Impact of HO₂ and OH in the NO_x-limited and NO_x-saturated regimes on modern Earth, adapted from Logan et al. (1981). In the NO_x-limited regime (white background), increasing NO_x allows for a more efficient smog mechanism, with the resulting increase in O₃ causing a corresponding rise in HO_x. In the NO_x-saturated regime (gray background), the abundance of NO_x rises to the point where smog O₃ production is suppressed as NO_x depletes HO_x (a necessary catalyst for the smog mechanism) by locking it up into reservoir species.