WHAT IS AIR POLLUTION ?

Pollution, has its origins in the Latin word polluere, means 'contamination of any feature of the environment'. Pollution has the following characteristics:

1. It is the addition of substance at a faster rate than the environment can accomodate. Certain pollutants like arsenic or mercury have natural levels in nature, but only if these levels exceed certain critical values can they consider to be pollutants.
2. Pollutants are not only chemicals, but also forms of energy like heat, sound (so noise pollution) and radioactive rays.

These are pollutants.

3.  
4. To be a pollutant, a material has to be potentially harmful to life.
5. Pollution is the only result of human activities.

The layer of air which supports life extends about 8 km above the Earth's surface and is known as troposphere. The composition of air remains remarkably constant except there may be small localised variations. Air pollution has existed since humans first used fire. The problem becomes significant since the industrial revolution in the 19th century. Almost all air pollutants is the result of burning fossil fuels, either in the home, by industry or in internal combustion engine.

 

POLLUTANTS

In this section we list some of the common air pollutants. You can find the source and the effect of these pollutants by clicking on the following sections:

Carbon monoxide Carbon dioxide Sulphur dioxide Nitrogen oxides

Hydrocarbons Ozone Particulates Lead Chlorofluorocarbons (CFCs)

Pollutant	Concentration (ppm)
Carbon monoxide	10
Hydrocarbons	3
Sulphur dioxide	0.08
Nitrogen oxides	0.05
Total oxidants (ozone and others)	0.02
Median concentrations of atmospheric pollutants in a typical urban atmosphere

1.     Affects human health
One may have symtomps such as headaches, irritation of eyes, nose and throat, nausea and general ill feeling. Respiratory diseases may also observed.

2.     Affects plants
Some plants such as lichens are sensitive to pollutants. Lichens can as a bio-indicator. Other plants may be destroyed by altered mud acidity and release of poison metal ions.

3.     Damaging effects on surroundings
Metals corrode, stoneworks weather by acid rain.

CARBON MONOXIDE

Source

Carbon monoxide [CO(g)] occurs in exhaust emissions from cars and other vehicles. The fuels that the vehicles used are mostly carbon-containing. Normal combustion of petrol produces only carbon dioxide and water.

Using heptane as an example of a fuel, complete combustion of this is:

With limited supply of oxygen, the exhaust may contain certain amount of carbon monoxide and carbon particles (known as soot). This is called incomplete combustion.

Health Effects

CO(g) is a poisonous gas on account of having an affinity of haemoglobin, the red pigment that carries oxygen in blood, some 250 times greater than oxygen itself. The combination, carboxyhaemoglobin, is not released and prevents oxygen from combining with it. Continued inhalation leads to death as all haemoglobin become attached to CO(g) (the condition known as asphyxia).

In small concentrations, it may cause dizziness and headache. The concentration rarely exceed 4% and it never accumulate due to bacterial and algal action. Cigarette smokers often have their concentration of CO(g) in blood up to 10%.

CARBON DIOXIDE

Source

Carbon dioxide [CO₂(g)] can be formed from 2 sources. The natural one is from the respiration of organisms. That produced by respiration is taken up by plants during photosynthesis, ensuring it does not accumulate.

The artificial generation of carbon dioxide is the burning of fossel fuels. The additional carbon dioxide in the atmosphere from such raises the atmospheric carbon dioxide concentration.

Social Effects

The change in carbon dioxide concentration in the air has triggered the so-called greenhouse effect. More heat from the sun is prevented to escape from the Earth, rather like the mechanism of a greenhouse.

Greenhouse effect will cause expansion of the oceans and the gradual melting of polar ice caps with a consequent rise in sea level. This would in turn cause flooding of low-lying land where many capital cities lie.

But greenhouse effect is not totally bad. Its influence maintains the Earth's temperature at an average of 15°C rather than -18°C which would be the case without any greenhouse gases.

