Stratospheric Ozone Photochemistry

The photochemical reactions that form and destroy ozone in the stratosphere and maintain the ozone layer.

Definition

Stratospheric ozone photochemistry is the set of sunlight-driven reactions producing and destroying ozone in the stratosphere, beginning with the oxygen-only Chapman mechanism.

Scope

Covers the Chapman mechanism of ozone formation and destruction from molecular and atomic oxygen, the concept of odd oxygen, the vertical distribution of ozone, the discrepancy between Chapman predictions and observations, and the absorption of ultraviolet radiation that heats the stratosphere.

Core questions

• How does the Chapman mechanism produce the ozone layer?
• Why does the Chapman cycle alone overpredict stratospheric ozone?
• How does ozone absorption shape the temperature structure of the stratosphere?

Key theories

Chapman mechanism

Photolysis of molecular oxygen creates atoms that form ozone, while ozone photolysis and recombination destroy it, defining a steady-state odd-oxygen abundance.

Mechanisms

Short-wavelength ultraviolet sunlight dissociates molecular oxygen into atoms that combine with oxygen molecules to form ozone. Ozone is itself photolysed back to molecular and atomic oxygen, and the family of atomic oxygen and ozone, termed odd oxygen, reaches a photochemical steady state that peaks near 25 kilometres. Because the oxygen-only Chapman cycle overpredicts ozone, additional catalytic loss cycles are needed to match observations; the absorption of ultraviolet radiation by ozone warms the stratosphere and creates its temperature inversion.

Clinical relevance

The ozone layer absorbs biologically harmful ultraviolet radiation, and quantifying its photochemistry is the basis for understanding both its natural state and its vulnerability to depletion.

History

Sydney Chapman proposed the photochemical theory of stratospheric ozone in 1930, providing the first quantitative explanation of the ozone layer that remains the starting point for stratospheric chemistry.

Key figures

• Sydney Chapman
• Guy Brasseur

Seminal works

• chapman1930
• brasseur2005

Frequently asked questions

What is odd oxygen?

Odd oxygen is the chemical family comprising atomic oxygen and ozone, treated together because they interconvert rapidly; its total abundance changes only through slower production and loss reactions.