Sandro Lanfranco
Department of Environmental Science, GF Abela Junior College, University of Malta, Msida
 
 

Introduction
Since the onset of the Industrial Revolution in the late 18^th century, most manufacturing and power-generation concerns have obtained their supplies of energy through the combustion of fossil fuels. Such combustion processes promote the release of various pollutants into the atmosphere. General usage of the term 'air pollution' refers to the anthropogenic input of such pollutants into the troposphere. Pollutants in the troposphere usually have quite a short residence time since they tend to be washed out by rain, altered by chemical reaction or deposited on the ground within a short while of their input. Pollutants that are injected directly into the stratosphere are usually characterised by a much longer residence time since removal mechanisms are weaker due to the comparative lack of water in this layer of the atmosphere. Natural sources, including volcanic eruptions, sea-spray and lightning also contribute large volumes of gases that are considered as pollutants.
 

Classification of atmospheric pollutants
Pollutants of the atmosphere may be classified according to their source or according to their chemical composition. Classification according to source is dependent on the site of formation of a pollutant: Primary pollutants are formed within ground-based sources (e.g. factories, cars, power plants, volcanoes, sea-spray) and released directly into the atmosphere. Secondary pollutants (e.g. acids, ozone), on the other hand, are formed in situ within the atmosphere as a result of the reaction of various primary pollutants with each other and with other constituents of the atmosphere.
 

Types of atmospheric pollutants
Various groups of pollutants may be identified according to chemical structure, and the following scheme should not be considered exclusive.

1. Suspended particulate matter (SPM): small solid particles such as dust, soot, ash, sand. These may be derived from combustion (soot, ash) or from natural sources (dust, sand). Particles smaller than 10m in diameter, called PM10, can pose a health risk.
2. Volatile organic compounds (VOC): mainly hydrocarbons, derived from boilers and central heating units, mainly due to burning of coal. VOC can also escape from vehicle engines into the atmosphere. Such pollutants may interfere with other atmospheric processes leading to a formation of tropospheric ozone.
3. Oxides: this class of pollutants comprises carbon dioxide (CO2), carbon monoxide (CO), sulphur dioxide (SO₂), nitrogen dioxide (NO₂) and nitric oxide (NO). All of these are products of combustion. Carbon monoxide is a toxic gas released from vehicles, and its concentration in the atmosphere has increased with the rise in traffic density. Sulphur dioxide is derived from the sulphur present in most fossil fuels. Burning of these fuels oxidises the sulphur and converts it to sulphur dioxide. This gas can contribute to the formation of acid rain. Nitrogen oxides are mainly derived from combustion and contribute to the formation of acid rain.
4. Lead: the use of lead as an anti-knocking agent in petrol leads to the emission of this metal (as part of a chemical compound such as PbCl₂) in automobile exhaust. Inhalation of lead compounds can produce serious effects on human health particularly in children.
5. Ozone (O₃): although stratospheric ozone (as the ozone layer) is considered beneficial to humans, it nevertheless remains a health hazard. Ozone can form in the troposphere through photochemical reactions involving the breakdown of nitrogen dioxide. Since it is not directly emitted into the atmosphere from ground-based sources it therefore qualifies as a secondary pollutant. Some tropospheric ozone also accumulates through transfer from the stratosphere.
6. Radon: this gas is a natural product of the radioactive decay of uranium that occurs in rocks and soil. Since radon is radioactive, it can decay to form small electrically-charged particles than can bind to small particles of dust present in the air. When this dust is inhaled, these particles (which emit radiation) are introduced into the respiratory system.

Main categories of air pollution
The different pollutants introduced into the atmosphere can give rise to distinct processes of change, the effects of which may be experienced on a global scale. These may be conveniently summarised into three classes: acid precipitation, the greenhouse effect and depletion of stratospheric ozone.

1. Acid precipitation: this refers to the occurrence of acidic rainfall, snow, fog, sleet, dew or hail, mainly as a consequence of air pollution. Normal, unpolluted rainfall is acidic (pH = 5.6) since rainwater combines with carbon dioxide in the atmosphere to form carbonic acid (H₂CO₃). Acid precipitation therefore refers to rainfall which is more acidic than this background level. Acids are not directly emitted into the atmosphere but form in situ and are therefore considered to be secondary pollutants. The formation of atmospheric acids follows from the emission of nitrogen oxides and sulphur oxides into the atmosphere. When these primary pollutants react with water vapour in the atmosphere they form nitric acid and sulphuric acid respectively.

formation of nitric acid (HNO₃):
NO₂ + OH^- ® HNO₃

the formation of sulphuric acid (H₂SO₄) follows a more complex pathway:
SO₂ + OH^- ® HSO₃^-
HSO₃- + O₂ ® SO₃ + HO₂^-
SO₃ + H₂O ® H₂SO₄

these acids subsequently precipitate as rain, dew, hail, fog, snow or sleet and may disrupt aquatic ecosystems, cause dieback of forests and damage buildings and monuments.