The long term emission (since industrial revolution) of carbon dioxide has posed great effects (50-70% contribution) on global warming. Though other gases like water vapour, methane and nitrogen oxides may absorb infra-red radiation from the sun more effectively and play a bigger role in the process. The long shell life of the gas in the atmosphere provides that it has the major influence on greenhouse effect.

The concentration of atmospheric CO₂ has risen more than 10% since the late 1950s. These data are obtained from the Mauna Loa Obseratory in Hawaii by monitoring the absorption of infrared radiation. The sawtooth shape of the graph is due to the seasonal changes in concentration each year.

SULPHUR DIOXIDE

Source

Fossil fuels contain between 1 and 4% sulphur. As a result, there are around 30 million tonnes of sulphur dioxide [SO₂(g)] emitted in Europe each year. The major sites of emission includes power station and metal smelters in which use coal as their fuel. Manufacturing industries often use fuel oil with high sulphur content.

Diesel of heavy vehicles generates much sulphur dioxide. In the mainland, the sulphur content of diesel is not as low as that in Hong Kong. Therefore the cross-border container vehicles imposes much air pollution.

Health Effects

Sulphur dioxide causes irritation of the respiratory system. It affects mainly the upper respiratory tract. It can attach to particles forming acidic coatings that damages the epithelial lining of the alveoli. It may also aggravate asthma attack.

The gas reduces the growth of many plants, e.g. barley, wheat, lettuce, while others such as lichens may be killed. So the tolerance of lichen and moss species to sulphur dioxide isvery valuable and they serve as the indicator species for measuring sulphur dioxide pollution.

Social Effects

Sulphur dioxide has the following benefits:

1. Most of the gas readily combines with water and ammonia that deposits in the soil to increase soil fertility.
2. It acts as a fungicide to help controlling diseases like blackspot of roses.

However, it dissolves in water to form acid rain.

NITROGEN OXIDES

Source

The most natural way of forming nitrogen oxides [NO_x(g), where x may be 1 or 2] is lightning. Atmospheric nitrogen reacts with nearby oxygen to form nitrogen monoxide.

Nitrogen monoxide further reacts with oxygen to form nitrogen dioxide.

But this only generate limited amount of nitrogen oxides. The pollution caused can be accounted for the engines of vehicles and furnaces. Due to their high temperature, these gases are evolved easily. The great number of cars on Hong Kong roads deepens the problem.

Health Effects

Nitrogen dioxide is much less soluble than sulphur dioxide. It penetrates deeply into smaller airways and lung parenchyma, causes pulmonary oedema at high concentrations. It may also damage the blood vessels.

Nitrogen dioxide is a major component of photochemical smog. Photochemical smog causes headaches, eye, nose and throat irritations, impaired lung function, coughing and wheezing.

Social Effects

Nitrogen dioxide is acidic in nature. Therefore, besides sulphur dioxide, it is another gas that will cause acid rain.

The gas, together with hydrocarbons and particulates, forms photochemical smog under sunlight. Its brown coloration obscure the sky with a yellow haze that lowers the visibility. This drives away tourists from coming.

HYDROCARBONS

Source

Hydrocarbons are compunds that have both the elements carbon and hydrogen. Motors of cars do not always burn the fuel completely. There is tiny amounts of unburnt hydrocarbons in car exhaust.

Another source of generation is from the careless of workers. Petrols and organic solvents are left unattended and exposed to air. Since those solvents are volatile in nature, these hydrocarbons pose danger to our health.

Health Effects

Certain hydrocarbons are carcinogenic. Prolonged exposure to such solvents can cause cancer (Click here for details of formation of cancer). The formation of photochemical smog renders other health problems possible.

 

Social Effects

Hydrocarbons are one of the components of photochemical smog. It affects our health and produces a brown haze around the city. Hydrocarbons are generally flammable. The existence of vapours induces dangers of catching fire. This leads to a lost in valuables and even lives!

