C1 Air Quality

There are already ways in which we can reduce air pollution caused by the burning of fuels, and others may be developed in the future.

Carbon Dioxide  : The fossil fuels include coal, natural gas and the products from the fractional distillation of crude oil, such as petrol and diesel. When any fossil fuel is burned, one of the combustion products is carbon dioxide.

Over the past 100 years, increasing amounts of fossil fuels have been burned each year. This has led to an increase in the percentage of carbon dioxide in the air. Many scientists believe this is causing global warming.

Reducing Carbon Dioixde in the Air : One way to reduce the amount of carbon dioxide in the air is to burn less fossil fuels. Unfortunately, we depend on the burning of fossil fuels for heating, electricity generation and transport. If we are to burn less fossil fuels, we will need alternatives for these essential activities.

Power stations that burn fossil fuels to produce electricity are one of the main contributors of carbon dioxide in the atmosphere.

One way to reduce the air pollution caused by power stations is to use less electricity. Some ways to do this are:

• turning down the heating at home
• insulating your home
• turning off lights when you leave a room
• using low-energy lamps
• using a microwave instead of an electric oven
• not leaving TV sets on standby.

Sulfur dioxide

Power stations give out sulfur dioxide, which is thought to be a cause of acid rain. The amount of sulfur dioxide given off by a power station can be reduced by:

• removing sulfur from natural gas and fuel oil
• removing sulfur dioxide from the flue gases emitted by coal-burning power stations.

Wet scrubbing

Wet scrubbing involves the removal of sulfur dioxide from flue gases before it escapes from the power station chimney. Two methods are employed:

• spraying the flue gases with seawater droplets (naturally alkaline) which react with the sulfur dioxide
• using powdered lime (calcium oxide) mixed with water to form an alkaline slurry. When mixed with air and sprayed with the slurry the sulfur dioxide in the flue gases reacts and forms solid calcium sulphate. This solid is removed allowing cleaned gases to escape.

Coal-powered stations give off solid particulates. These can also be removed from the flue gases. Waste gases pass through an electrostatic precipitator allowing particulates to be collected and removed.

Atmospheric pollution caused by the exhaust gases given out by motor vehicles can be reduced by:

• burning less fuel by having more efficient engines
• using low sulfur fuels
• using catalytic converters, which convert nitrogen monoxide to nitrogen and oxygen, and carbon monoxide to carbon dioxide
• adjusting the balance between public and private transport, with more people using buses and trains instead of cars
• setting legal limits on exhaust emissions that are enforced through Ministry of Transport (MOT) tests

When there is a chemical reaction, air pollutants are produced. You can understand how these pollutants are formed by looking at what happens to atoms during these chemical reactions.

In a chemical reaction, the substances that react together are called the reactants, while the substances that are formed are called the products.

The products have different properties from the reactants.

As a chemical reaction takes place, atoms in the reactants are rearranged to make the products. No atoms are added or taken away. This is called the conservation of atoms.

Count the number of atoms of each element in the reactants, and in the products. They are the same. As the atoms are conserved and all atoms have mass, the mass of the reactants will be the same as the mass of the products. This is known as conservation of mass.

Burning fuels releases harmful pollutants into the air. Pollutants can kill plants, harm our lungs, cause acid rain and are thought to contribute to global warming.

A combustion reaction is when a chemical reacts with oxygen and releases energy.

We burn fuels to produce energy. When these fuels burn, the atoms in the fuels combine with oxygen from the air to make new molecules.

Coal is made mainly of carbon. This burns to produce carbon dioxide.

Petrol, diesel fuel and fuel oil are hydrocarbons. Their molecules are made of carbon and hydrogen atoms. When these fuels burn, the carbon and hydrogen atoms combine with oxygen atoms to produce carbon dioxide and water vapour.

Fuels are burned in the vehicles we use for transport, and in power stations to make electricity. When fuels are burned, pollutants are formed and released into the air.

This combustion of fuels increases the concentration of carbon dioxide in the air. This is thought to cause global warming.

