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air pollution
Introduction / Particulate matter / Hydrocarbons / Carbon monoxide / Sulphur oxides / Nitrogen oxides / Photochemical oxidants / References

There are various definitions for pollution in general and air pollution in the literature. Peavy et al. (1985) defines the pollution of ambient (outdoor) air as:

Air pollution is the presence in the outdoor atmosphere of one or more air contaminants (that is, dust, fumes, gas, mist, odour, smoke or vapour) in sufficient quantities, of such characteristics, and of such duration as to be or to threaten to be injurious to human, plant, or animal life or to property, or which reasonably interferes with the comfortable enjoyment of life or property.

This definition includes both natural and anthropogenic (manmade) sources. However, except in extreme cases like a volcanic eruption, pollution from natural sources does not usually pose problems severe enough to endanger life and property. Colls (1997) thus defines pollution, based on the tenth report of the Royal Commission on environmental pollution, as:

The introduction by man into the environment of substances or energy liable to cause hazard to human health, harm to living resources and ecological systems, damage to structure or amenity or interference with legitimate use of the environment.

Modern day legislation to address the effects of air pollution has been present since the 1950s, when the United States passed their Air Pollution Control Act in 1955. This act was later supplanted by the Clean Air Act of 1963, further strengthened by amendments in 1970 and 1977 (Peavy et al., 1985). Britain passed their Clean Air Act in 1956 and South Africa brought out the Air Pollution Prevention Act in 1965 (Act 45 of 1965). Due to these acts and public support during the latter years of the previous century, it is probable that the move towards air pollution control has gathered sufficient momentum to ensure a cleaner and healthier atmosphere for future generations.

In South Africa the current legislation governing air pollution in the country is the National Environmental Management: Air Quality Act (No 39 of 2004). At present the legislation provide ambient air quality standards for, inter alia, the following pollutants:

  • particulate matter,
  • sulphur oxides,
  • nitrogen oxides, and
  • ozone (photochemical oxidants).

A brief overview of these pollutants, as well as hydrocarbons and carbon monoxide are given in this section.

Particulate matter or particulates may be classified according to their physical, chemical or biological characteristics. Physical characteristics include size, mode of formation, settling properties and optical qualities. Chemical characteristics include organic or inorganic composition, and biological characteristics relate to their classification as bacteria, viruses, spores, pollens, etc. (Peavy et al., 1985)

2.1 Physical characteristics

Size is one of the most important physical properties of particulates and are measured in micrometres (μm), where one micrometre is equal to 10-6 metres. Particles larger than 50 μm can normally be seen with the naked eye.

Particulates are also classified according to their mode of formation as dust, smoke, fumes, fly-ash, mist or spray. The first four are solid particles, while the last two are liquid. Peavy et al. (1985) defines each of these modes of formation as follows:

  • Dust is small, solid particles created by the break-up of larger masses through processes such as grinding, crushing or blasting, and may come directly from the processing or handling of materials such as coal, cement or grains.

  • Smoke is described as fine, solid particles resulting from the incomplete combustion of organic particles such as coal, wood or tobacco.

  • Fumes are fine, solid particles formed by the condensation of vapours.

  • Fly ash consists of finely divided, non combustible particles contained in flue gases arising from combustion of coal.

  • Mist consists of liquid particles or droplets formed by the condensation of a vapour, the dispersion of a liquid (as in foaming or splashing), or the enactment of a chemical reaction.

  • Spray consists of liquid particles formed by the atomisation of parent liquids, such as pesticides and herbicides.

Settling properties are one of the most important properties of particulates, since settling is the major natural self-cleansing process for removal of particulates from the atmosphere. Peavy et al. (1985) classifies particulates, based on their settling properties, as follows:

  • suspended particulates vary in size from less than 1 μm to approximately 20 μm and remain suspended in the atmosphere for long periods of time.

  • settleable particles, or dustfall, are larger and heavier and settle out close to their sources. They are generally greater than 10 μm in size.

The surface properties of particulates, including adsorption, absorption, chemisorption, and adhesion, are particularly important factors in the settling process of particles less than 1 &Mu m. Settling of even smaller particles, those less than 0,1 μm in diameter, tends to be affected by a phenomenon known as Brownian motion. The random movement, or Brownian motion, of particles in the submicron range, causes them to collide with the surrounding molecules, then to coagulate, flocculate, and eventually settle out. (Peavy et al., 1985)

Optical qualities of particulates relates to one of the most obvious effects of air pollution, that of reduced visibility. The scattering of light by particulate matter is primarily responsible for such a reduction. Since visibility is affected by particle shape and surface characteristics, as well as by the distribution of the particles by size, accurate calculation of transmission and scattering of light is a highly complex procedure. However, an empirical mathematical relationship can be derived between visibility and particulate matter under highly specialised circumstances (Peavy et al., 1985):

2.2 Chemical characteristics

The chemical composition of particulates found in the atmosphere varies greatly and particulates contain both organic and inorganic components. Organics include, inter alia, phenols and alcohols and common inorganics found in particulates include nitrates, sulphates and certain metals such as iron and lead.

2.3 Biological characteristics

According to Peavy et al. (1985), the biological particles in the atmosphere include protozoa, viruses, bacteria, fungi, spores, pollens, and algae. Microorganisms generally survive for only a short time in the atmosphere because of the lack of nutrients and ultraviolet radiation from the sun. However, certain bacteria and fungi form spores and can survive for long periods.

2.4 Sources of particulates

Sources of particulates can either be natural, for example, pollen, spores, bacteria, viruses and volcanic dust, or anthropogenic, for example, fly ash, smoke, soot particles, metallic oxides and salts, silicates and other inorganic dusts, and metallic fumes.

2.5 Control of particulates

Although the control of particulate matter can be undertaken either at the source or by dilution, the idea that "dilution is the solution to pollution" is no longer applicable and cannot be considered a viable control method. Control at the source, the only acceptable method, may depend upon principles of sedimentation, centrifugation, impaction, filtration, or electric charge. (Peavy et al., 1985)

Organic compounds containing only carbon and hydrogen are classified as hydrocarbons. Most of the major chemicals in gasoline and other petroleum products are hydrocarbons, which are, according to Peavy et al. (1985), divided into two major classifications, namely aliphatic and aromatic.

3.1 Aliphatic hydrocarbons

The aliphatic hydrocarbon group contains alkanes, alkenes, and alkynes. The alkanes, saturated hydrocarbons (for example, methane) are fairly inert and generally not active in atmospheric photochemical reactions. However, the alkenes, often called olefins, are unsaturated and highly reactive in atmospheric photochemistry. The reactivity of alkenes such as ethylene makes them much more important in the study of air pollution than alkanes, because in the presence of sunlight they react with nitrogen dioxide at high concentrations to form secondary pollutants such as peroxyacetyl nitrate (PAN) and ozone (O3). The third series of aliphatics are the alkynes, which, though highly reactive, are relatively rare and thus not of major concern in air pollution studies. (Peavy et al., 1985)

3.2 Aromatic hydrocarbons

Aromatic hydrocarbons are all derived from, or related to, benzene and are biochemically and biologically active, and some potentially carcinogenic.

3.3 Sources of hydrocarbons

Hydrocarbons present in the atmosphere are from both natural and anthropogenic sources. Most natural hydrocarbons are from biological sources, though small amounts of these hydrocarbons come from geothermal areas, coal fields, natural gas from petroleum fields, and natural fires. The more complex, naturally produced hydrocarbons found in the atmosphere, such as volatile terpenes and isoprene, are produced by plants and trees. The terpene molecules combine to form aerosols that produce the "blue haze" over forested areas. The major anthropogenic source of hydrocarbons is from industrial installations. Until recently, transportation, including incomplete combustion from car engines, along with evaporative emissions from fuel tanks, crankcases, and carburetors, contributed the largest percentage of hydrocarbons. (Peavy et al., 1985)

3.4 Control of hydrocarbons

According to Peavy et al. (1985), the available control technology for hydrocarbons from stationary sources can be divided into five general classifications, namely:

  • incineration,
  • adsorption,
  • absorption,
  • condensation, and
  • substitution of other materials.

Colourless, tasteless, and odourless, carbon monoxide gas is chemically inert under normal conditions and has an estimated atmospheric mean life of about two and a half months. Carbon monoxide at present ambient levels has little if any effect on property, vegetation, or materials. However, at higher concentrations it can seriously affect human health.

4.1 Sources of carbon monoxide

Carbon monoxide sources are both natural and anthropogenic. Three and a half billion ton of CO is produced in nature yearly by the oxidation of methane gas from decaying vegetation (Peavy et al., 1985). The major source of anthropogenic origin is from transportation.