OZONE

Source

Ozone [O₃(g)] is a pale blue gas that exists in the stratosphere of our atmosphere as the ozone layer. It is formed from atmospheric oxygen by the absorption of ultraviolet radiation of the right energy (wavelength 250 nm).

Ozone itself undergoes photodissociation with 215-295 nm ultraviolet radiation.

Thus, ozone is constantly created and destroyed. The above reaction is responsible for the vital screening effect of ozone.

The presence of chlorofluoroarbons (CFCs) generate reactive chlorine radicals that constantly destroy ozone. The depletion of ozone layer will be discussed later.

Ozone can also be formed in the lower part of the atmosphere. It is formed by reactions between nitrogen oxides and hydrocarbons under sunlight, or by electric sparks which occur in car engines or electrical appliances like photocopiers. In nature, the gas can be generated during lightning.

Health Effects

Ozone is harmless when it is in low concentrations. The gas itself is a respiratory irritant. It penetrates into small airways and the lung, causes pulmaonary oedema at high concentrations. Other illnesses include lung fibrosis and reduces lung function. It precipitates asthma attacks.

Social Effects

Ozone layer in the stratosphere filters out 99% of dangerous ultraviolet radiation from the sun. The thnning of the ozone layer may lead to an increase of skin cancer and eye cataract. The presence of 'ozone hole' over Antarctica and the southern hemisphere has introduced incidence of increasing victums of skin cancer. The yield of crops may also decrease. The expanse in medical cure and economical loss in food is uncountable.

PARTİCULATES

Source

Particulates make up smoke. They may be ashes from burning of fuels. If fossil fuels (like coal and oil) are burnt, the tiny particules formed are mainly soot (carbon) from incomplete combustion. Incineration plants, factories and diesel vehicles are sources of emission.

Health Effects

Smaller particulates can penetrate into small airways and lung bypassing the 'muco-ciliary defence' (The trapping of germs and dust by secreted mucus and cilia on epithelium along the respiratory tract.). Soot particles may blacken the alveoli, causing damage to the delicate epithelial linings. Toxicity depends on the concentration accumulated and linked to the increased risks of respiratory and cardiovascular illnesses and death. Cancer may also be induced.

The suspension of particulates in the air reduces the ground level light intensity. This lowers the rate of photosynthesis. The deposit of smoke may coat plant leaves, reducing photosynthesis by preventing light from penetrating or by blocking stomata.

Social Effects

Smoke, ash and soot become deposited on clothes, cars and buildings. They are costly to clean. The visibility reduces when there is smoke. This highly affects the safety of navigation and aviation. It is a reduction of appeal on tourists to come to a smoky, choky place

LEAD

Source

Lead is a heavy metal that is used to make water pipes, but most lead comes from emission of cars. A lead compound, tetraethyl lead(IV) [TEL, (C₂H₅)₄Pb] is added to petrol acts as an anti-knock agent so as to assist the even burning of fuel inside the engine. The combustion of this leaded petrol deposits metallic lead and lead(II) oxide in the engine surface. So some 'lead scavenger' like 1,2-dibromoethane is also added to petrol. This converts the lead into volatile lead(II) bromide that carries away in the exhaust.

Health Effects

Each year, around 50 000 tonnes of lead is released to the atmosphere by UK. Much of this is deposited close to roads. The remains would be absorbed by the lungs that cause the following adverse effects:

1. Digestive problems, e.g. intestinal colic
2. Impairing the functioning of the kidney
3. Anaemia
4. Nervous problems, including convulsions
5. Brain damage and mental retardation in children

CHLOROFLUOROCARBONS

Source

Chlorofluorocarbons (CFCs) are a family of compounds containing chlorine, fluorine and carbon. They are in general unreactive, having low flammability, toxicity and solubility in water.

The major sources of CFCs are aerosol propellants, cleaning solvents, refrigerants and plastic blowing agents.

