Smog and Ozone

Ground-level ozone, a main component of smog, is formed by a chemical reaction between volatile organic chemicals and nitrogen oxides in the presence of UV light. Ozone concentration can reach unhealthy levels when the weather is hot and sunny with little or no wind

Ozone can irritate the eyes and trachea, causing breathing difficulties and may increase susceptibility to infection. It is a highly reactive chemical, capable of attacking surfaces, fabrics and rubber materials. Ozone is also toxic to some crops, vegetation and trees.

Ground-level ozone is not emitted directly into the atmosphere, but is a secondary pollutant produced by a reaction between nitrogen dioxide, hydrocarbons and sunlight.


As ozone itself is photodissociated (split up by sunlight) to form free radicals it
catalyses its own formation. Consequently, high levels of ozone are generally observed during hot, still, sunny, summertime weather in locations where the air already has a lot of hydrocarbons and nitrogen oxides (e.g. urban areas with traffic).

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The resulting ozone pollution or “summertime smog” may persist for several days and be transported over long distances.

Catalytic Converters

There is widespread concern about the emissions from motor vehicles that can cause smog and produce O3 forming gases. One attempt to stop this problem has been the introduction of catalytic converters in car exhaust systems.

A catalytic converter sits inside a car and uses a catalyst to convert three harmful compounds in car exhaust into harmless compounds.

The three harmful compounds are:

  • Hydrocarbons (in the form of unburned gasoline)
  • ...

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