Properties

The products have different properties from the reactants.

Many fuels contain small amounts of sulfur compounds. When these fuels are burned sulfur dioxide is released into the air. Sulfur dioxide causes acid rain that can damage buildings and kill plants.

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Nitrogen Oxide

In the high temperatures of vehicle engines, nitrogen and oxygen react to form nitrogen oxides. These gases irritate the lungs and cause acid rain.

Incomplete combustion

If there is not enough oxygen present to burn the fuels completely, incomplete combustion takes place. Carbon monoxide, a very poisonous gas, is formed.

Incomplete combustion also releases very small particles of carbon into the air. This particulate carbon makes buildings dirty and can cause breathing difficulties

When fuels are burned in vehicle engines, high temperatures are reached. At these high temperatures nitrogen and oxygen from the air combine to produce nitrogen monoxide.

nitrogen + oxygen → nitrogen monoxide

Coal is made mainly of carbon. This burns to produce carbon dioxide.

When this nitrogen monoxide is released from vehicle exhaust systems, it combines with oxygen in the air to form nitrogen dioxide.

 nitrogen monoxide + oxygen → nitrogen dioxide

Nitrogen dioxide causes acid rain. It also causes breathing problems and can worsen the effects of asthma.

Nitrogen monoxide and nitrogen dioxide are jointly referred to as NO_x

Pollutants that enter the air do not simply stay there. They can be removed in several different ways.

Particulates 

Particulates are very small pieces of solids, mainly carbon, that are released into the air during incomplete combustion. Since they are solids, they will stick to other solid materials that they come into contact with. They coat the walls of buildings making them look dirty. 

Sulfur Dioxide

Sulfur dioxide is formed when fuels containing sulfur compounds are burned. Sulfur dioxide reacts with water and oxygen in the air to produce acid rain. This removes sulfur dioxide from the air, but the acid rain corrodes buildings and kills plants.

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Nitrogen Monoxide

Nitrogen monoxide reacts in the atmosphere to form nitrogen dioxide.

Nitrogen Dioxide

Nitrogen dioxide also reacts with water and oxygen in the air to produce acid rain.

Carbon Monoxide

Carbon monoxide is produced when fuel is burned in an insufficient supply of oxygen. It is a poisonous gas that alters the level of oxygen in the blood.

Carbon dioxide is produced by burning fossil fuels, such as like coal, oil, petrol and natural gas.

Plants remove some of the carbon dioxide from the atmosphere, because they use it in photosynthesis. Here is the word equation for photosynthesis:

carbon dioxide + water → glucose + oxygen

This process uses energy from sunlight. The energy has been absorbed into the plant by the green pigment chlorophyll.

Carbon dioxide is also removed from the atmosphere when it dissolves in both rain water and sea water. As a result rain water becomes slightly acidic, and the oceans are a huge reservoir of dissolved carbon dioxide.

Not all the carbon dioxide we produce is removed from the atmosphere. The level of CO₂ is steadily increasing, and this contributes to global warming.

A correlation is a link between two things. If an outcome happens when a factor is present, and does not happen when the factor is absent, there is a correlation. Or, if an outcome increases or decreases when a factor increases or decreases, there is a correlation. We say there is a correlation between the factor and the outcome. For example, studies show that there is a correlation between how much a person smokes and how likely they are to get lung cancer. As the number of cigarettes smoked increases, the percentage of people who smoke that number and get lung cancer also increases. But a person may smoke cigarettes and never get lung cancer. However, one individual case does not provide evidence for or against a correlation. A correlation is only valid when there is a large amount of data to support it.

The fact that there is a correlation between a factor and an outcome does not necessarily mean this factor causes the outcome.

There is a correlation between the pollen count in the air and the incidence of hay fever, for instance. The pollen count increases from spring onwards, reaching a peak in mid-summer. It is therefore possible that pollen causes hay fever.

There is also a correlation between the amount of ice cream sold during the summer and the number of hay fever cases. But nobody would suggest that eating ice cream causes hay fever.

