Atmospheric pollutants

When fuels are burned, a number of atmospheric pollutants are produced.

Here are some of the pollutants from burning fuels, and where they come from:

Pollutant Source
Carbon dioxide, CO2Complete combustion of any fuel containing carbon atoms
Carbon monoxide, CO Incomplete combustion of any fuel containing carbon atoms
Particulate carbon, C (soot)Incomplete combustion of any fuel containing carbon atoms
Unburned hydrocarbonsHydrocarbon fuel molecules which have not been oxidised at all
Sulfur dioxide, SO2Combustion of a fossil fuel which contains sulfur impurities
Nitrogen oxides, NOxOxidation of atmospheric nitrogen inside the engine of a car, lorry, etc

Sulfur dioxide as a pollutant

Sulfur dioxide is caused when sulfur atoms which are present in some fossil fuels are oxidised:

sulfur + oxygen → sulfur dioxide

S + O2 → SO2

Sulfur dioxide is then further oxidised in the atmosphere to sulfur trioxide, SO3. This gas dissolves in rainwater to make acid rain, which is a dilute solution of sulfuric acid, H2SO4.

The two reactions are:

sulfur dioxide + oxygen → sulfur trioxide

2SO2 + O2 → 2SO3

And then:

sulfur trioxide + water → sulfuric acid

SO3 + H2O → H2SO4

Acid rain harms and kills plants and animals, especially those that live in aquatic environments. It can also damage man-made objects like statues and buildings.

NOx gases as pollutants

Nitrogen is not present in fuels, but the high temperatures and pressures inside a car engine can cause the nitrogen and oxygen in the air to react together to make oxides of nitrogen.

There are several compounds formed when nitrogen bonds with oxygen, but the two which are made inside engines are NO and NO2. These two pollutants are grouped together with the general formula NOx.

NOx gases can cause acid rain, and they also react in the atmosphere with other pollutants to make photochemical smog. Smog can have major health effects, causing asthma attacks and even death.


Diesel sometimes contains sulfur impurities. List the pollutants which could be produced when it is burned incompletely inside a car engine.

Carbon monoxide, particulate carbon, sulfur dioxide and nitrogen oxides.

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