Relationship between NO_x and HO_x in the lower atmosphere on modern Earth, adapted from Logan et al. (1981). In the NO_x-limited regime (white background) an increase of NO_x will lead to an increase of HO_x and thus increased productivity of the smog mechanism. In the ΝΟ_x-saturated regime (gray background) the NO_x/HO_x ratio reaches a tipping point where NO_x begins to deplete HO_x by locking it up into stable reservoir species, therefore suppressing smog formation.