Protection without poison: why does ozone maximize in the stratosphere?

Official Version to appear.

The number density of ozone, [O3], maximizes around 26 km in the tropics, protecting life from harmful ultraviolet light (UV) without poisoning it at the surface. Textbooks explain this interior maximum with two paradigms: (1) the source-controlled paradigm explains [O3] to maximize where its source maximizes between abundant photons aloft and abundant [O2] below, and (2) the source/sink competition paradigm, inspired by the Chapman cycle, explains ozone to scale with [O2] and the photolytic source/sink ratio. Yet, each paradigm’s prediction for the altitude of peak [O3] is off by 10 km, reflecting their well-known omission of ozone sinks from catalytic cycles and transport. We present a minimal, steady-state theory for the tropical stratospheric [O3] maximum, accurate to within 1 km, formulated in terms of the dominant ozone sinks. These sinks are represented simply by augmenting the Chapman cycle with linear damping of O and O3, leading to the Chapman+2 model. The Chapman+2 model correctly simulates peak tropical [O3] at 26 km, yet this peak is not explained by either paradigm. Instead, the peak is newly explained by the transition from an O-damped regime aloft to an O3-damped regime below. An explicit analytical expression is derived for ozone under gray radiation. This theory accurately predicts an interior maximum of ozone and correctly predicts that an increase in top-of-atmosphere UV leads to a downward shift of peak [O3], due to a downward shift in the regime transition, a result not even qualitatively predicted by the existing paradigms.