4.2 Control of carbon monoxide

Adsorption, absorption, condensation, and combustion are four basic technical control methods used for CO, and use of these methods can control almost all carbon monoxide emissions. Control of CO at source is far more desirable than control by dilution in the ambient air. (Peavy et al., 1985)

Sulphur oxides (SOx) are probably the most widespread and the most intensely studied of all anthropogenic air pollutants. They include six different gaseous compounds: sulphur monoxide (SO), sulphur dioxide (SO2), sulphur trioxide (SO3), sulphur tetroxide (SO4), sulphur sesquioxide (S2O3), and sulphur heptoxide (S2O7). Sulphur dioxide (SO2) and sulphur trioxide (SO3) are the two oxides of sulphur of most interest in the study of air pollution. (Peavy et al., 1985)

Sulphur dioxide is a colourless, non-flammable, nonexplosive gas with a suffocating odour and highly soluble in water. It is estimated that SO2 remains airborne an average of 2 to 4 days, during which time it may be transported as far as 1 000 km. Thus, the problem of SO2 pollution can become an international one. Relatively stable in the atmosphere, SO2 acts either as a reducing or an oxidising agent. SO2 can react with water to form sulphurous acid (Equation 1) and SO3 to form sulphuric acid (Equation 2).

SO2 + H2O <--> H2SO3 [Equation 1]

SO3 + H2O <--> H2SO4 [Equation 2]

Effects of sulphur oxides

High levels of sulphur dioxide causes injury to vegetation that can be classified as acute or chronic. The SO2 concentration in acute exposure is high for a short period, resulting in damage characterised by clearly marked dead tissue between the veins or on the margins of the leaves.

Sulphuric acid aerosols will also readily attack building materials, especially those containing carbonates such as marble, limestone, roofing slate and mortar. The carbonates are replaced by sulphates, which are water soluble, as the following equation indicates.

CaCO3 (limestone) + H2SO4 <--> CaSO4 + CO2 + H2O [Equation 3]

The calcium sulphate, or gypsum (CaSO4), formed in this process is washed away, leaving a pitted, discoloured surface. Sulphuric acid mists can also damage cotton, linen, rayon, and nylon. Leather also weakens and disintegrates in the presence of excess amounts of byproducts of SO2. Paper absorbs SO2, the SO2 is oxidized to H2SO4, and the paper yellows and becomes brittle. Excess exposure to SO2 accelerates corrosion rates for many metals such as iron, steel, zinc, copper, and nickel, especially at relative humidities over 70%. (Peavy et al., 1985)

Sources of sulphur oxides

The burning of solid and fossil fuel contributes more than 80% of anthropogenic SO2 emissions. Fuel combustion in stationary sources (primarily electric utilities) and industrial processes are the principal contributors of sulphur oxides from human sources. Transportation contributes little to the anthropogenic SOx in the atmosphere, because the sulphur content of gasoline is low (about 0,03% by mass). Present concern about automotive catalytic converters oxidizing SO2 to SO3 is of small consequence when compared to the potential dangers of carbon monoxide and hydrocarbon emissions. However, the SO3 can react with moisture in the air to produce H2SO4 mist. (Peavy et al., 1985)

Control of sulphur oxides

The broad-based methods for control of sulphur oxide emissions include burning fuel with less sulphur, removing sulphur from fuel, converting coal by liquefaction or gasification, substitution of another energy source, cleaning up the combustion products, or dispersion by tall stacks. (Peavy et al., 1985)

Nitrogen oxides (NOx) include six known gaseous compounds: nitric oxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O), nitrogen sesquioxide (N2O3), nitrogen tetroxide (N2O4), and nitrogen pent oxide (N2O5). The two nitrogen oxides of primary concern in air pollution are nitric oxide (NO) and nitrogen dioxide (NO2), the only two oxides of nitrogen that are emitted in significant quantities to the atmosphere. Heavier than air, nitrogen dioxide (NO2) is readily soluble in water, forming nitric acid and either nitrous acid or nitric oxide as indicated in the following equations (Peavy et al., 1985):

2NO2 + H2O <--> HNO3 + HNO2 (nitrous acid) [Equation 4]

3NO2 + H2O <--> 2HNO3 + NO (nitric oxide) [Equation 5]

Both nitric and nitrous acid will fall out in the rain or combine with ammonia (NH3) in the atmosphere to form ammonium nitrate (NH4NO3). In this instance, the NO2 will produce a plant nutrient. A good absorber of energy in the ultraviolet range, NO2 consequently plays a major role in the production of secondary air contaminants such as ozone (O3). Nitric oxide (NO) is emitted to the atmosphere in much larger quantities than NO2. It is formed in high temperature combustion processes when atmospheric oxygen and nitrogen combine according to the following reaction (Peavy et al., 1985):

N2 + O2 <--> 2NO [Equation 6]