2. Greenhouse effect: various gases (referred to as greenhouse gases) that are present in the Earth's atmosphere can absorb infra-red radiation (heat). Were it not for these gases, the average temperature at the surface of the Earth would be -18°C. Such greenhouse gases therefore make the Earth more easily habitable. The principal greenhouse gases are carbon dioxide, methane (CH₄), water vapour, nitrous oxide (N₂O), ozone (O₃) and halocarbons (including CFC). A significant proportion of the light energy incident from the sun travels through the atmosphere and strikes the surface of the Earth. This energy is re-radiated outward from the surface as infra-red radiation (heat). Whilst the atmosphere is transparent to visible wavelengths of light, the presence of greenhouse gases means that it is opaque to such long wavelength radiation. Much of the heat energy is therefore retained, raising the average temperature of the surface to 15°C. An excess of greenhouse gases may lead to greater retention of heat in the atmosphere causing global warming. The consequences of global warming are not well characterised but would almost certainly involve rising sea-levels and climatic change. Processes that cause production of greenhouse gases include combustion of fossil fuels, enteric fermentation in livestock, cultivation of rice in paddies and production of cement. Natural processes, including volcanic eruptions also contribute large quantities of greenhouse gases to the atmosphere.
3. Depletion of stratospheric ozone: approximately 90% of the ozone in the atmosphere of the Earth occurs in the stratosphere at altitudes of between 15km and 50km a.s.l. where it filters ultra-violet radiation from the sun, removing most of the radiation of wavelengths less that 300nm. UV radiation is a cause of sunburn and skin cancer and any depletion of ozone would magnify this threat. Destruction of stratospheric ozone may occur through the use of CFC (freons). Freons are used as aerosol propellants, refrigerants and as gases for the production of foamed plastics. Their use is widespread since they are chemically inert, non-toxic and non-flammable. The two commonest gases in this respect are CFC 11 (CFCl₃) and CFC 12 (CF₂Cl₂). Small amounts of these gases may travel through to the stratosphere. Once in the stratosphere, attack by UV radiation may lead to the photodissociation of chlorine atoms from CFC. These free chlorine atoms catalyse the destruction of ozone. The overall process being as follows:

Cl^- + 2O₃ ® 3O₂ + Cl^-
Each chlorine atom destroys two ozone molecules without itself being altered. In such a way, it can subsequently catalyse the destruction of further ozone.

GLOSSARY OF IMPORTANT TERMS
 
 

Anthropogenic: caused by human activity

Combustion: burning; chemically, this involves the addition of oxygen to the material that is being burnt.

Oxidation: addition of oxygen to another chemical. This may also occur through burning (e.g. oxidation of coal or oil) but also through other processes (e.g. rusting of iron, which is the oxidation of iron).

Photochemical reactions: chemical reactions driven by solar energy.

Photodissociation: breaking down by means of light.

Pollutants: in its most basic sense, chemicals that can cause pollution. No chemical is an absolute pollutant, since pollution is a construct of human perception. However, a pollutant may generally be considered a chemical that is in a position to lower the quality of the environment, as perceived by humans.

residence time: the length of time for which a particular chemical remains in the atmosphere.

Stratosphere: the layer of air directly above the troposphere. This extends up to a height of 50km above mean sea-level (a.s.l.). Very low concentrations of water are present (hence little moisture is available). Production of ozone occurs, peaking at an altitude of 22km a.s.l.; this is termed the 'ozone layer'

Troposphere: the lower part of the atmosphere, comprising the air from sea level to a height of 8-15km. This layer of air contains 99% of the water vapour present in the atmosphere.

Important data concerning atmopsheric pollution
 

Table 1: atmospheric levels and rates of change of various greenhouse gases

Gas	1980 level (ppb)	2030 level (ppb)	Rate of increase
Carbon dioxide	339000	450000	2.4% yr -1
Methane	1650	2340	0.7% yr -1
Nitrous oxide	300	375	0.45% yr -1
CFC 11	0.18	1.1	3% yr -1
CFC 12	0.28	1.8	3% yr -1
O3 (tropospheric)			0.23% yr -1

ppb = parts per billion

Table 2: approximate elemental composition of fossil fuels (% by weight)

 

Fuel	Carbon	Hydrogen	Oxygen	Sulphur	Nitrogen
Wood	50	6	43	0.5	0.5
Bituminous coal	82-92	4.0-5.5	2.5-8.0	0.7-1.0	1.5-1.9
Natural gas	74.8	24.5	0.4	0	0.2
Gasoline	85-88	12-15	0	<0.1	0
Diesel fuel	86.5	13.2	0	0.3	0
Fuel oils	85-86	11.4-13.2	0.1-0.4	0.5-3.0	0.1-0.4

 
 

Table 3: air quality classification (Department of the Environment, UK)

	NO2 (ppb)	SO2 (ppb)	O3 (ppb)
Very good	0-50	0-60	0-50
Good	50-100	60-125	50-100
Poor	100-300	125-500	100-200
Very poor	300+	500+	200+

 

Table 4: natural emission of compounds of Sulphur and Nitrogen

 

Source	Tg S yr -1	Source	Tg N yr -1
Volcanoes	<2	Lightning	8
Biogenic gases from land	35	NH3 oxidation	1-10
Biogenic gases from water	35	From stratosphere	0.5
Sea spray	171	Biogenic production	8
		Biomass burning	12
Natural total	243	Natural total	33
Anthropogenic	75	Anthropogenic	21

Tg S yr ^-1: teragrams of sulphur per year

Tg N yr ^-1 teragrams of nitrogen per year

All tables from: Clarke, A.G. (1992). The Atmosphere. In Harrison R.M. [ed.] Understanding our Environment: An Introduction to Environmental Chemistry and Pollution. Royal Society of Chemistry. Second Edition, 1992.