Stratospheric Chemistry and Ozone

The chemistry that creates and destroys the stratospheric ozone layer, including the catalytic cycles responsible for ozone depletion.

Definition

Stratospheric chemistry and ozone is the study of the reactions governing the abundance of ozone and related species in the stratosphere, where the ozone layer shields the surface from ultraviolet radiation.

Scope

This area covers the photochemical formation and destruction of stratospheric ozone described by the Chapman mechanism, the catalytic cycles involving hydrogen, nitrogen, chlorine and bromine species, the role of halogenated source gases, heterogeneous chemistry on polar stratospheric clouds, and the springtime polar ozone holes and their recovery under international regulation.

Sub-topics

• Halogen and Catalytic Ozone-Loss Cycles
• Polar Ozone Depletion
• Stratospheric Ozone Photochemistry

Core questions

• How is the stratospheric ozone layer formed and maintained?
• Which catalytic cycles destroy ozone, and why are they so efficient?
• What caused the polar ozone holes and how is the layer recovering?

Key theories

Chapman mechanism

A set of oxygen-only photochemical reactions that produce and destroy ozone, explaining the existence of the ozone layer though overpredicting its abundance.

Catalytic ozone destruction

Trace radicals from hydrogen, nitrogen and especially halogen species catalytically destroy ozone, with chlorine from chlorofluorocarbons identified as a major anthropogenic threat.

Mechanisms

Ultraviolet sunlight splits molecular oxygen, and the atoms combine with oxygen to form ozone; the Chapman cycle of formation and photolysis sets a baseline ozone abundance. Real ozone levels are lower because catalytic cycles involving hydroxyl, nitric oxide, chlorine and bromine radicals destroy ozone without being consumed. Long-lived halogenated source gases transport chlorine and bromine to the stratosphere, where photolysis releases the catalysts, and heterogeneous reactions on polar stratospheric clouds activate them to produce severe seasonal depletion.

Clinical relevance

Stratospheric ozone absorbs harmful ultraviolet radiation, so understanding its chemistry underpinned the Montreal Protocol and continues to guide assessment of ozone-layer recovery.

History

Chapman proposed the oxygen-only ozone mechanism in 1930; Crutzen, Molina and Rowland revealed the catalytic roles of nitrogen oxides and chlorofluorocarbons in the 1970s, work recognized by the 1995 Nobel Prize in Chemistry, and the 1985 discovery of the Antarctic ozone hole spurred the Montreal Protocol.

Key figures

• Mario Molina
• F. Sherwood Rowland
• Paul Crutzen
• Susan Solomon

Seminal works

• molina1974
• brasseur2005

Frequently asked questions

Is the ozone hole recovering?

Yes. Following the Montreal Protocol's phase-out of ozone-depleting substances, stratospheric chlorine and bromine are declining and observations indicate the ozone layer is slowly recovering.