Maintaining Air Quality

Air may look empty, but every breath contains an enormous number of gas particles. Most of these gases are harmless or essential to life. The problem begins when combustion, industry, transport and other activities add unwanted substances to the atmosphere.

Maintaining air quality is therefore not simply about keeping the air looking clean. Some of the most dangerous pollutants are colourless and invisible. To understand how air pollution can be controlled, we first need to know what clean air normally contains, where pollutants come from and what chemical reactions they undergo.

What Is Air Made Of?

Air is a mixture, not a compound. Its gases are physically mixed together and are not chemically bonded in a fixed ratio.

The composition normally quoted in Chemistry refers to dry air. This means that water vapour has been excluded because the amount of water vapour changes with humidity, temperature and weather conditions.

Why does the composition add up to approximately 100%?

Nitrogen and oxygen already account for about 99% of dry air. The remaining approximately 1% consists mainly of argon, together with very small quantities of carbon dioxide and other gases.

The percentages are approximate because atmospheric composition is not perfectly identical everywhere. For example, carbon dioxide levels may be slightly higher near busy roads, factories or large fires.

What Makes a Substance an Air Pollutant?

An air pollutant is a substance present in the atmosphere that can cause harm to health or the environment. The O-Level syllabus gives examples including carbon monoxide, methane, nitrogen oxides, ozone, sulfur dioxide and unburnt hydrocarbons.

Carbon dioxide is discussed mainly through the carbon cycle and the greenhouse effect. It occurs naturally and is needed for photosynthesis, but increasing its atmospheric concentration strengthens the greenhouse effect and may contribute to global warming. For examination revision, it is useful to keep this role separate from pollutants such as CO, NOₓ and SO₂, whose specific harmful effects are discussed elsewhere in this topic.

The harm caused by atmospheric pollution may affect:

• human health;
• plants and animals;
• buildings and monuments;
• visibility and local air quality;
• the Earth’s climate.

It is important to consider both the source of a pollutant and its effect. An examination question may describe an activity and ask you to identify the gas released, or name a pollutant and ask what damage it causes.

The Main Air Pollutants

The substances in the table below are the examples named in the O-Level Chemistry syllabus. Notice that particulate matter is not ozone: particulate matter consists of tiny solid particles or liquid droplets, whereas ozone, O₃, is a gas.

Carbon monoxide: the product of incomplete combustion

When a carbon-containing fuel burns in a plentiful supply of oxygen, complete combustion occurs. Carbon dioxide and water are formed.

When the oxygen supply is limited, the fuel may not burn completely. Carbon monoxide and soot may then be produced.

Carbon monoxide is especially dangerous because it is colourless and odourless. A person may inhale it without realising that it is present.

Carbon monoxide combines strongly with haemoglobin in red blood cells. This prevents haemoglobin from carrying enough oxygen around the body. Exposure can cause headaches, unconsciousness and, at sufficiently high concentrations, death.

Sulfur dioxide from sulfur-containing fuels

Coal and petroleum are fossil fuels. Some deposits contain sulfur compounds as impurities. When the fuel is burned, the sulfur is oxidised to sulfur dioxide.

Sulfur dioxide irritates the respiratory system. It is particularly harmful to people with breathing difficulties such as asthma.

It can also react further in the atmosphere and eventually form sulfuric acid. This makes sulfur dioxide one of the gases responsible for acid rain.

Nitrogen oxides from high-temperature engines

Nitrogen and oxygen are both present in air. Under ordinary conditions, they do not react easily because a large amount of energy is needed to begin the reaction.

Inside a vehicle engine, however, temperatures become extremely high. This provides enough energy for nitrogen and oxygen molecules to react.

Nitrogen monoxide can then react with more oxygen to form nitrogen dioxide.

The different oxides of nitrogen are commonly represented together as NOₓ. They irritate the respiratory system and can contribute to acid rain.

Methane: a pollutant and a greenhouse gas

Methane, CH₄, is released when organic matter decomposes in places where little or no oxygen is present. Important sources include landfill sites, waterlogged rice fields and the digestive systems of livestock.

Methane is not treated here as a pollutant because it directly irritates the lungs. Its main environmental concern in this topic is that it is a greenhouse gas. An increase in atmospheric methane strengthens the greenhouse effect and may contribute to global warming.

Ground-level ozone and photochemical smog

Ground-level ozone, O₃, is not normally emitted directly by vehicle engines or factories. Instead, it forms later in the atmosphere from other pollutants. A pollutant formed in this way is called a secondary pollutant.

Vehicle engines can release nitrogen oxides, NOₓ, and unburnt hydrocarbons. In strong sunlight, these pollutants take part in a series of photochemical reactions in the air. One important product is ground-level ozone.

Where does photochemical smog fit in?

