Chemistry of the air

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  • Created by: Buffya
  • Created on: 29-05-13 21:06

The greenhouse effect

The greenhouse effect is the process in which the absorption and subsequent emission of infrared radiation by greenhouse gases warms the lower atmosphere and the planet's surface.

Carbon dioxide can be produced by volcanic eruptions, respiration of animals and burning of organic matter. Methane can be produced by rotting organic waste, the production of fossil fuels and from certain animals. There are large quantities of methane trapped in ice-like structures below the sea called clathrates.

The bonds in carbon dioxide, water and methane are able to absorb infrared radiation causing the molecules to vibrate. They will then emit some of this energy. 

The ability of a trace gas to cause global warming is called it's Global Warming Potential (GWP). GWP is related to the lifetime of the gas, the concentration and it's ability to absorb IR.

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Climate change

The combustion of fossil fuels is releasing billions of tonnes of carbon dioxide into our atmosphere which is increasing the rate of climate change. The effects can be seen through rising sea levels, disastrous harvests, storms and floods.

In 1997 the Kyoto Protocol was introduced which meant that committed countries had to reduce their emissions by 5%. The USA which is responsible for 25% of carbon dioxide emissions refuse to sign. The emissions must be reduced by 70-80% to stabilise atmospheric carbon dioxide concentrations.

The EU agreed a long term strategy where 20% of energy must come from renewable resources, 10% of transport fuel must come from biofuel and carbon dioxide emissions must be reduced by 20% by 2020.

Cars must also emit less carbon dioxide.

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Solutions to the greenhouse effect

The familiar solution is alternative fuels such as wind turbines, tidal power, solar power and nuclear plants.

Another solution is carbon capture and storage (CCS) or sequestration. This can be the removal of waste carbon dioxide underground where porous rock can act as a sponge to store carbon dioxide.

Another project by BP is decarbonised fuels. The carbon dioxide will be separated and piped offshore to an oilfield nearing the end of its productive life. This will enable the rest of the oil to be extracted and hydrogen to be burn as a clean fuel.

Mineral storage aims to convert carbon dioxide into a carbonate rock by reacting with metal oxides. However, this natural reaction is very slow.

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Ozone layer

Ozone is a molecule consisting of three oxygen atoms. 

The ozone layer is found in the stratosphere which is the second layer of the Earth's atmosphere and about 10-50km above the surface. 

UV-a is not as damaging as it has less energy and only 5% is absorbed by the ozone layer. UV-b has shorter wavelengths of 280-320nm and can cause skin cancer - 95% is filtered out. All of UV-c is absorbed by the ozone layer.

Ozone is continuously being formed and broken down in the stratosphere by UV with wavelengths of less than 240nm. The high energy causes an oxygen molecule to split into two atoms. The atom then reacts with a molecule forming O3 and heat. 

The ozone molecules then absorb UV radiation less than 310nm. The atomic oxygen is again released. This immediately reacts with other oxygen molecules, reforming the ozone molecule. The overall effect is to turn UV radiation into heat.

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Ozone depletion

The ozone depletion potential of a substance is the relative amount of breakdown to the ozone layer caused by the substance, compared to that for trichlorofluoromethane (1).

Once CFC's reach the ozone layer, the UV radiation provides the energy needed to break them down. It breaks the C-Cl bond forming CFCL2 and a chorine radical.

Propagation: O3 + ~CL ---> CLO~ + O2

CLO~ + O ---> ~CL + O2

The reaction leaves a chlorine radical to attack another ozone molecule.

Nitrogen oxide does a similar reaction

Propagation: ~NO + O3 ----> ~NO2 + O2

~NO2 + O ---> ~NO + O2

The Montreal Protocol stopped the production of CFCs, replaced by HFCs.

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Controlling air pollution

A car emits atmospheric pollutants:

  • Carbon monoxide which is harmful to humans
  • Nitrogen dioxide which contributes to acid rain
  • Unburnt hydrocarbons which combine with NO2 to form low-level ozone

A catalytic converter is used to turn these harmful products into less harmful substances.

An oxidisation catalyst adds oxygen forming carbon dioxide and burning hydrocarbons. A three way catalyst combines NO and CO making carbon dioxide and nitrogen.

The catalyst provides a surface for the reaction to take place. The CO and NO diffuse over the surface where they held to the surface by adsorption. They are held by temporary bonds in a place that makes them react together. The CO2 and N2 produced is then desorbed from the surface.

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