Speaker: Dr. Aaron Match, NSF Postdoctoral Research Fellow, New York University

Title: Revisiting the photochemistry of the ozone layer: structure, self-healing, and sensitivity to global warming

Abstract: We revisit three classic questions about ozone: 1) Why is the ozone layer in the stratosphere? 2) How does ozone self-healing work? 3) Why is ozone predicted to decrease in the tropical lower stratosphere under global warming? Conventional answers to these questions are common knowledge in textbooks and scientific reports: (1) Ozone occupies a sweet spot between abundant photons aloft and abundant oxygen below, (2) ozone depletion allows more photons to reach lower altitudes, causing partially-compensating increases in ozone, and (3) strengthening stratospheric upwelling advects ozone-poor air from below. Distilling ozone photochemistry to its bare essentials reveals qualitative limitations in the first two conventional explanations. We present a new framework for interpreting the vertical structure of ozone in terms of the joint distribution of ozone and oxygen absorption coefficients. Our idealized framework facilitates an analytical solution for Chapman photochemistry that exhibits an interior maximum (for a suitably-constrained joint distribution of absorption coefficients). This solution highlights the role of competition between the photolytic source and sink in shaping the ozone layer. Self-healing is newly conceptualized as a remapping of this photolytic source and sink lower in altitude due to ozone depletion, which unifies self-healing with lesser-known positive column ozone feedbacks. Lastly, we revisit the ozone response to global warming. Robustly-predicted reductions in ozone in the tropical lower stratosphere have been explained to result from a strengthening Brewer-Dobson circulation. Our idealized photochemical-transport model suggests that tropospheric expansion also contributes at leading order to these reductions by eroding the ozone layer from below.