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Air pollution

Air composition

The atmosphere is the mixture of gases surrounding Earth's surface.

Air is the part of the atmosphere that living things interact with (through breathing and photosynthesis).

Air is mostly a mixture of nitrogen ($\ce{N2}$) and oxygen ($\ce{O2}$) gases.

Dry air is mainly composed of nitrogen and oxygen. The remaining gases are mainly argon and carbon dioxide.

Carbon monoxide

Carbon monoxide ($\ce{CO}$) is a colourless and odourless gas that is highly toxic.

The incomplete combustion of fuel releases carbon monoxide into the air.

In incomplete combustion, there is not enough oxygen gas available to produce carbon dioxide ($\ce{CO2}$). Carbon monoxide is produced instead.

 Complete combustion of ethanol $\ce{C2H5OH + 3O2 -> 2CO2 + 3H2O}$ Incomplete combustion of ethanol $\ce{C2H5OH + 2O2 -> 2CO + 3H2O}$

Carbon monoxide is poisonous because it obstructs the transport of oxygen in blood. It bonds strongly to haemoglobin, the molecule that carries oxygen in the blood.

Haemoglobin cannot carry oxygen to the body's cells when carbon monoxide bonds to it. Cells and even the organism as a whole will die if not enough oxygen is delivered.

Nitrogen oxides

A sample of nitrogen dioxide in a glass tube.

Nitric oxide ($\ce{NO}$) and nitrogen dioxide ($\ce{NO2}$) are pollutants known as nitrogen oxides.

These two gases are described by the general formula $\ce{NO}{_x}$.

$\ce{NO}{_x}$ gases are produced in internal combustion engines (such as cars). In these engines, fuel reacts with oxygen in the air to release energy.

The high temperatures in the engines cause nitrogen ($\ce{N2}$) and oxygen to react, forming $\ce{NO}{_x}$ gases.

$\ce{NO}{_x}$ gases are also formed during lightning. Lightning provides the energy necessary for nitrogen to react with oxygen to form $\ce{NO}{_x}$ gases.

$\ce{NO}{_x}$ gases react with water ($\ce{H2O}$) in air to form nitric acid ($\ce{HNO3}$).

Nitric acid contributes to acid rain, which kills plants, corrodes metal and can damage limestone and marble structures.

Directly breathing in $\ce{NO}{_x}$ gases can cause breathing problems because the gases damage lung tissue.

Catalytic converters

A catalytic converter is part of the exhaust system of most vehicles. It reduces the amount of carbon monoxide and nitrogen oxide pollutants from the exhaust fumes.

Catalytic converters contain solid catalysts that speed up the oxidation of $\ce{CO}$ to $\ce{CO2}$ and the reduction of $\ce{NO}{_x}$ to $\ce{N2}$.

As a result, a car exhausts more $\ce{CO2}$ and $\ce{N2}$ and very little $\ce{CO}$ and $\ce{NO}{_x}$.

These reactions are redox reactions because they involve changes in oxidation states (carbon is oxidised and nitrogen is reduced).

Transition metals such as platinum and rhodium are used as catalysts in catalytic converters.

One of the catalysts used in a metal-core catalytic converter is platinum.

Different catalysts are needed to speed up the oxidation and reduction reactions. Therefore, multiple metal catalysts are present in the exhaust.

Sulphur dioxide

Sulfur dioxide ($\ce{SO2}$) is an air pollutant that contributes to acid rain.

Sulfur dioxide is produced from the combustion of sulfur or of sulfur compounds contained as impurities in fossil fuels. Fossil fuels are commonly burned for energy in industries.

Flue gas desulfurisation was introduced to remove $\ce{SO2}$ from the waste gases released by industries. As a result, $\ce{SO2}$ emissions from industrial processes have been greatly reduced.

The largest source of $\ce{SO2}$ emissions is the eruption of volcanoes. During an eruption, sulfur compounds react with oxygen to release millions of tonnes of $\ce{SO2}$.

In air, $\ce{SO2}$ can be oxidised to $\ce{SO3}$. $\ce{SO3}$ readily combines with water to form sulfuric acid ($\ce{H2SO4}$) in air.

Along with nitric acid, sulfuric acid is a major component of acid rain. It also irritates the lungs if inhaled.

Effects of acid rain on soil pH

Acid rain severely damages forests.

A major consequence of acid rain is the lowering of soil pH. This in turn results in the destruction of forests and other vegetation.

This happens because plants are sensitive to changes in soil pH. Soil pH is a measure of acidity or basicity of the soil. Soil that is either too acidic or too basic is unsuitable for plant growth.

Acidic soil can be neutralised using calcium hydroxide ($\ce{Ca(OH)2}$). Calcium hydroxide increases the pH because of the released hydroxide ions that react with $\ce{H^+}$ ions.

The amount of calcium hydroxide added has to be controlled as an excess raises the soil pH, making it more basic than desired.

Calcium oxide ($\ce{CaO}$) or calcium carbonate ($\ce{CaCO3}$) can also be added to neutralise the $\ce{H^+}$ ions in the soil.

Calcium carbonate

A dry sample of calcium carbonate.

Calcium carbonate ($\ce{CaCO3}$) is used to reduce the effects of acid rain and pollutants.

Acid rain can lower the pH of rivers and streams.

Adding calcium carbonate to these environments neutralises the acid from acid rain, producing salts, water and carbon dioxide.

The pH of the water environment reverts to normal levels as the acid is neutralised.

Calcium carbonate is used in flue gas desulfurisation. This process removes $\ce{SO2}$ from the gas mixture (the flue gas) released from fossil fuel combustion plants.

Calcium carbonate reacts with $\ce{SO2}$ to form calcium sulfite ($\ce{CaSO3}$) and carbon dioxide ($\ce{CO2}$).

$$\ce{CaCO3 {(s)} + SO2 {(g)} -> CaSO3 {(s)} + CO2 {(g)}}$$

This removes sulfur dioxide from the emissions.