Beyond self-healing: Stabilizing and destabilizing photochemical adjustment of the ozone layer

Abstract. The ozone layer is often noted to exhibit self-healing, whereby a process that depletes ozone can nonetheless lead to increased ozone at lower altitudes. Self-healing has been explained to occur because ozone depletion aloft allows more ultraviolet (UV) light to reach lower levels, where it enhances ozone production. Similarly, a process that increases ozone can nonetheless reduce ozone below, known as reverse self-healing. This paper considers self-healing and reverse self-healing to manifest a more general mechanism we call photochemical adjustment, whereby ozone perturbations lead to a downward cascade of anomalies in ultraviolet fluxes and ozone. Conventional explanations for self-healing suggest that photochemical adjustment is stabilizing, i.e., the initial perturbation in column ozone is damped towards the surface. However, if the enhanced ultraviolet transmission due to ozone depletion disproportionately increases the ozone sink, then photochemical adjustment can be destabilizing. We use the coefficients of the Cariolle v2.9 linear ozone model to analyze photochemical adjustment in the chemistry-climate model MOBIDIC. We find that: (1) photochemical adjustment is destabilizing in the upper stratosphere, and (2) self-healing is often just the tip of the iceberg of large photochemical stabilization throughout the mid- and lower-stratosphere. The photochemical regimes from MOBIDIC can be reproduced by the Chapman Cycle, a classical model of ozone photochemistry whose simplicity admits theoretical insight. Photochemical regimes in the Chapman Cycle are controlled by the spectral structure of the perturbed ultraviolet fluxes. The transition from photochemical destabilization to stabilization occurs at the slant column ozone threshold where ozone becomes optically saturated in the overlap window of absorption by O₂ and O₃, i.e., 10¹⁸ molec cm^-2 (around 40 km in the tropics).