Chemicals in the Air
- The Earth is surrounded by a thin layer of gases called the atmosphere.
- Air forms part of the atmosphere.
- It is a mixture of different gases consisting of small molecules with large spaces between them.
- Air contains about 78% nitrogen, 21% oxygen, 1% argon and other noble gases.
- There are small amounts of water vapour, carbon dioxide and particulates.
- The amount of water vapour and polluting gases varies as a result of human activity or by natural processes such as volcanoes.
- Since the Earth was formed the atmosphere has changed a lot...
- 4 billion years ago - the earliest atmosphere contained ammonia, water vapour and carbon dioxide. These gases came from inside the Earth and were often released from volcanoes.
- 3 billion years ago - as the temperature of the planet fell, the water vapour in the atmosphere condensed to form oceans and seas.
- 2 billion years ago - the evolution of photosynthesising organisms started to reduce the amount of carbon dioxide and increase the amount of oxygen in the atmosphere.
- 1 billion years ago - carbon from carbon dioxide in the air became locked up in sedimentary rocks such as carbonates and fossil fuels after dissolving in the oceans.
Pollutants in the Air
- Pollutants are chemicals that can harm the environment and our health.
- Pollutants that harm the environment can also harm humans indirectly.
- Carbon dioxide is harmful to the environment as it traps heat in the Earth's atmosphere.
- Nitrogen oxides are harmful to the environment and humans because it causes acid rain and causes breathing problems and can make asthma worse.
- Sulfur dioxide is harmful to the environment as it causes acid rain.
- Particulates (small particles of solids) are harmful to the environment and humans because it makes buildings dirty and can make asthma and lung infections worse if inhaled.
- Carbon monoxide is harmful to humans as it displaces oxygen in the blood, which can result in death.
- By measuring the concentrations of pollutants in the air, it is possible to assess air quality.
- The units of measurement used are ppb (parts per billion) or ppm (parts per million).
- Elements are the 'building blocks' of all materials.
- There are more than 100 elements and each one is made up of very tiny particles called atoms.
- All the atoms of a particular element are the same and are unique to that element.
- Each element is represented by a different chemical symbol.
- Atoms can join together to form bigger building blocks called molecules.
- Compounds are formed when the atoms of two or more different elements are chemically combined.
- The properties of a compound are very different to the properties of the individual elements it is made from.
- Chemical symbols and numbers are used to write formulae.
- Formulae show the different elements that make up a compound and the number of atoms of each different element in one molecule.
- During a chemical reaction new substances are formed from old ones because the atoms in the reactants are rearranged in some way:
- Joined atoms may be separated
- Separate atoms may be joined
- Joined atoms may be separated and then joined again in different ways.
- These chemical changes are not easily reversible.
- An example of oxidation is a chemical reaction that occurs when oxygen joins with an element or compound.
- An example of reduction is when oxygen is lost from a substance.
- Combustion is a chemical reaction that occurs when fuels burn, releasing energy as heat.
- For combustion to take place, oxygen must be present.
- No atoms are lost or produced in a chemical reaction.
- There will always be the same number of atoms on each side of the reaction, therefore conserving mass.
- This means there will always be some pollutants formed during the combustion of fuels.
- The conservation of atoms during combustion reactions has implications for air quality since some atoms in the fuel may react to give products that are pollutants.
Burning Fossil Fuels
- Many of the pollutants in the atmosphere are produced through the combustion of fossil fuels.
- Fossil fuels consist mainly of compounds called hydrocarbons.
- A hydrocarbon contains only hydrogen atoms and carbon atoms.
- When it is burned in air, carbon dioxide and water are produced.
- Carbon dioxide is a pollutant so this causes pollution.
- If the fuel burns in pure oxygen, the reaction is more rapid than when it burns in air.
Methane + Oxygen -------// Carbon dioxide + Water
CH4 + 2O2 ---------// CO2 + 2H2O
- If a fuel is burned and there is not enough oxygen in the air, carbon particulates or carbon monoxide may be produced.
Methane + Oxygen ---------// Carbon + Water
CH4 + O2 ----------// C + 2H2O
Methane + Oxygen --------// Carbon monoxide + Water
2CH4 + 3O2 ----------// 2CO + 4H2O
- Incomplete combustion occurs in car engines, so exhaust emissions contain carbon particulates and carbon monoxide, as well as carbon dioxide.
- Many samples of coal contain sulfur so sulfur dioxide is released into the atmosphere.
- Sulfur + Oxygen -------// Sulfur dioxide S + O2 -------// SO2
- During the combustion of fuels, high temperatures can cause nitrogen in the atmosphere to react with oxygen and produce nitrogen oxides.
The nitrogen oxides are formed in two steps:
- The nitrogen reacts with oxygen to form nitrogen monoxide
Nitrogen + Oxygen -------// Nitrogen monoxide N2 + O2 --------// 2NO
- Nitrogen monoxide is then oxidised to produce nitrogen dioxide.
Nitrogen monoxide + Oxygen -------// Nitrogen dioxide 2NO + O2 --------// 2NO2
- When NO and NO2 occur together they are called oxides of nitrogen and they are written as NOx
Identifying Health Hazards
- Because humans need to breathe in air to get oxygen, it is reasonable to assume that air quality will have some effect on the body.
