06 Jun 2024
 | 06 Jun 2024
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

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

Aaron Match, Edwin P. Gerber, and Stephan Fueglistaler

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.

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Aaron Match, Edwin P. Gerber, and Stephan Fueglistaler

Status: open (until 18 Jul 2024)

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Aaron Match, Edwin P. Gerber, and Stephan Fueglistaler

Model code and software

Chapman Cycle Photochemical Equilibrium Solver Aaron Match

Aaron Match, Edwin P. Gerber, and Stephan Fueglistaler


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Short summary
Explanations for the tropical ozone maximum at 26 km have fragmented into two paradigms, shown to represent limiting regimes of ozone photochemistry with production by UV and generalized destruction by catalytic cycles and transport. Paradoxically, neither paradigm explains the observed ozone peak, motivating a new theory: peak ozone occurs precisely at the transition between these regimes. An idealized analytical ozone profile is derived, helping to interpret sensitivities to UV perturbations.