Aerosol propellants

CFCs such as trichloromethane (CCl₃F) and dichlorodifluoromethane (CCl₂F₂) are normally packed with materials like paint, insecticide or cosmetic preparations in pressurised containers. Upon depressurisation by opening the valve, the propellant vaporises and expels the materials inside the can in the form of aerosol spray.

Cleaning solvents

CFCs like trichlorofluoroethane (CCl₂FCClF₂) can dissolve grease and are widely used as solvents in cleaning electric components and metals.

Refrigerants

Freon is a series of CFCs. Dichlorodifluoromethane (CCl₂F₂) is the most important one. Freons absorb heat of vaporisation on evaporation resulting in the cooling of the surroundings. they are widely used as refrigerants in refrigerators and air conditioning units.

Foam plastic blowing agent

In making foam plastic, a volatile CFC, trichlorofluoromethane (CCl₃F) is incorperated in the plastic. The heat evolved during the polymerisation reaction vaporises the CFC which then forms tiny bubbles in the plastic.

Social Effects

Due to their inert behaviour, the once emitted CFCs can stay in the atmosphere for a long time (a few tens of years). They will not be broken down by rainwater. The prolonged effect of CFCs is the depletion of ozone layer. Ultraviolet light generates chlorine radicals that destroy ozone molecules and thus the equilibrium of ozone in the atmosphere. The increased intensity of sunlight may induce health complications (skin cancer, eye cataract) and ecological disasters. (Global warming leads to melting of ice caps in the polar regions. The low-lying land becomes flooded.)

Unpolluted rain water is slightly acidic owing to the presence of carbon dioxide in the air. Its pH is at 5.7.

Therefore, rain water with pH values lower than 5.7 is called acid rain. In some parts of the world,a pH as low as 2.5 in rainwater has been recorded.

The Cause

Sulphur dioxide

It is the primary cause of acid rain. Sulphur dioxide is released from burning of fossil fuels and industrial plants.

The sequence of changes from the emitted gas to acid is at follows:

[Note: The actual pathways are more complex.]

The forming of sulphur trioxide from sulphur dioxide is influenced by the prevailing atmospheric conditions:

• sunlight
• temperature
• humidity
• presence of hydrocarbons
• nitrogen oxides
• particulates

Besides the formation of sulphuric acid [sulphuric(VI) acid], sulphurous acid [sulphuric(IV) acid] is also formed.

Nitrogen oxides

It is another cause of acid rain. Nitrogen oxides are released from vehicles and power plants.

The sequence of changes are as follows:

[Note: The actual pathways are more complex.]

Effects

Vegetation

Acid rain washes away esssential nutrients that the plants are in need. In adition, it makes the soil acidic and aids the release of aluminium and copper ions which are harmful to plants.

A woodland in Germany in 1972.		The same spot in 1983. Notice that acid rain has destroyed the forest.

Aquatic life

When pH is less than 4.5, calcium metabolism in fresh water fish will be affected, leading to poor health and stunted growth. As a result, diversity and population of some fish species will be reduced.

Building materials

Acid rain will cause damage to common building materials (such as limestone and marble), statues and monuments.

Many metals will become oxidised. Iron corrodes with the presence of acid rain to form rust. The cost of maintainance of iron structures is high in highly polluted areas.

Photochemical smog is a mixture of pollutants which includes particulates, nitrogen oxides, ozone, aldehydes, peroxyethanoyl nitrate (PAN), unreacted hydrocarbons, etc. The smog often has a brown haze due to the presence of nitrogen dioxide. It causes painful eyes.

The Cause

The condition needed for the formation of the smog are present in modern cities. They included sunlight, hydrocarbons, nitrogen oxides and particulates which act as catalyst.

These are the equations in the formation of photochemical smog.

Case Study

On 9 December 1952, foggy conditions developed over London. Being very cold, most houses kept fires burning, with coal as the major fuel. The smoke from these fires mixed with the fog and was unable to disperse, resulting in a smog which persisted for 4 days. The pH of air during the Great London Smog was as low as 1.6. During this period some 4000 more people died than would expected at this time of the year. Most of these additional deaths were due to respiratory disorders.