- To find out exactly how air quality affects us, scientists look for correlations that might link a factor (e.g. a pollutant in the air) to an outcome (e.g. a respiratory complaint like asthma).
- An example of this is the correlation between pollen in the air and hay fever.
- Another condition that is linked to air quality is asthma but this is more complicated.
- Studies of asthma have shown that when the concentration of nitrogen dioxide increases in the air, more asthma attacks are triggered.
- People still have asthma attacks when the levels of nitrogen dioxide are very low which means that although nitrogen dioxide can increase the chance of an asthma attack, it is not the primary cause.
- There are many factors that can trigger an asthma attack so to fully understand which factors cause asthma and which factors may aggravate the condition, scientists need to study a large sample of people.
Improving Air Quality
Air pollution affects everyone, so we all have a responsibility to reduce it.
Motor vehicles and power stations that burn fossil fuels are two major sources at atmospheric pollution, so we need to look at how emissions from these sources can be reduced.
Emissions from power stations can be reduced by:
- using less electricity so fewer fossil fuels need to be burned
- removing toxic chemicals before they are burned, e.g. removing the sulfur from natural gas and fuel oil
- using alternative renewable sources of electricity, e.g. solar energy, wind energy etc.
- using a filter system to remove sulfur dioxide and particulates (carbon and ash) from flue gases before they leave a coal-burning power station's chimney.
The sulfur dioxide is removed from flue gases by wet scrubbing, using an alkaline slurry or seawater.
During wet scrubbing, the flue gas containing the sulfur dioxide is brought into contact with a slurry of either limestone (calcium carbonate) or lime (calcium oxide) and water. The alkaline slurry is....
Improving Air Quality.
..usually sprayed onto the pollutant gas flow, where the sulfur dioxide is absorbed onto its surface. The absorbed sulfur dioxide is then converted to calcium sulfate.
During seawater scrubbing, flue gases are washed with seawater, which dissolves the sulfur oxides. The acidic dissolved sulfur oxides will react with alkalis to produce sulfate salts.
Emissions from motor vehicles can be reduced by:
- buying a car with a modern engine that is more efficient and burns less fuel
- buying a hybrid car, which uses electric power in the city centre and can then switch to running on petrol for longer journeys
- using a low-sulfur fuel to reduce the amount of sulfur dioxide released
- converting the engine to run on biodiesel, which is a renewable fuel
- using public transport to reduce the number of vehicles on the road
- making sure cars are fitted with catalytic converters which reduce the amount of carbon monoxide and nitrogen monoxide emitted
Improving Air Quality..
The reactions that occur in a catalytic converter are:
- Carbon monoxide is oxidised to carbon dioxide by gaining oxygen.
Carbon monoxide + Oxygen --------// Carbon dioxide 2CO + O2 ---------// 2CO2
- Nitrogen monoxide is reduced to nitrogen by losing oxygen.
Nitrogen monoxide + Carbon monoxide ------// Nitrogen + Carbon dioxide
2NO + 2CO -----// N2 + 2CO2
The only way of reducing carbon dioxide emissions is to burn fewer fossil fuels.
In 1997 there was an international meeting about climate change in Kyoto, Japan. People from many nations agreed to reduce carbon dioxide emissions and targets were set for individual countries. The governments of the countries are required to take appropriate measures to meet the targets.
Here are some of the initiatives that are helping the UK meet its target:
- Setting legal limits for vehicle exhaust emissions, which are enforced by statutory MOT tests
- Making catalytic converters compulsory on new vehicles
- Using subsidies (grants) or reduced taxes to encourage power companies to use 'cleaner' fuels
- Introducing a car tax system that encourages drivers to buy smaller cars with smaller engines
- Encouraging investment in non-polluting renewable energy such as wind and solar energy.
These initiatives will impact on many areas of science and industry, e.g. when new cars are developed, the technology used must meet all the legal requirements. Some governments are concerned that steps taken to reduce carbon dioxide emissions will result in a decline in manufacturing and production, employment and the national economy.
Different countries may make different decisions after weighing up the benefits and problems. Biofuels benefits- renewable source that is carbon zero, problems- a lot of land is needed to grow biomass, transportation of biomass to the generator. Electricity benefits- no exhaust fumes and a quiet engine, problems- lots of charging stations needed, length of time to recharge batteries, battery life, cost.
Many local authorities are trying to encourage us to make environmentally friendly choices by providing:
- doorstep collections of paper, bottles, metals and plastics for recycling
- regular bus and train services
- electric trams (in some cities)
- congestion charges
- 'park and ride' schemes
- cycle paths and cycle parks
It is clear that the choices we make as individuals affect the amount of pollution in the air.
Using less energy in the home reduces the demand for energy from power stations, e.g. turning off televisions and not leaving them on standby.
Making sure your car is energy efficient and has a catalytic converter or choosing an alternative mode of transport, cuts down on emissions.
Recycling materials like paper, bottles, metals and plastics helps to conserve natural resources but also saves energy, e.g. it takes about 95% less energy to recycle an aluminium can than to make a new one.
There are other benefits to the 'green' options too. For example, walking and cycling instead of travelling by car help to keep us fit!