Atmospheric Pollution and Emission Control

Why combustion pollution matters

Burning fossil fuels supplies energy for transport, heating and electricity generation, but it also releases pollutants into the atmosphere. Increased amounts of these substances can damage human health, ecosystems and buildings. The effects depend on which substances are present: not all pollutants cause the same problems.

Four important examples are carbon monoxide, particulates, sulfur dioxide and oxides of nitrogen. Their origins help explain how their emissions can be reduced, but first we need to distinguish their effects.

Carbon monoxide and particulates harm health

Carbon monoxide, $CO$, is a poisonous gas produced by incomplete combustion. It binds to haemoglobin, the substance in red blood cells that normally carries oxygen. This reduces the blood's ability to supply oxygen to body tissues. Severe exposure can cause unconsciousness or death. Carbon monoxide is colourless and odourless, so people cannot reliably detect it using their senses.

Particulates are tiny particles suspended in air. Incomplete combustion can produce carbon particles, or soot. Breathing polluted air containing these particles can cause respiratory problems. Soot also settles on surfaces, blackening buildings and monuments.

These are different types of harm: carbon monoxide interferes with oxygen transport, whereas inhaled particulates affect respiratory health.

Sulfur dioxide and nitrogen oxides cause acid rain

Sulfur dioxide, $SO_2$, forms when sulfur impurities in fuels burn. Oxides of nitrogen, often written as $NO_x$, form when nitrogen and oxygen react at the high temperatures found in engines and furnaces.

Both can irritate the respiratory system and worsen breathing problems such as asthma. They also enter water droplets in clouds and undergo reactions that form acids. Rain containing these acids is called acid rain.

Acid rain has several connected effects:

• It damages plants and can reduce their growth.
• It makes lakes and rivers more acidic, harming aquatic organisms and disrupting food chains.
• It reacts with carbonate building materials such as limestone, wearing away buildings and statues. It can also increase corrosion of metal structures.

Nitrogen oxides also contribute to photochemical smog. Sunlight drives reactions involving nitrogen oxides and other pollutants, including unburned hydrocarbons. The resulting mixture can irritate the eyes, throat and lungs.

Removing sulfur before the fuel burns

One approach is to prevent a pollutant from forming. Sulfur compounds can be removed from fuels during processing, producing low-sulfur petrol.

The connection is straightforward: less sulfur in the petrol means less sulfur dioxide is formed when it burns. This reduces the contribution of the exhaust gases to acid rain and respiratory irritation.

Low-sulfur petrol is not pollution-free petrol. It still burns, so it can still produce other combustion products. Removing sulfur specifically tackles sulfur dioxide emissions.

Removing sulfur dioxide from waste gases

Power stations can use gas scrubbers to treat waste gases before they leave the chimney. These waste gases are also called flue gases.

In a wet scrubber, the gases contact an alkaline mixture, such as powdered lime mixed with water. Sulfur dioxide is acidic, so it reacts with the alkaline material in a neutralisation reaction. For example, calcium hydroxide reacts with sulfur dioxide to form calcium sulfite and water.

The sulfur-containing product is collected rather than allowing the sulfur dioxide to escape into the atmosphere. Further treatment can turn calcium sulfite into calcium sulfate, which can be used to make gypsum for building products.

The key distinction is the stage at which action is taken: low-sulfur petrol reduces sulfur in the fuel, while a scrubber removes sulfur dioxide from the waste gases.

Changing pollutants in vehicle exhausts

A catalytic converter is fitted in a vehicle's exhaust system. Exhaust gases pass over catalysts, such as platinum and rhodium, which speed up reactions without being used up overall.

These reactions convert several pollutants into less harmful substances:

• Carbon monoxide is converted into carbon dioxide.
• Nitrogen oxides are converted into nitrogen.
• Unburned hydrocarbons are converted into carbon dioxide and water.

For example, carbon monoxide can react with nitrogen monoxide to produce carbon dioxide and nitrogen. This removes two harmful exhaust pollutants in the same reaction.

The catalyst is spread over a large surface area so that the gases have plenty of contact with it. Catalytic converters work best when hot. They reduce harmful emissions, but do not eliminate all environmental effects: carbon dioxide is still released and contributes to global warming.

Reducing the amount of fuel burned

Treating fuels and exhaust gases tackles particular pollutants. Another approach is to burn less fossil fuel in the first place. Using energy more efficiently, reducing fuel use in transport and replacing fossil-fuel electricity generation with renewable sources can reduce combustion-related emissions.

These approaches complement one another. Reducing fuel consumption lowers the amount of combustion, while low-sulfur fuels, scrubbers and catalytic converters reduce particular pollutants from the combustion that still takes place.

Match the pollutant to the problem

• Carbon monoxide: poisonous; binds to haemoglobin and reduces oxygen transport.
• Particulates: respiratory problems; soot blackens buildings.
• Sulfur dioxide: respiratory irritation and acid rain.
• Nitrogen oxides: respiratory problems, acid rain and photochemical smog.

Acid rain: damages plants, harms aquatic organisms and attacks limestone buildings and metal structures.