To claim a causal link between a factor and an outcome there has to be a mechanism that explains how the factor causes the outcome. In the case of hay fever, skin tests show that people who get hay fever are allergic to pollen. This is supporting evidence that hay fever can be caused by an allergic reaction to pollen.

The atmosphere surrounding the Earth is a mixture of gases. In some places, human activities have added other gases to the atmosphere, which are called pollutants.

The Earth is surrounded by a mixture of gases called the atmosphere. The main gases in the atmosphere, and their approximate percentages in dry air, are shown below.

Gases in the Atmosphere

Nitrogen = 78%  Oxygen = 21% Argon = 1%

The atmosphere also contains a small but important amount of carbon dioxide, approximately 0.04 per cent, and tiny amounts of a few other gases.

In addition to these, air also contains water vapour. The percentage of water vapour varies from one location to another, and according to weather conditions. This is why the percentages of the other gases are usually quoted for dry air.

Human activities produce a number of gases that are released into the atmosphere. Most of these pollutant gases are produced by the burning of fuels.

The burning of fuels releases a large amount of carbon dioxide into the atmosphere. This is thought to cause global warming.

Other pollutant gases that are released when fuels are burned include carbon monoxide, nitrogen oxides and sulfur dioxide. The incomplete combustion of fuels also releases small particles of solids, such as carbon, into the air. This makes buildings dirty and affects the lungs.

The effects of poullants

Some of these pollutants are directly harmful to humans. For example, carbon monoxide is toxic and, if breathed in, can cause death.Others pollutants are harmful to the environment, so cause harm to humans indirectly. For example, sulfur dioxide causes acid rain that can damage or kill trees and crops.

The composition of the present day atmosphere is very different from that of the early Earth. Evidence can be interpreted in different ways leading to a number of explanations as to how the atmosphere has changed.  The atmosphere of the early Earth was largely made up of carbon dioxide and water vapour, probably coming from volcanoes. When the Earth cooled the water vapour condensed to form the oceans.  Carbon dioxide dissolved in the oceans and began to form sedimentary rocks. A lot of carbon dioxide was trapped underground and eventually formed fossil fuels.

Indirect Evidence

The early atmosphere cannot be measured directly so scientists look for indirect evidence. This includes the chemical make-up of rocks, examining air bubbles in ice cores and looking at fossils.  Some fossils suggest early organisms underwent the process of photosynthesis, using the carbon dioxide and releasing oxygen. This meant carbon dioxide levels in the atmosphere went down and oxygen levels went up.

When measuring the composition of gases in the atmosphere, or the concentrations of atmospheric pollutants, it is important that the data obtained is accurate and reliable. Many people may have ideas that they think explain what is happening in the atmosphere. But these opinions are not very useful unless they are supported by data. To justify an explanation, you need to have data to support it. This data may be obtained by taking measurements.

Accuracy  The accuracy of each measurement depends on the quality of the measuring apparatus and the skill of the scientists taking the measurement. If the apparatus is faulty or the scientists make a mistake, the measurement may be inaccurate

Reliability  For the data to be reliable, the variation within the values must be small. There is always some variation in any set of measurements, whatever is being measured.

Range

The highest and lowest values in a set of measurements show the range.The less variation in the measurements, the more reliable they are. In other words, the smaller the range, the more reliable the data.

Mean

The results from all of the samples can be added together, and then the total divided by the number of samples. This gives a value called the 'mean'.

Sample no. 1 2 3 4 5 Nitrogen dioxide (ppb) 17 19 36 17 15

In this set of data, samples 1, 2, 4 and 5 are very close to each other, but sample 3 is very different from the others. It is an outlier. Perhaps the measuring apparatus malfunctioned when this measurement was taken, or perhaps the person taking the measurement made a mistake.

If only one sample is used to produce a best estimate, it may be incorrect. If several samples are used, however, the incorrect ones (the outliers) can be ignored. The others can be used to calculate the mean, which is then used as the best estimate.