Halogen and Catalytic Ozone-Loss Cycles

The catalytic radical cycles that destroy stratospheric ozone far more efficiently than the oxygen-only Chapman reactions.

Definition

Halogen and catalytic ozone-loss cycles are the chains of reactions in which trace radicals repeatedly destroy ozone while being regenerated, accounting for most stratospheric ozone loss.

Scope

Covers the hydrogen-oxide, nitrogen-oxide, chlorine and bromine catalytic cycles, the concept of a catalyst that destroys many ozone molecules without being consumed, the chemistry of chlorofluorocarbon and halon source gases, reservoir species, and the relative efficiencies of the different families in destroying ozone.

Core questions

• How can a trace radical destroy thousands of ozone molecules?
• Which catalytic families dominate ozone loss at different altitudes?
• How do long-lived source gases deliver chlorine and bromine to the stratosphere?

Key theories

Catalytic ozone-destruction cycles

Radicals such as nitric oxide, chlorine and bromine react with ozone and atomic oxygen in cycles that net-destroy odd oxygen while regenerating the catalyst, so a small reservoir destroys vast amounts of ozone.

Mechanisms

In a catalytic cycle a radical X reacts with ozone to form XO, which then reacts with atomic oxygen or another XO to regenerate X, with the net effect of converting odd oxygen into molecular oxygen. The hydroxyl, nitric oxide, chlorine and bromine families each drive such cycles, dominant at different altitudes. Chlorine and bromine reach the stratosphere mainly as chemically inert chlorofluorocarbons and halons that are photolysed there, releasing catalysts; reservoir species such as chlorine nitrate temporarily sequester them, modulating the loss rate.

Clinical relevance

Identifying the catalytic role of chlorofluorocarbons revealed the human threat to the ozone layer and provided the scientific basis for the Montreal Protocol's phase-out of ozone-depleting substances.

History

Crutzen described the nitrogen-oxide catalytic cycle in 1970, and Molina and Rowland identified chlorine-atom catalysis from chlorofluorocarbons in 1974; together with Antarctic observations this work earned the 1995 Nobel Prize in Chemistry.

Key figures

• Paul Crutzen
• Mario Molina
• F. Sherwood Rowland

Seminal works

• crutzen1970
• molina1974

Frequently asked questions

Why are catalytic cycles so destructive to ozone?

Because the catalyst is regenerated at the end of each cycle, a single chlorine or bromine atom can destroy many thousands of ozone molecules before it is removed from the stratosphere.