Composition of clean, dry air

Clean, dry air is a mixture of gases. It contains approximately 78% nitrogen, $$\ce{N2}$$, and 21% oxygen, $$\ce{O2}$$, by volume.

The remaining approximately 1% contains noble gases and carbon dioxide, $$\ce{CO2}$$. Nitrogen is therefore the largest component of air, while oxygen is the second largest.

Dry means that water vapour has been excluded; clean means that pollutants have been excluded.

Greenhouse gases and climate change

Carbon dioxide and methane are greenhouse gases. They absorb some of the infrared radiation that the Earth emits and re-emit it in different directions. This reduces the rate at which energy escapes into space, warming the Earth's surface and lower atmosphere.

Sources of carbon dioxide and methane

When a carbon-containing fuel burns completely in sufficient oxygen, its carbon forms carbon dioxide, $$\ce{CO2}$$, which enters the air.

Methane, $$\ce{CH4}$$, enters the air when vegetation decomposes. It is also released in waste gases produced during digestion in animals.

From greenhouse gases to climate change

Higher levels of either carbon dioxide or methane increase the greenhouse effect: less energy escapes into space, so the Earth warms. This rise in the Earth's average temperature is called global warming.

Global warming leads to climate change: long-term changes in climate patterns, including temperature and rainfall.

Reducing climate change

Climate change can be limited by releasing less carbon dioxide and methane into the atmosphere, or by removing more carbon dioxide from it.

Reducing greenhouse gas emissions

Burning fossil fuels releases carbon dioxide. Using less coal, oil and natural gas therefore reduces emissions from their combustion. Electricity generated from renewable sources such as wind and solar power can replace some electricity generated by burning fossil fuels.

Hydrogen can also be used as a fuel in place of carbon-containing fossil fuels. When hydrogen burns, it produces water rather than carbon dioxide. However, producing hydrogen may release carbon dioxide, depending on how the hydrogen is made.

Livestock, particularly cattle, release methane during digestion. Reducing livestock farming can therefore reduce methane emissions associated with these animals.

Removing carbon dioxide with trees

As trees grow, they take in carbon dioxide from the air for photosynthesis and store carbon in their biomass. Planting more trees can therefore increase the uptake of carbon dioxide from the atmosphere.

Nitrogen oxide pollution from car engines

Inside a car engine, high temperatures cause some nitrogen and oxygen from the air to react. The resulting oxides of nitrogen leave the engine in the exhaust gases and can pollute the air.

Effects of nitrogen oxide pollution

Oxides of nitrogen react in the atmosphere to form acids. These acids can dissolve in rainwater, contributing to acid rain. The pollutants also contribute to photochemical smog: a polluted haze formed when substances in the air react in sunlight. Breathing air containing oxides of nitrogen can irritate the respiratory system and cause breathing problems.

Reducing emissions with catalytic converters

A catalytic converter fitted to a vehicle treats exhaust gases before they are released. It helps convert oxides of nitrogen into nitrogen, reducing the amount of nitrogen oxide pollution entering the air. This also reduces the vehicle’s contribution to acid rain.

Sulfur dioxide pollution from fossil fuels

Some fossil fuels contain sulfur compounds. When the fuel burns, sulfur is oxidised to form sulfur dioxide, $$\ce{SO2}$$, which can escape into the atmosphere with the combustion gases.

Sulfur dioxide contributes to acid rain. In the atmosphere, it dissolves and reacts to form acids, which can fall to the ground in rain.

Reducing sulfur dioxide emissions

Using low-sulfur fuels reduces the amount of sulfur available to form sulfur dioxide during combustion. Less sulfur dioxide is therefore produced.

Sulfur dioxide can also be removed after combustion. In flue gas desulfurisation, combustion gases are treated with calcium oxide, $$\ce{CaO}$$, before they are released into the air. The calcium oxide reacts with sulfur dioxide, forming calcium compounds that can be removed from the gases.

Photosynthesis

Photosynthesis is a process in which light energy is used to make glucose, a sugar. Chlorophyll absorbs light, providing the energy for carbon dioxide to react with water. The products are glucose and oxygen.

The word equation for photosynthesis is: