Dry air contains:
-The remaining 1% is mainly Argon and very small amounts of other gases
The Earth's atmosphere
The earth's atmosphere was probably formed about 4 billion years ago by gases given out by volcanoes.
Volcanoes realease huge amounts of carbon dioxide and water vapour. They also realease lava and dust.
4 billion years ago, the earth's atmosphere was very hot. As the earth cools, oceans formed from the condensed water. About 3 billion years ago, simple bacteria-like creatures evolved to use photosynthesis. This removed carbon dioxide from the air and released oxygen, allowing animals to evolve. Carbon dioxide was also removed by plants and animals dying and becoming buried. over millions of years, some of the buried material has become fossil fuels which are now burnt for energy.
How has human activity changed air quality?
Humans are changing the gases in the atmosphere by burning fossil fuels. This realeases carbon dioxide and solid particulates that float in the air eg. soot (carbon). Human activity like burning down forests increases carbon dioxide and particulate levels.
Gases known as pollutants are harmful to our health and the environment. Examples are carbon monoxide, nitrogen monoxide, nitogen dioxide, sulphur dioxide.
In the last 50 years, the amount of carbon dioxide in the air has increased by 25%.
Sulphur dioxide and nitrogen dioxide are pollutants that make acid rain which is harmful to plants and animals.
Small amounts of carbon dioxide in the air is measured in ppm (parts per million). 1ppm means that there is 1 gram of a pollutant in 1 million grams of air. The amount of pollutant gases are measured in air quality monitoring stations across the UK. The data is transmitted automatically to a central computer for analysis.
What happens when fuels burn?
Oxygen is needed for any fuel to burn and release energy. (Pure oxygen can make fuels burn more quickly and at higher temperatures)
Fossil fuels are mainly hydrocarbons such as petrol, diesel and fuel oil.
Coal is a fossil fuel made of carbon atoms.
Hydrocarbon fuel + Oxygen --> Carbon dioxide + Water (+energy)
Oxidation is when oxygen is added to a substance. eg combustion
Reduction is when oxygen is removed form a substance.
Solid yellow suphur burns and makes a colourless gas called sulphur dioxide.
Sulphur is insoluble but sulphur dioxide dissolves in water to form an acid solution.
Fossil fuels contain small amounts of sulphur from the plants and animals that formed them.
When fossil fuels burn, the sulphur burns:
Sulphur + Oxygen --> Sulphur dioxide
Coal tends to contain the most sulphur so burning it will produce the most sulphur dioxide.
Sulphur dioxide causes acid rain. Acid rain lowers the pH when it falls on land or enclosed water and can harm living things or erode carbonate rock.
Acid rain does not affect humans directly so is called an indirect pollutant.
How pollutants are formed
Fossil fuels all contain impurities which when burnt can produce pollutants.Carbon dioxide is always formed when fuels burn.
If not enough air is aviliable to burn the fuel: (incomplete combustion)
-poisonous carbon monoxide is made
-bits of solid carbon (soot) called particulates are made, making surfaces dirty.
Car engines make nitrogen oxides when nitrogen and oxygen from the air react at high temperatures; this contributes to acid rain.
Nitrogen monoxide is formed in furnaces and engines at temperatures over 1000 degrees c. When nitrogen monoxide is relased into the atmosphere it cools and then reacts with more oxygen in the air to form toxic nitrogen dioxide, a brown gas.
Pollutants are removed from the air when...
-Particulate carbon settles on surfaces making them dirty
-Sulphur and nirtigen oxides react with water and oxygen to produce a mixture of sulphuric acid and nitric acid in rain (acid rain)
-Carbon dioxide is used by plants for photosynthesis
-carbon dioxide dissolves in the sea and rainwater
Improving power stations
Burning oil and gas make less sulphur dioxide than burning coal. Sulphur can be removed from oil and gas before it is burnt but this is harder to do for coal.
Power staions are developing ways of reducing pollution by cleaning waste gases. They can remove solid particulates using electrostatic filters. Sulphur dioxide can be removed from waste gases by 'flue gas desulphurisation'.
Two 'wet-scrubbing' methods to remove sulphur dioxide from power staions waste gases are:
-using an alkaline slurry of calcium oxide (lime) and water to make gypsum (calcium sulphate) which can be sold as plaster.
-using seawater, a natural alkaline which absorbs sulphur dioxide.
Reducing carbon dioxide and replacing fossil fuels
Ways to reduce our use of fossil fuels include...
-Using alternative energy sources: one alternative to fossil fuels is using biofuels, which are made from plants. examples are woodchips, palm oil and alcohol made from sugar. Biofules are carbon neutral.
-Improving building insultaion
Modern vehicles use more efficient engines which require less fuel. Catalytic converters contain a platinum catalyst that allows pollutant gases to react with each other. Low sulphur fuels are needed as sulphur damages the catalyst. Using low sulphur fuels also reduces sulphur dioxide emisssions.
Inside a catalytic converter, this reactin takes place: Carbon monoxide + Nitrogen monoxide--> Nitrogen + Carbon dioxide