Photochemical smog is the polluted mixture that can develop when sunlight acts on pollutants such as nitrogen oxides and unburnt hydrocarbons. Ground-level ozone is an important harmful component of this smog.

This means that ozone and photochemical smog are connected, but they are not the same thing. Ozone, O₃, is one chemical substance. Photochemical smog is a mixture of pollutants formed and transformed by sunlight-driven photochemical reactions.

For O-Level revision, treat photochemical smog as useful context for understanding ground-level ozone, not as a separate reaction equation to memorise. The key relationship is the link between NOₓ, unburnt hydrocarbons, sunlight and ground-level O₃.

How can an ozone molecule form?

The simplified steps below are useful for understanding where O₃ comes from. They are not a complete mechanism for photochemical smog and should not distract you from the overall O-Level relationship above.

Sunlight can supply enough energy to split nitrogen dioxide:

The oxygen atom released can then combine with an oxygen molecule already present in air:

These equations help explain how ozone can form, but the full photochemical mechanism contains several linked reactions. Unburnt hydrocarbons take part in those reaction sequences and help conditions develop in which ozone can accumulate in sunny, polluted air.

Ground-level ozone can irritate the eyes and respiratory system and may damage plants. This harmful ground-level ozone must not be confused with ozone high in the stratosphere, which protects life by absorbing harmful ultraviolet radiation.

Unburnt hydrocarbons from incomplete combustion

Hydrocarbon fuels should ideally undergo complete combustion to form carbon dioxide and water. Inside a vehicle engine, however, some fuel molecules may leave without being completely oxidised. These are described as unburnt hydrocarbons.

Unburnt hydrocarbons are primary pollutants because they are released directly into the air. Ground-level ozone is a secondary pollutant because it forms later in the atmosphere.

In sunlight, unburnt hydrocarbons and nitrogen oxides take part in atmospheric reaction sequences that can lead to the formation and accumulation of ground-level ozone. This is why vehicle emissions can contribute to photochemical smog even though ozone itself is not normally emitted directly from the exhaust.

Extension: particulate matter and soot

Particulate matter is not another name for ozone. It consists of tiny solid particles or liquid droplets suspended in the air. Soot is a form of particulate matter produced when carbon-containing fuels burn incompletely.

Very small particles can travel deep into the lungs and may worsen respiratory and cardiovascular problems. Particulates can also scatter and absorb light, reducing visibility and contributing to haze.

Particulate matter is an important real-world air pollutant, but it is not one of the named examples students are specifically required to recall in the 6092 syllabus.

How Acid Rain Forms

Normal rainwater is slightly acidic because carbon dioxide dissolves in it. However, this is not what is usually meant by acid rain.

Acid rain is significantly more acidic than normal rain because sulfur dioxide and nitrogen oxides undergo reactions in the atmosphere that produce acids.

Formation of sulfuric acid

Sulfur dioxide can be oxidised to sulfur trioxide, which then reacts with water.

Formation of nitric acid

Nitrogen dioxide reacts with oxygen and water in the atmosphere to form nitric acid.

Students do not always need to reproduce every atmospheric equation. The central idea is that SO₂ and NOₓ form acidic substances that dissolve in rainwater.

Effects of acid rain

Once acidic rainwater reaches the ground, it can affect both living and non-living parts of the environment.

• It lowers the pH of lakes and rivers, making conditions unsuitable for some aquatic organisms.
• It damages leaves and may remove useful mineral ions from soil.
• It corrodes metals.
• It reacts with limestone and marble buildings.

Limestone and marble contain calcium carbonate. Like other carbonates, calcium carbonate reacts with acids to form a salt, water and carbon dioxide.

This reaction slowly wears away the surface of limestone buildings, statues and monuments.

Reducing Pollution from Vehicles

Catalytic converters

A catalytic converter is fitted into the exhaust system of many vehicles. Its surface contains catalysts that speed up reactions which convert harmful exhaust gases into less harmful products.

One important reaction removes both carbon monoxide and nitrogen monoxide:

Carbon monoxide is oxidised to carbon dioxide, while nitrogen monoxide is reduced to nitrogen.

Catalytic converters also help oxidise unburnt hydrocarbons:

A catalytic converter does not make vehicle exhaust completely harmless. It reduces emissions of pollutants such as CO, NOₓ and unburnt hydrocarbons, but it does not mean that every pollutant molecule is removed. Any NOₓ and unburnt hydrocarbons that still reach the atmosphere can contribute to the sunlight-driven photochemical reactions that produce ground-level ozone and photochemical smog.

Carbon dioxide is also still produced when carbon-containing fuels are burned. Catalytic converters are therefore important for controlling several harmful exhaust pollutants, but they do not remove every environmental impact of fossil-fuel use.

