Mr Warne

C1 Air Quality

Revision Notes

1) The Atmosphere

The atmosphere is made up of a mixture of gases which are small molecules with large spaces between them. The main gases are:

Nitrogen 78% Oxygen 21% Argon 1% Small amounts of other gases e.g. carbon dioxide

(0.037%) and water vapour

2) Evolution of the Atmosphere

The early atmosphere would have been made of mainly carbon dioxide and water vapour released by volcanic eruptions and volcanic activity. As the earth cooled the water vapour condensed and formed the oceans. The evolution of photosynthesising organisms caused the production of oxygen and the reduction of carbon dioxide. Carbon dioxide also dissolved in the oceans, which was used by organisms in the sea to create their shells. These shells were converted into sedimentary rock over millions of years and eventually turned into fossil fuels. These processes produced the atmosphere as we know it today.

3) Air Pollutants

The major air pollutants are:

Pollutant Formula Particle diagram Produced by… ProblemCarbon dioxide CO2 The combustion

of coal and fuels (i.e. hydrocarbons)

Causes global warming

Carbon monoxide

CO Incomplete combustion of coal and fuels

Poisonous to humans by stopping the red blood cells

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carrying oxygenCarbon particulates

C Incomplete combustion of coals and fuels

Linked to asthma and global dimming. Cover buildings with soot

Sulfur dioxide SO2 Combustion of coal and fuel containing impurities of sulfur

Causes acid rain by dissolving in rain clouds

Nitrogen dioxide

NO2 Combustion of nitrogen in car engines produces nitrogen monoxide which is oxidised in the atmosphere to form nitrogen dioxide

Causes acid rain by dissolving in rain clouds

4) Combustion of Coal:

Coal is made up mainly of carbon. Coal is often burnt in power stations and the energy given off is used to make electricity. When coal burns (combusts) it reacts with oxygen in the air to produce carbon dioxide. The chemical equation for this is:

Carbon + Oxygen Carbon dioxide

The substances reacting together are the reactants (i.e. carbon and oxygen) and the substance produced is the product (i.e. carbon dioxide). A chemical reaction where a substance combines with oxygen is called oxidation. Combustion is an example of oxidation. Coal burns more rapidly in pure oxygen than in the air.

If there isn’t enough oxygen present incomplete combustion occurs. The products of incomplete combustion are carbon monoxide or carbon particulates:

+ +

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Notice that the number of atoms of each element on the left hand side of the reaction (the reactants) is the same as the number of atoms of each element on the right-hand side of the reaction (the products). This is called conserving mass.

5) Combustion of Hydrocarbon fuels

Most fuels are made up of hydrocarbons, for example petrol, natural gas (methane) and diesel. Hydrocarbons are compounds containing only hydrogen and carbon atoms. When hydrocarbons combust (i.e. react with oxygen in the air) carbon dioxide and water are produced. The carbon in the hydrocarbon fuel reacts with oxygen to form carbon dioxide and the hydrogen in the hydrocarbon fuel reacts with oxygen to form water. The chemical equation for the combustion of methane would be:

Notice that the number of atoms of each element is that same on both sides of the equation (conserving mass). If there wasn’t enough oxygen present carbon monoxide or carbon particulates could be formed instead of carbon dioxide (incomplete combustion).

6) Production of other pollutants

Sulfur is found as an impurity in coal and hydrocarbon fuels. When coal and other fuels are burnt, sulfur is able to react with oxygen forming sulfur dioxide. The chemical equation is:

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In car engines, air is drawn in from the atmosphere which provides oxygen for the combustion of petrol or diesel. At the high temperatures found in car engines nitrogen from the air is able to react with oxygen to form nitrogen oxide. Nitrogen oxide is released into the atmosphere which is oxidised. This means it reacts with oxygen in the air to form nitrogen dioxide. Nitrogen dioxide is able to react with water and oxygen to form acid rain which damages trees and crops.

