Ozone and Photochemical Smog

How sunlight, nitrogen oxides and hydrocarbons combine to produce ground-level ozone and the secondary pollutants of photochemical smog.

Definition

Photochemical smog is the mixture of ozone and other oxidants formed when sunlight drives reactions among nitrogen oxides and volatile organic compounds in polluted air.

Scope

Covers the photostationary state relating ozone, nitric oxide and nitrogen dioxide, the role of volatile organic compounds in shifting this balance toward net ozone production, the nonlinear dependence of ozone on nitrogen oxide and hydrocarbon levels, and the formation of secondary pollutants such as peroxyacetyl nitrate.

Core questions

• Why does ozone accumulate at ground level on sunny days in polluted cities?
• How do volatile organic compounds and nitrogen oxides jointly control ozone?
• Why is ozone reduction sometimes achieved by cutting hydrocarbons rather than nitrogen oxides?

Key theories

Photochemical ozone production cycle

Oxidation of volatile organic compounds converts nitric oxide to nitrogen dioxide without consuming ozone, so its photolysis yields a net catalytic build-up of ground-level ozone.

Mechanisms

In clean air ozone, nitric oxide and nitrogen dioxide reach a photostationary state with no net ozone production. When volatile organic compounds are present, the peroxy radicals from their oxidation convert nitric oxide to nitrogen dioxide without destroying ozone; subsequent photolysis of nitrogen dioxide then generates ozone, producing a net accumulation. The yield depends nonlinearly on the ratio of volatile organic compounds to nitrogen oxides, giving rise to distinct nitrogen-oxide-limited and hydrocarbon-limited regimes.

Clinical relevance

Ground-level ozone is a major air pollutant harmful to respiratory health and vegetation, and understanding its nonlinear chemistry guides effective emission-control strategies.

History

Arie Haagen-Smit identified the photochemical origin of Los Angeles smog in the early 1950s, showing that sunlight acting on car exhaust and hydrocarbons produces ozone, a discovery that founded the study of urban photochemical air pollution.

Key figures

• Arie Haagen-Smit
• James Pitts

Seminal works

• haagenSmit1952
• finlaysonPitts2000

Frequently asked questions

Is ground-level ozone the same as the ozone layer?

No. Stratospheric ozone shields life from ultraviolet radiation, whereas ground-level ozone is a harmful pollutant formed by photochemistry near the surface; the two are governed by different processes.