6.1 Effects of nitrogen oxides

There is no evidence that NO is damaging to plants outside the laboratory, and while NO2 and primary pollutants can cause some injury to vegetation, peroxyacetyl nitrates (PAN) and O3, secondary pollutants produced during photochemical reactions involving NOx, are far more likely to be damaging to plants. Exposure to high levels of NO2 can cause fading of textile dyes, yellowing of white fabric, and oxidation of metals. (Peavy et al., 1985)

6.2 Formation of secondary pollutants

Photochemically, nitrogen oxides are one of the two groups of chemical compounds, which are the necessary ingredients for the production of photochemical smog, according to the following oversimplified equation (Peavy et al., 1985):

hydrocarbons + NOx + sunlight <--> smog [Equation 7]

According to Peavy et al., (1985), there are many complex reactions taking place, and the exact reactions, which lead to smog, are as yet, unknown. However, it is known that NO2 functions primarily as the light energy absorber and that in the presence of smog there are elevated levels of the oxidants. The major process by which NO2 is formed in the atmosphere is:

O3 + NO <--> O2 + NO2 [Equation 8]

Hydroperoxyl radicals may also react with NO to generate NO2 and hydroxyl radicals:

HO2 + NO <--> HO + NO2 [Equation 9]

and alkylperoxyl radicals can react to oxidize NO to generate alkyloxyl radicals and NO2:

RO2 + NO <--> RO + NO2 [Equation 10]

The effect is rapid cycling of NO2, and no overall effect would result if it were not for a series of competing reactions involving the hydrocarbons. The hydrocarbon reactions cause the photolytic cycle to be unbalanced. The O (atomic oxygen) reacts with the hydrocarbons to produce a reactive intermediate species called alkylperoxyl radicals (RO2). These free radicals react rapidly with NO to produce NO2. This removes the NO from the cycle and, thus, the reaction that would remove O3 from the system is eliminated, causing the O3 concentration to increase in the atmosphere. The end product of these photochemical reactions is photochemical smog consisting of air contaminants such as O3, PAN, aldehydes, alkyl nitrates, ketones, and carbon monoxide. (Peavy et al., 1985)

6.3 Sources of nitrogen oxides

Some nitrogen oxides are produced naturally and others are anthropogenic in source. Small concentrations of the NOx produced in the upper atmosphere by solar radiation reach the lower atmosphere through downward diffusion. Small amounts of NOx are produced by lightning and forest fires. Bacterial decomposition of organic matter also releases NOx into the atmosphere. In fact, the naturally occurring forces of NOx produce approximately 10 times as much NOx as do anthropogenic sources, which are concentrated in urban areas. Primary origins of human induced NOx are fuel combustion in stationary sources and in transportation. Both NO and NO2 also exhibit distinct diurnal variations, depending upon solar radiation, meteorological phenomena, and traffic volume. (Peavy et al., 1985)

6.4 Control of nitrogen oxides

In general, most control measures for NOx emissions have been directed at modification of combustion conditions to decrease NOx production and at utilisation of various devices to remove NOx from exhaust gas streams. (Peavy et al., 1985)

Oxidants or total oxidants, two terms used to describe levels of photochemical oxidants, generally indicate the net oxidizing ability of the ambient air. Ozone (O3), the major photochemical oxidant, makes up approximately 90% of the oxidant pool.

7.1 Effects of oxidants

The major photochemical oxidants that can cause damage to plants are ozone (O3) and peroxyacetyl nitrate (PAN). (Peavy et al., 1985)

7.2 Sources of oxidants

Ozone, the photochemical oxidant of major concern in air pollution, is produced in the upper atmosphere by solar radiation, and small concentrations of this gas diffuse downwards. Also, small concentrations are produced by lightning and forest fires. Anthropogenic precursors are usually involved where oxidant levels in rural areas have exceeded ambient standards. Ozone concentrations often are higher in suburban and even rural areas than in urban areas because O3 in urban areas is lost to reactions with NO. (Peavy et al., 1985)

7.3 Control of oxidants

Oxidant control strategy has traditionally been aimed at limiting hydrocarbon and nitrogen oxide emissions, the precursors of the oxidants. However, it is now known that that even if no hydrocarbons or aldehydes are present in the atmosphere, significant concentrations of ozone can still be generated as long as CO and NOx are present Currently, despite concerted efforts to control CO, HC and NOx emissions, quantities of these air contaminants, sufficient to photochemically generate ozone, are still present. (Peavy et al., 1985)

COLLS J (1997) Air Pollution, an introduction. E & FN Spon, London.

PEAVY HS, ROWE DR and TCHOBANOGLOUS G (1985) Environmental Engineering. McGraw Hill, New York.
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