atmospheric pollutants

Higher and foundation tier

Most of the fuels we use today such as petrol, diesel and kerosene (aviation fuel) are all hydrocarbons. Hydrocarbons are compounds containing the elements hydrogen and carbon. The products of combustion of fuels are carbon dioxide gas and water (hydrogen oxide) e.g. the equation for the complete combustion of pentane is:

sulfur burning to produce the acidic gas sulfur dioxide
pentane(l) + oxygen(g) carbon dioxide(g) + hydrogen oxide(g)
C5H12(l) + 8O2(g) 5CO2(g) + 6H2O(g)

However fossil fuels such as coal, oil and gas not only contain hydrocarbons but there are many impurities present as well. One of the main impurities is sulfur. Sulfur causes lots of problems when it is burned. Being a non-metal when it burns it obviously forms a non-metal oxide, sulfur dioxide. Non-metal oxides are acidic. This means that they will dissolve in water to form acids, e.g.

sulfur(s) + oxygen(g) sulfur dioxide(g)
S + O2(g) SO2(g)
The sulfur dioxide gas produced can dissolve in water, for example rain water to form an acid:
sulfur dioxide(g) + water(l) sulfurous acid(aq)
When factories and other industries burn fuels contaminated with sulfur the toxic, choking gas sulfur dioxide will be released into the atmosphere, this will then dissolve in the rain water and form a dilute solution of sulfurous acid which then undergoes futher oxidation to form sulfuric acid. Sulfuric acid is one of the main components of acid rain.

The products of combustion

A diagram similar to the one below was used in the section on complete and incomplete combustion of fuels. The set-up is slightly modified from the previous one in that a third test-tube has been added. This test-tube contains universal indicator. So when fuels (hydrocarbons) burn they will produce hydrogen oxide (water) which will condense in the first test-tube. Carbon dioxide gas is also produced and this can be detected using the limewater solution in test-tube 2. If the hydrocarbon is contaminated with sulfur then when it burns the acidic gas sulfur dioxide will be produced, this will turn the universal indicator red, as shown in test-tube 3

The products of combustion of a fuel

The problems with acid rain

Acid rain will kill aquatic life in ponds, streams, lakes and rivers. The plants and animals are unable to survive in water when the pH begins to drop and the water becomes more acidic. Acid rain will also erode pipe work and stonework on buildings causing long term damage. Many historical building e.g. churches have been very badly affected by erosion caused by acid rain. Acid rain will also damage many natural ecosystems and reduce soil fertility by leaching essentail minerals from the soil.

acid rain

Erosion caused by acid rain

statue eroded by acid rainLimestone, marble and sandstone are common rocks used in building and construction. Limestone and marble are soft and attractive rocks which are easily carved by stonemasons, so they are often used in statues, monuments and gravestones. However over many years acid rain can attack these soft rocks and slowly erode them. The writing on gravestones can become unreadable due to acid rain erosion, statues can become featureless blobs and even stone walls can be eroded by acid rain.
Unfortunately acid rain also effects metals. Metals are also very common materials used in the building and construction industries. Acid rain will corrode metals e.g. bronze, copper, zinc and mild steels are common metals/alloys used in the UK however these metals/alloys are all highly susceptible to attack by acid rain

More acid rain

air buring inside an engineThe air is a mixture mainly of nitrogen (80%) and oxygen (20%). Nitrogen is a very unreactive gas, however inside a car or motorcycle engine the high temperatures and the high voltages produced by the spark plugs which is designed to burn the fuel can also cause the air itself to burn. The nitrogen and oxygen gases in the air can react to form various oxides of nitrogen, often called NOx.
These oxides of nitrogen are a serious hazard to health and also a contributor to acid rain. One of these oxides of nitrogen is nitrogen dioxide, which is a very acidic gas and will dissolve in water to form nitric acid e.g.

nitrogen(g) + oxygen(g) nitrogen dioxide(g)
The nitrogen dioxide gas produced can dissolve in water, for example rain water to form an acid:
nitrogen dioxide(g) + water(l) nitric acid(aq)

More combustion problems

When hydrocarbon fuels burn they do not always burn efficiently inside the engine, often incomplete combustion occurs and the products of this are a mixture of carbon monoxide gas, unburnt fuel and tiny particles of carbon (soot), often called particulates. Some people are worried that these tiny particulate particles can damage your lungs and may even be linked to some types of cancer. These tiny particulate particles are particularly worrying for a number of reason, not only do they affect the health of people adversely but in the upper atmosphere these particulate particles actually reflect away light from the Sun back into space, this means that less light is actually reaching the surface, scientists estimate that between 5-20% of the light from the Sun is reflected back into space, this means that the days today are dimmer than they were say 50 years ago - this phenomenon is called global dimming.

Finally the other product of incomplete combustion, carbon monoxide is an odourless, colourless and tasteless gas that is very toxic and also difficult to detect. If you are unfortunate enough to breath in carbon monoxide gas then it binds strongly to the haemoglobin in your blood and prevents it from carrying vital oxygen around your body. This can cause serious damage to the cells and tissues in your body and if it is inhaled in large enough volumes it can cause death. Every year in the UK around 60 people die from carbon monoxide poisoning, many of them from faulty gas fires and boilers.

Key Points

Practice questions

Check your understanding - Questions on Atmospheric pollution