (a, c) and TRAPPIST-1 (b, d) for varying CO and NO production rates and the biotic scenario with CO deposition (T_surf = 288 K). The second x-axis gives the corresponding lightning flash rate (in units of modern Earth’s flash rate), as estimated for the respective atmospheric composition.