Protection without poison: Why tropical ozone maximizes in the interior of the atmosphere

Abstract. Ozone is the most significant radiatively-active gas whose number density maximizes in the interior of the atmosphere, at an altitude of around 26 km in the tropics. Textbook explanations for this interior maximum begin by invoking the Chapman Cycle, a photochemical system that reproduces the altitude of maximum ozone despite omitting leading-order sinks from catalytic cycles and transport. Yet, these textbook explanations subsequently fragment into (1) a source-controlled paradigm, explaining ozone to maximize where its production rate maximizes, between abundant photons aloft and abundant O₂ below, and (2) a source/sink competition paradigm explaining ozone to maximize due to competition between the photolytic source and photolytic sink. Augmenting the Chapman Cycle with destruction by generalized catalytic cycles and transport, we demonstrate that these paradigms correspond to different regimes of ozone destruction, distinguished by whether photolysis of O₃ contributes at leading order to the sink. The tropical stratosphere is estimated to occupy a photolytic sink regime above 26 km and a non-photolytic sink regime below. Paradoxically, each paradigm predicts ozone to maximize outside its altitude range of applicability, motivating a new explanation, the regime transition paradigm: the interior maximum of ozone occurs at the transition from the photolytic sink regime aloft to the non-photolytic sink regime below. An explicit solution is derived for ozone under gray radiation, which produces an interior maximum at an endogenously-determined regime transition, and elucidates the ozone response to top-of-atmosphere UV perturbations.