Catalytic Ozone Destruction Cycles

Introduction

The Chapman mechanism alone predicts about twice as much ozone as we observe. The missing loss comes from trace radicals — present at parts per billion or less — that catalyse the destruction of odd oxygen. Because each radical is regenerated, a single chlorine atom can remove thousands of ozone molecules before it is locked away. Understanding these chain cycles, and the reservoirs that interrupt them, explains both natural ozone balance and human-caused ozone depletion.

Core explanation

The general cycle. A radical X reacts with ozone, and the product XO reacts with an oxygen atom:

X + O₃ → XO + O₂ XO + O → X + O₂ Net: O + O₃ → 2O₂

This is exactly Chapman reaction 4, but catalysed. X is regenerated, so it acts as a chain carrier. Both steps are fast radical reactions with small activation energies, while the uncatalysed O + O₃ reaction has a relatively high barrier, so the catalytic route dominates.

The four families.

Family Carrier pair Main source gas Most important region --- --- --- --- HOx OH / HO₂ H₂O, CH₄ (via O(¹D)) Upper stratosphere and lowest stratosphere NOx NO / NO₂ N₂O (via O(¹D)) Middle stratosphere, about 25–40 km ClOx Cl / ClO CFCs, CH₃Cl, CCl₄ Upper stratosphere, and polar regions BrOx Br / BrO CH₃Br, halons Lower stratosphere

For NOx the cycle is NO + O₃ → NO₂ + O₂ followed by NO₂ + O → NO + O₂. NOx is the largest single natural catalyst in the middle stratosphere. For HOx, one cycle needs no oxygen atoms at all: OH + O₃ → HO₂ + O₂ and HO₂ + O₃ → OH + 2O₂, net 2O₃ → 3O₂, which makes HOx important in the lower stratosphere where O atoms are scarce.

Coupled cycles. Families interact. The ClO + BrO reaction regenerates Cl and Br without needing O atoms, and it makes bromine, atom for atom, roughly 60 times more destructive than chlorine in the lower stratosphere.

Reservoir species. If every radical kept cycling, ozone would be destroyed far faster. In practice most active chlorine is stored in reservoirs :

Cl + CH₄ → HCl + CH₃ ClO + NO₂ + M → ClONO₂ + M

HCl and chlorine nitrate (ClONO₂) do not attack ozone. Normally more than 95% of stratospheric inorganic chlorine sits in these reservoirs. Slowly, photolysis or reaction with OH releases chlorine again. Similarly, NOx is stored as HNO₃ and N₂O₅, and HOx is removed by forming H₂O. Note that the families interfere: NO₂ ties up ClO as ClONO₂, so NOx both destroys ozone and limits chlorine destruction.

Chain length. The number of cycles completed before termination — the chain length — can reach 10³ to 10⁵. This is why CFC-derived chlorine, at only about 3 ppb, has such a large effect on ozone at parts per million.

Formulae

Rate of odd-oxygen loss by a cycle = 2 × rate of its rate-limiting step. For ClOx in the upper stratosphere: loss rate ≈ 2k[ClO][O]. Chain length ≈ rate of propagation ÷ rate of termination.

Step-by-step reasoning

To identify a catalytic ozone-destruction cycle:

1. Write the individual radical steps. 2. Add them and cancel species appearing on both sides. 3. Check that the catalyst is regenerated unchanged. 4. Check that the net reaction removes odd oxygen (O or O₃). 5. Identify the rate-limiting step, usually the XO + O reaction, and consider which reservoirs compete for X or XO.

Visual explanation

Draw a circle with X at the top and XO at the bottom. On the left arrow, O₃ enters and O₂ leaves; on the right arrow, O enters and O₂ leaves. Off to the side, a dashed arrow leads from the circle to a box labelled "reservoir (HCl, ClONO₂)", with a slow return arrow back.

Real-world analogy

A single pickpocket working a crowd can rob hundreds of people in a day, because each theft leaves the pickpocket free to strike again. Only arrest — the reservoir — stops the chain, and if the prisoner is released later the thefts resume.

Real-world example

Measurements by high-altitude aircraft in the late 1980s and 1990s showed ClO levels rising sharply exactly where ozone fell, a direct fingerprint of the ClOx cycle. This evidence strongly supported the case for phasing out CFCs, halons and methyl bromide.

Why?

Why is bromine more damaging per atom than chlorine? Bromine's reservoirs (HBr and BrONO₂) are formed slowly and destroyed quickly, especially by photolysis, so most inorganic bromine remains active as Br or BrO, whereas most chlorine is held in reservoirs.

Common misconception

"CFC molecules themselves destroy ozone." CFCs are inert in the troposphere. They must reach the stratosphere and be photolysed by short-wavelength UV to release chlorine atoms; it is those atoms, cycling catalytically, that destroy ozone.

Worked example

Question: Show that the reaction sequence Cl + O₃ → ClO + O₂, ClO + O → Cl + O₂ is catalytic, and state its net effect.

Reasoning: Adding the steps: Cl + O₃ + ClO + O → ClO + O₂ + Cl + O₂. Cancelling Cl and ClO, which appear on both sides, leaves O + O₃ → 2O₂. Chlorine is regenerated unchanged.

Answer: It is a catalytic cycle whose net effect is O + O₃ → 2O₂, destroying two odd-oxygen species per cycle.

Quick check

1. Name two chlorine reservoir species and explain why their formation protects ozone. Answer: HCl and ClONO₂; they hold chlorine in forms that do not react with ozone, interrupting the catalytic chain.

Exam focus

Be ready to write the general X/XO cycle and one specific example for each family, and to show the net reaction. Many questions test reservoirs: explain how NO₂ can both destroy ozone and protect it by forming ClONO₂.

Advanced insight

Because the families are coupled, the ozone response to a change is not additive. Increasing stratospheric N₂O, for example, adds NOx destruction but also locks more chlorine in ClONO₂. Today, with CFCs declining under the Montreal Protocol, N₂O emissions are the largest remaining human-made contribution to stratospheric ozone loss.

Summary

Radicals X (OH, NO, Cl, Br) destroy ozone catalytically: X + O₃ → XO + O₂ and XO + O → X + O₂, net O + O₃ → 2O₂. Each family has characteristic sources and dominant altitudes. Reservoirs such as HCl, ClONO₂ and HNO₃ store radicals in inactive forms. Long chain lengths let trace radicals control ozone, which is why CFCs and halons were so harmful.

Practice questions

1. Write the NOx catalytic cycle and its net reaction. Answer: NO + O₃ → NO₂ + O₂; NO₂ + O → NO + O₂; net O + O₃ → 2O₂. 2. Why is the HOx cycle OH + O₃ → HO₂ + O₂, HO₂ + O₃ → OH + 2O₂ important in the lower stratosphere? Answer: It needs no oxygen atoms, which are very scarce at low altitude, so it can operate where O-dependent cycles are slow. 3. Explain how the reaction ClO + NO₂ + M → ClONO₂ + M reduces ozone loss. Answer: It removes both ClO and NO₂ from their active families, storing them as an unreactive reservoir and shortening both chains. 4. What is the main source gas of stratospheric NOx? Answer: Nitrous oxide, N₂O, which reacts with O(¹D) in the stratosphere to form NO.