7) Reducing Air Pollution

a) Reducing pollution from burning coal

The amount of polluting gases produced by power stations that burn fossil fuels can be reduced by:

Using less electricity so that less fossil fuels are burnt and therefore less carbon dioxide and other harmful gases are produced.

Removing sulfur from natural gas and fuel oil which prevents sulfur dioxide being produced when they are burnt

Removing sulfur dioxide from flue gases by wet scrubbing: an alkaline slurry (made of calcium oxide and water) or sea water is sprayed onto the waste gases causing the sulfur dioxide to be neutralised and preventing the release of sulfur dioxide into the atmosphere

Removing carbon particulate from flue gases preventing the release of carbon particulate which land on surfaces (e.g. of buildings) making them dirty

b) Reducing pollution from car exhausts

Various pollutants are produced from the combustion of fuels in cars (carbon dioxide, sulfur dioxide and nitrogen oxide). There are a number of ways of reducing these emissions:

Having more efficient engines which burn less fuels Using low sulfur fuels Using more public transport, or walking and cycling instead of driving Having legal limits on exhaust emissions (tested during the MOT of cars)

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Using biofuels or electric vehicles (you should be aware there are benefits and problems to using alternative fuels. For example biofuels are produced by growing large amounts of crops which means the land is not able to be used to grow food, however they are carbon neutral. Electric vehicles still produce emissions as the majority of electricity is produce from fossil-fuel burning power stations).

Using catalytic convertors: catalytic convertors are found in car exhausts. They convert carbon monoxide into carbon dioxide by reacting the carbon monoxide with oxygen (oxidation). They also reduce nitrogen monoxide by converting it into nitrogen and oxygen.

The only way of reducing carbon dioxide from both cars and power stations is to burn less fossil fuels.

8) Analysing data about air pollution

You must be able to (the bold lettering summaries what you need to know, the lettering in italics gives examples and further explanation):

Use data to back up (justify) an explanation: E.g. “I know that Car A is better for the environment because the data shows that it produces less carbon dioxide emissions that car B”.

Explain why it is important to repeat results (reproducibility (you can repeat your own results in multiple tests) and repeatability (that someone else can repeat your results) and give reasons why, if you were to perform only 1 test, the results may not be the true value: E.g. You perform a test by a busy main road to measure air pollution in one area by collecting soot particles on a square of sticky paper. You find that you do not collect any air pollution which seems strange as it is next to a busy road. You repeat the test using multiple squares of paper and find that on average 20 carbon particulates are found in each square. You discover that the first time you performed the test water had dropped onto the sticky paper washing the pollution away.

Calculate the mean of a set of data and state that the mean is the best estimate of the true value.

Understand that by repeating a test you can identify the range of your data and explain that the true value is likely to fall within that range. E.g. The emissions of nitrogen oxide from a car exhaust is measured 5 times. The mean is found to be 33ppm and the range 30-35ppm. The actual amount of nitrogen oxide released from the car (the true value) is very likely to be within your range and your best estimate of the actual amount is your calculated mean. The reason you cannot measure the exact amount every time will be due to errors in your measuring equipment and because of the procedure itself (i.e. the way go about measuring them).

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Identify outliers and explain your decision to discard them from your results: you should recognise that an outlier lies well outside your range and is very different to the other results. You may be asked to explain why an outlier for a particular set of data has occurred.

Discuss if there is a real difference between 2 sets of data: there is only a real difference if the ranges do not overlap

9) Correlation and cause

You need to recognise that there may be a correlation between 2 sets of data. For example during the summer ice cream sales increase and hayfever cases increase. However, one does not necessarily cause the other. This means that whilst there may be a correlation between 2 sets of data (e.g. hayfever and ice cream sales), one may not cause the other (i.e. eating ice cream does not cause hayfever). When looking at correlations you need to try and find out if there is a link between the two, for example if you look at the graph you will see there is a correlation between smoking and getting lung cancer. This correlation was spotted in the 1975, however it wasn’t until years later that the link between the two was found and a mechanism was determined to explain how smoking causes lung cancer.