Composition of clean, dry air

1. Air is a mixture of gases, because the gases in it are not chemically combined with one another.
2. Its composition is given for clean, dry air, because the amount of water vapour and the amount of pollution in air vary from place to place.
3. Clean, dry air contains approximately:
   1. $78\%$ nitrogen, ${\mathrm{N}}_2$.
   2. $21\%$ oxygen, ${\mathrm{O}}_2$.
   3. A remainder of about $1\%$, which is a mixture of noble gases and carbon dioxide, $\mathrm{C}{\mathrm{O}}_2$.
4. Most of the remaining $1\%$ is the noble gas argon, while carbon dioxide makes up only about $0.04\%$ of the air.
5. Nitrogen is the main gas in air, so air is not mostly oxygen.

Sources of air pollutants

1. Most air pollutants come from burning carbon-containing fuels, such as coal, natural gas, petrol and diesel.
2. Carbon dioxide comes from the complete combustion of carbon-containing fuels.
   1. With a plentiful supply of oxygen, all of the carbon in the fuel is converted into carbon dioxide, for example $\mathrm{C}{\mathrm{H}}_4+2{\mathrm{O}}_2\to \mathrm{C}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}$.
3. Carbon monoxide and particulates come from the incomplete combustion of carbon-containing fuels.
   1. In a limited supply of oxygen, some of the carbon forms carbon monoxide instead of carbon dioxide, for example $2C{\mathrm{H}}_4+3{\mathrm{O}}_2\to 2CO+4{\mathrm{H}}_2\mathrm{O}$.
   2. With even less oxygen, some of the carbon is released as tiny solid particles of soot, for example $\mathrm{C}{\mathrm{H}}_4+{\mathrm{O}}_2\to \mathrm{C}+2{\mathrm{H}}_2\mathrm{O}$.
   3. Incomplete combustion happens in vehicle engines and in faulty gas boilers and heaters, where the fuel burns without enough air.
4. Methane comes from the decomposition of vegetation and from waste gases from digestion in animals.
   1. Rotting plant material in marshes, rice fields and landfill sites releases methane.
   2. Cattle and sheep release methane as a waste gas produced while they digest their food.
5. Oxides of nitrogen come from car engines.
   1. The main oxides of nitrogen are nitrogen monoxide, $\mathrm{NO}$, and nitrogen dioxide, $\mathrm{N}{\mathrm{O}}_2$, which are often written together as $\mathrm{N}{\mathrm{O}}_{\mathrm{x}}$.
6. Sulfur dioxide comes from the combustion of fossil fuels which contain sulfur compounds.
   1. Coal and some oils contain sulfur compounds as impurities, and the sulfur burns to form sulfur dioxide, $\mathrm{S}+{\mathrm{O}}_2\to \mathrm{S}{\mathrm{O}}_2$.
   2. Power stations that burn coal are a major source of sulfur dioxide.

Effects of air pollutants

1. Carbon dioxide
   1. Higher levels of carbon dioxide in the atmosphere lead to increased global warming, which leads to climate change.
   2. Climate change includes more frequent extreme weather, such as droughts, floods and storms, together with melting ice and rising sea levels.
2. Carbon monoxide
   1. Carbon monoxide is a toxic gas.
   2. It combines with the haemoglobin in red blood cells, so the blood carries less oxygen around the body.
   3. It has no colour and no smell, so a person can breathe it in without noticing.
3. Particulates
   1. Particulates are small enough to be breathed deep into the lungs.
   2. They cause an increased risk of respiratory problems, such as asthma, and an increased risk of cancer.
4. Methane
   1. Higher levels of methane in the atmosphere lead to increased global warming, which leads to climate change.
5. Oxides of nitrogen
   1. Oxides of nitrogen cause acid rain, because they dissolve in the water in clouds to form acidic solutions.
   2. They cause photochemical smog, a brown haze that hangs over busy cities on sunny days.
   3. They cause respiratory problems, because they irritate the lining of the lungs and airways.
6. Sulfur dioxide
   1. Sulfur dioxide causes acid rain, because it dissolves in the water in clouds to form acidic solutions.