Ground-Level Ozone Formation

Ground-level ozone forms when sunlight drives chemical reactions among pollutants in the air, rather than being emitted directly from vehicles or factories. It is a secondary pollutant produced through photochemical reactions involving nitrogen oxides (NOx) and volatile organic compounds (VOCs), and is a major component of photochemical smog.

Sunlight initiates ozone formation

The starting step is photolysis of nitrogen dioxide (NO₂): sunlight supplies the energy needed to split the molecule and release an oxygen atom.

• The first reaction is NO₂ + sunlight → NO + O, producing nitric oxide and an oxygen atom.
• The oxygen atom then combines with molecular oxygen: O + O₂ + M → O₃ + M. Here, M is another air molecule that carries away excess energy and stabilises the ozone formed.
• Ozone can react with nitric oxide through NO + O₃ → NO₂ + O₂. This reaction consumes ozone and regenerates nitrogen dioxide.

VOCs allow ozone to accumulate

The reactions above alone mainly cycle nitrogen oxides and ozone. Sustained net ozone production occurs when reactions involving VOCs convert nitric oxide into nitrogen dioxide without consuming existing ozone.

• Oxidation of VOCs, often initiated by hydroxyl radicals, produces reactive intermediates including peroxy radicals, such as RO₂.
• A representative reaction is RO₂ + NO → RO + NO₂. Unlike the reaction between NO and ozone, this pathway produces NO₂ without removing an ozone molecule.
• The newly formed NO₂ undergoes photolysis, allowing additional ozone to form. Repeated cycling can therefore increase ozone concentrations in polluted, sunlit air.

Conditions favouring urban ozone episodes

Strong sunlight drives the photochemistry, while warm conditions often favour ozone episodes. Actual concentrations also depend on precursor availability, atmospheric mixing and pollutant transport.

• Vehicles and combustion sources emit nitrogen oxides, while fuels, solvents and industrial activities supply VOCs.
• Stagnant air can allow precursors and ozone to build up, while winds can transport them to downwind areas.
• Fresh nitric oxide emissions can locally consume ozone, so the highest ozone concentrations need not occur beside the largest emission sources.
• Ozone control requires attention to both NOx and VOCs because their chemical interactions make the response to emission reductions nonlinear.