New York City, before and after a photochemical smog.

Effects

1. It can cause headaches, eye, nose and throat irritations. It may cause the lung function impaired, coughing and wheezing.
2. It can cause rubbers and fabrics to deteriorate.
3. It can damage plants, leading to the loss of crops.

The Cause

Ozone layer is situated in the stratosphere of the atmosphere. Ozone is a blue gas that gives our sky blue in colour when the weather is fine. The ozone there is kept in constant concentration by the action of ultraviolet light.

Reactions responsible for the destroy of ozone:

Reactions responsible for the formation of ozone:

Under this natural equilibrium, the rate of destroy equals the rate of formation. Therefore, the concentration of ozone is fairly constant in the stratosphere.

Chlorofluorocarbons (CFCs) is the killer of ozone. They generate chlorine radicals that attack and destroy ozone molecules. CFCs are important compounds in industry and domestic uses. This renders the depletion of ozone layer severe. Now a region of low ozone concentration (commonly called the 'hole') is situated over Antarctica and the Arctic region.

Why CFCs are accumulated?

There are 3 major sources of escape of CFCs:

1. escape into the atmosphere during use
2. scrapping of refrigerators and air conditioners
3. disposal of foam plastics

CFCs have a relatively long lifetime. They are inert in nature that gives enough time for them to be transported into the stratosphere. For instance, trichlorofluoromethane (CCl₃F) has a lifetime in the atmosphere of 75 years. Their low solubility in water makes them difficult to be removed by rainfall or dissolution in the vast ocean.

Mechanism

It is a complex chemical process. The following just gives an outline.

Chain initiation

Chain propagation

The depletion of ozone molecules is greater than the formation of ozone. The balance of concentration is disturbed here. Also the chlorine radical is regenerated, the presence of one such radical can effectively destroy many ozone molecules.

The map showing on the right shows us the total ozone present in the Southern Hemisphere in October 1994. This satellite map represent the different ozone concentration by means of colours. The vast pink area is the area of lowest ozone concentration.

Chain termination

Stratospheric clouds & ozone depletion

Various chemists and atmospheric scientists have been contributed in the study of ozone depletion over Antarctica from the mid-1970s. A puzzling phenomenon happens in which the rate of depletion maximises in September and October, the beginning of Antarctica spring. About 70% of ozone is destroyed in the stratosphere (the height of 12 and 30 km).

Recent studies shows that the ozone hole is formed by the interaction of both chemical and physical effects. Photodissociation of CFCs generates free chlorine radicals (Cl·). They destroy ozone molecules and form chlorine monoxide (ClO·). The two radicals undergo reactions with other atmospheric contents, like methane (CH₄) and nitrogen dioxide (NO₂):

Neither HCl nor ClONO₂ reacts directly with ozone. As more Cl· and ClO· are tied up in these so-called chlorine 'reservoir'. No further destruction of ozone happened.

The presence of polar stratospheric clouds (PSCs) accelerates the depletion of ozone. This form of uncommon clouds removes nitrogen dioxide from the atmosphere. This stops the removal of ClO·. The crystalline surfaces of clouds acts as seeds which catalyse the formation of chlorine (Cl₂).

When spring approaches (in September), the chlorine formed is photodissociated by sunlight into its free radicals. They begin to attack ozone molecules again. Due to the lack of nitrogen dioxide in the atmosphere, the ClO· produced is not tied up in the chlorine 'reservoir'. It self-combines to form a molecule of Cl₂O₂. And then it photodissociates into oxygen (O₂) and free chlorine radicals.

The cycle of destruction begins anew. The PSCs make the free chlorine radicals more readily to destroy more ozone molecules. Each single radical may destroy thousands of ozone molecules before deactivated by a hydrocarbon or nitrogen dioxide.

The mass destruction continues into November. The warming temperature and wine currents break up the clouds. The destruction ends and the ozone-lack air flows out into the southern hemisphere. The overall ozone concentration in these areas decreases.

