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The Chemistry of Photochemical Smog

Key Concepts

Photochemical smog is a type of air pollution produced when sunlight acts upon motor vehicle exhaust gases to form harmful substances such as ozone (O3), aldehydes and peroxyacetylnitrate (PAN).

Photochemical smog formation requires the following conditions:

  • a still, sunny day

  • temperature inversion (pollutants accumulate in the lower inversion layer)

Ozone causes breathing difficulties, headaches, fatigue and can aggrevate respiratory problems.

The peroxyacetylnitrate (CH3CO-OO-NO2) in photochemical smog can irritate the eyes, causing them to water and sting.

Ozone Production

Motor vehicles produce exhaust gases containing oxides of nitrogen such as nitrogen dioxide (NO2) and nitric oxide (NO).

At the high temperatures of the car's combustion chamber (cylinder), nitrogen and oxygen from the air react to form nitric oxide (NO):

N2(g) + O2(g) → 2NO(g)

Some of the nitric oxide (NO) reacts with oxygen to form nitrogen dioxide (NO2):

2NO(g) + O2(g) → 2NO2(g)

The mixture of nitric oxide (NO) and nitrogen dioxide (NO2) is sometimes referred to as NOx.

When the nitrogen dioxide (NO2) concentration is well above clean air levels and there is plenty of sunlight, then an oxygen atom splits off from the nitrogen dioxide molecule:

NO(g) + O(g)

This oxygen atom (O) can react with oxygen molecules (O2) in the air to form ozone (O3):

O + O2 → O3

Nitric oxide can remove ozone by reacting with it to form nitrogen dioxide (NO2) and oxygen (O2):

NO(g) + O3(g) → NO2(g) + O2(g)

When the ratio of NO2 to NO is greater than 3, the formation of ozone is the dominant reaction. If the ratio is less than 0.3, then the nitric oxide reaction destroys the ozone at about the same rate as it is formed, keeping the ozone concentration below harmful levels.

The reaction of hydrocarbons (unburnt petrol) with nitric oxide and oxygen produce nitrogen dioxide also in the presence of sunlight, increasing the ratio of nitrogen dioxide to nitric oxide.

Peroxyacetylnitrate Production

Nitrogen dioxide (NO2), oxygen (O2) and hydrocarbons (unburnt petrol) react in the presence of sunlight to produce peroxyacetylnitrate (CH3CO-OO-NO2):

NO2(g) + O2(g) + hydrocarbons

Catalytic Converters

Catalytic converters on motor vehicle exhausts are a way of trying to reduce the carbon monoxide and nitrogen oxide emissions.

The catalyst used is either platinum or a combination of platinum and rhodium.

The platinum catalyses the reaction of unburnt hydrocarbon (such as pentane) and oxygen (O2) to produce carbon dioxide (CO2) and water vapour (H2O):

platinum catalyst

The rhodium catalyses the reaction of carbon monoxide (CO) and nitric oxide (NO) to form carbon dioxide (CO2) and nitrogen gas (N2):

2CO + 2NO
rhodium catalyst
2CO2 + N2

The reduction of nitric oxide (NO) to nitrogen gas (N2) must proceed more quickly than the oxidation of carbon monoxide (CO) to carbon dioxide (CO2) or else all the carbon monoxide will be oxidised to carbon dioxide before it can be used to reduce the nitric oxide.

Motor vehicles can only use catalytic converters if they use unleaded petrol since the lead in petrol renders the catalyst inactive.

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