Reducing Sulfur Dioxide from Power Stations

Using fuels with less sulfur

Sulfur dioxide emissions can be reduced by removing sulfur compounds from fuels before combustion or by choosing fuels that naturally contain less sulfur.

This works by addressing the source of the problem: less sulfur in the fuel means less sulfur dioxide can form when the fuel burns.

Flue gas desulfurisation

Power stations that burn coal release large volumes of hot waste gases called flue gases. These gases can be treated before they leave the chimney.

In flue gas desulfurisation, powdered limestone or a limestone suspension is brought into contact with sulfur dioxide. Limestone contains calcium carbonate, CaCO₃, which reacts with the acidic oxide.

Calcium sulfite can then be oxidised to calcium sulfate.

Hydrated calcium sulfate is commonly known as gypsum and may be used in building materials.

The Ozone Layer and Ultraviolet Radiation

Ozone, O₃, is a form of oxygen containing three oxygen atoms per molecule. Most atmospheric ozone is found high above the Earth in the stratosphere.

The ozone layer absorbs much of the Sun’s harmful ultraviolet radiation. Excessive ultraviolet exposure can increase the risk of skin cancer, damage eyes and harm living organisms.

How chlorofluorocarbons damage ozone

Chlorofluorocarbons, or CFCs, were once widely used in refrigerants, aerosol propellants and foam production. They are stable in the lower atmosphere, so they can remain there long enough to reach the stratosphere.

High-energy ultraviolet radiation breaks CFC molecules apart and releases chlorine atoms. These chlorine atoms react with ozone and convert it into ordinary oxygen.

The chlorine atom is regenerated during the reaction sequence, so one chlorine atom can help destroy many ozone molecules. This is why even a relatively small quantity of CFCs can cause significant ozone depletion.

The Carbon Cycle

Carbon atoms are continually transferred between the atmosphere, living organisms, oceans, soil and rocks. This movement is called the carbon cycle.

The carbon cycle does not mean that the amount of atmospheric carbon dioxide can never change. It means that natural processes both add and remove carbon dioxide.

Photosynthesis removes carbon dioxide

Green plants absorb carbon dioxide and water. Using light energy, they produce glucose and oxygen.

Some of the carbon becomes part of carbohydrates, proteins, fats and other substances in the plant. Carbon can then pass through food chains when animals eat plants or other animals.

Respiration returns carbon dioxide

Plants, animals and microorganisms respire. During aerobic respiration, glucose reacts with oxygen and releases energy.

The carbon dioxide produced returns to the atmosphere.

Decomposition releases carbon compounds

When organisms die, decomposers break down their remains. Respiration by decomposers releases carbon dioxide.

In oxygen-poor environments, decomposition may also produce methane. This occurs in places such as waterlogged soils, rice fields and landfill sites.

Fossilisation stores carbon for very long periods

Most dead organisms are decomposed, but some organic material can become buried and, over geological timescales, contribute to the formation of fossil fuels such as coal, petroleum and natural gas. This transfers carbon into a long-term store.

When these fossil fuels are later burned, the stored carbon is returned to the atmosphere mainly as carbon dioxide.

Combustion releases stored carbon

Wood, natural gas, petroleum and coal contain carbon. When they undergo complete combustion, carbon dioxide is released.

Burning fossil fuels is particularly important because it transfers carbon that has been stored underground for millions of years into the atmosphere within a relatively short time.

The Greenhouse Effect and Global Warming

Energy from the Sun reaches the Earth mainly as short-wavelength radiation. The Earth’s surface absorbs some of this energy and becomes warmer. It then emits energy as infrared radiation.

Greenhouse gases absorb some of this outgoing infrared radiation and re-emit it in different directions. Some energy is returned towards the Earth’s surface, keeping the lower atmosphere warmer than it would otherwise be.

This natural greenhouse effect is essential. Without it, the Earth would be too cold for most present forms of life.

What causes the enhanced greenhouse effect?

Human activities have increased the atmospheric concentrations of greenhouse gases, including carbon dioxide and methane.

• Burning fossil fuels releases additional carbon dioxide.
• Deforestation reduces the number of trees available to remove carbon dioxide by photosynthesis.
• Landfill sites and some agricultural activities release methane.

Higher greenhouse-gas concentrations cause more outgoing infrared radiation to be absorbed. This strengthens the greenhouse effect and raises the Earth’s average temperature. The long-term increase in average global temperature is called global warming.

Possible consequences of global warming

• Melting of glaciers and polar ice.
• Thermal expansion of seawater and rising sea levels.
• Changes in rainfall patterns.
• More frequent or severe extreme-weather events in some regions.
• Disruption of ecosystems and food production.

Environmental Problems That Must Not Be Confused

Common Mistakes and Why They Are Wrong

Final Revision Checklist