The effect of ozone depletion in Antarctica is severe, however, the ozone in the Arctic region should not be neglected. There is a trend showing the concentration of ozone there starts decreasing (over 30%). Global warming is a problem that our human race must face.

Effects

Ozone layer in the stratosphere filters out 99% of dangerous ultraviolet radiation from the sun. The thinning of the ozone layer may lead to an increase of skin cancer and eye cataract. The presence of 'ozone hole' over Antarctica and the southern hemisphere has introduced incidence of increasing victims of skin cancer. The yield of crops may also decrease. The expanse in medical cure and economical loss in food is uncountable.

The increase in intensity of sunlight may induce global warming that melt the ice caps at the poles. If this happens, the sea level will rise from a few centimetre to a few metre. The low-lying areas will become underwater. It is an ecological disaster. However, the economic centres are always located along the coast. Hence flooding can lead to great economical loss.

The cause

A greenhouse is a glass-built building for growing plants. With illumination from the sun, heat enters the house with little of it escaped as the glass traps it. The temperature inside the house ismuch higher than that of air outside. Plants grow there can have more warmth and healthier.

Our atmosphere is like a huge greenhouse. Greenhouse gases, like carbon dioxide, acts like the glass of a greenhouse. The changes in climate of the Earth and the subsequent effects are called greenhouse effect.

What are greenhouse gases?

Name of gas	Contribution	Source(s)
Carbon dioxide (CO2)	50%	From burning coal and oil, and the removal of vegetation
Chlorofluorocarbons (CFCs)	20%	From air conditioners, refrigerators and aerosols
Methane (CH4)	16%	From rice growing, animal waste, swamps and landfills
Ozone (O3)	8%	From air pollution
Nitrous oxide (N2O)	6%	From fertilizers and burning of coal and oil

Source: Global Ecology Handbook

How the problem intensifies

1. The burning of coal and oil in cars, power stations and factories gives out huge amounts of carbon dioxide and nitrogen oxides into the atmosphere.
2. The rapid growth of population needs more food. More cattles, sheep, etc., are reared. More people breathe out more carbon dioxide, and the waste of animals generates methane.
3. Forrests are cleared or burnt for development. Fewer plants can contribute for the absorption of carbon dioxide that maintains the balance of oxygen and carbon dioxide in the atmosphere. The level of carbon dioxide increases.

Carbon dioxide and greenhouse effect

Much carbon dioxide is absorbed into oceans or used by plants for photosynthesis. Nevertheless, the production rate is higher..

Chemists have monitored atmospheric carbon dioxide concentration since 1958. Analysis of air trapped in ice cores from the polar regions permits us to know the past carbon dioxide levels from about 160,000 years ago. The level is fairly constant since the past ice ages some 10,000 years ago. Until the industrial revolution some 300 years ago, the concentration of carbon dioxide has increased by about 25%.

The accumulation or the gas has caused absorption of radiant heat. The climate changes.

The impact

Greenhouse effect may be beneficial. When the Earth is warmer, there may be an increase in agricultural production. But the problem is that the amount of greenhouse gases increases dramatically. It grew 3 times in the past 100 years. Such rising may induce harmful effects to the environment:

1. Rise of sea-level
   Climates do change naturally over long periods of time. However, the Earth becomes warmer in the recent decades. It was found that 9 hottest years on record occured during the past 14 years. The mean Earth temperature in 1997 was 0.43°C higher than the past 26 years.
   
   Global warming causes the polar ice sheets melt. It increases the sea-level. It will possibly rise 18 cm by 2030 and 58 cm by 2090. Flooding will probably occur in lowlands. Many cities along the coast may be under sea water.
2. Changing rainfall patterns
   Warming also affects rainfall and food production. Some places have an increment in rainfall and some will decrease. There may be flooding or drought in various farmlands. Food is important for lives. That means more people, especially in the developing countries, suffer from hunger.