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Introduction / Structure and composition of the Atmosphere / Units for pollutant concentration / Natural and anthropogenic emissions / Classification of pollutants / Indoor air pollution / Air quality indexes / References

Although air management, and more particular, air pollution concepts have developed in a highly technical and scientific orientated field, education in such matters should start from an early age. With this in mind, an overview of certain air management concepts are provided in this section, keeping in mind that it unfortunately most of the time is slanted towards technical/scientific explanations. Apart from concepts explained in the air pollution and health effects sections, some additional information with regard air management is given in this section. Issues covered are as follows:

  • structure and composition of the atmosphere,
  • units for pollutant concentration,
  • natural and anthropogenic emissions,
  • classification of pollutants,
  • indoor air pollution, and
  • air quality indexes.

A necessary first step toward understanding air pollution and its control is understanding the structure and composition of the atmosphere. The atmosphere consists of four layers, namely the troposphere, stratosphere, mesosphere and the thermosphere (see Figure 1). Of these layers, the troposphere is of greatest interest in pollution control as this is the layer in which most living things exist. According to Giddings (1973), the air in the troposphere consists, inter alia, of about 78% nitrogen (N2), 21% oxygen (O2), 1% argon (Ar), and 0,03% carbon dioxide (CO2), on a volume basis.

fig 1.1 One of the more recent changes in the troposphere involves the phenomenon of acid rain. Acid rain, or acid deposition, results when gaseous emissions of sulphur oxides (SOx) and nitrogen oxides (NOx) interact with water vapour and sunlight and are chemically converted to strong acidic compounds such as sulphuric acid (H2SO4) and nitric acid (HNO3). These compounds, along with other organic and inorganic chemicals, are deposited on the earth as aerosols and particulates (dry deposition) or are carried to the earth by raindrops, snowflakes, fog or dew (wet deposition). (Peavy et al., 1985)

The damage to buildings and monuments, related to acid rain, is not limited to the immediate area of release. Tall stacks disperse pollutants into the upper reaches of the troposphere where they may remain for days, often being carried long distances. Pollutants that are generated in one country and deposited in another have become a matter of international concern.

Peavy et al. (1985) states that the ozone (O3) layer in the stratosphere is being depleted as ozone reacts with chlorine released from the fluorocarbons used as aerosol spray propellants. Since the O3 in the atmosphere reduces the ultraviolet radiation that reaches the earth's surface, and since ultraviolet radiation at high levels can damage plants and animals, loss of O3 represents a potentially serious problem. In light of this danger, some industrialised nations have banned the use of fluorocarbons.

The amount of tropospheric carbon dioxide (CO2) is reported by Peavy et al., (1985) to be increasing at a rate of 1,8 mg/m3 per year, a process that may not be reversible. This increase has also been accompanied by an equivalent decrease in atmospheric oxygen (O2).

In air quality management, both volumetric and gravimetric concentration units are used. The recommended unit of measurement for suspended particles and gaseous contaminants are micrograms per cubic metre (μg/m3). However, gaseous contaminants are often given in parts per million (ppm) by volume. To convert parts per million (ppm) to micrograms per cubic meter (μg/m3), the following equation is used:

[Equation 1]

where the l/mol term, according to Avogadro's law, is equal to 22,4 at standard conditions (temperature 0 °C or 273 K and pressure is 101,3 kPa or 1 atmosphere or 760 mm Hg). However, most often other than standard conditions exist and the following equation, derived from the ideal gas law, is used:

[Equation 2]

All air contains natural contaminants such as pollen, fungi spores, salt spray, and smoke and dust particles from forest fires and volcanic eruptions. It also contains naturally occurring carbon monoxide (CO) from the breakdown of methane (CH4); hydrocarbons in the form of terpenes from pine trees; and hydrogen sulphide (H2S) and methane (CH4) from the anaerobic decomposition of organic matter. (Peavy et al., 1985) The five most prevalent air pollutants from anthropogenic (manmade) origin are:

  • total suspended particulates,
  • sulphur oxides,
  • nitrogen oxides,
  • hydrocarbons, and
  • carbon monoxide.

The sources of these pollutants are so numerous that they have been categorised into five main groups, namely (Peavy et al., 1985):

  • transportation (motor vehicles, aircraft, railroads, ships, and the handling and/or evaporation of gasoline),

  • stationary combustion (residential, commercial, and industrial power and heating, including steam-powered electric power plants),

  • industrial processes (chemical, metallurgical, and pulp-paper industries and petroleum refineries),

  • solid-waste disposal (household and commercial refuse, and coal refuse), and

  • miscellaneous (forest fires, agricultural burning, etc).

All air pollutants may be classified according to origin, chemical composition, and state of matter (Peavy et al., 1985).

5.1 Origin

Pollutants are considered as either primary or secondary contaminants, based on their origin. Primary pollutants such as sulphur oxides (SOx), nitrogen oxides (NOx), and hydrocarbons (HC) are those emitted directly to the atmosphere and found there in the form in which they were emitted. Secondary pollutants such as ozone (O3) and peroxyacetyl nitrate (PAN) are those formed in the atmosphere by a photochemical reaction or by hydrolysis or oxidation. (Peavy et al., 1985)

5.2 Chemical composition

Primary and secondary pollutants may be further classified according to their chemical composition, as either organic or inorganic. Organic compounds contains carbon and hydrogen, and many also contain elements such as oxygen, nitrogen, phosphorus and sulphur. Inorganic materials found in contaminated atmosphere include carbon monoxide (CO), carbon dioxide (CO2), carbonates, sulphur oxides, nitrogen oxides, ozone, hydrogen fluoride and hydrogen chloride. (Peavy et al., 1985)

5.3 State of matter

Particulate pollutants are finely divided solids and liquids which include dust, fumes, smoke, fly ash, mist and spray. These pollutants, under proper conditions, will settle out of the atmosphere. Gaseous pollutants, formless fluids that completely occupy the space into which they are released, behave much as air and do not settle out of the atmosphere. Among common gaseous pollutants are carbon oxides, sulphur oxides, hydrocarbons, and oxidants. (Peavy et al., 1985)

Until recently, the principal actions taken to control air pollution have emphasised emissions to the atmosphere. However, since most people spend a large percentage of their time indoors, attention has recently been given to the health problems posed by air pollutants that originate from building materials, furnishings, equipment, and such human activities as cooking, cleaning, and smoking. The National Academy of Sciences (NAS) in the United States has identified several potential indoor air pollutants for which there is evidence of adverse health effects. Formaldehyde, tobacco smoke, radon and radon daughters, unvented combustion appliances, aero pathogens, pesticides, and asbestos are included in the NAS listings. (Peavy et al., 1985)

Although air quality indexes have not yet been set for South Africa, it is interesting to note that, as an example, the Environmental Protection Agency, the Council on Environmental Quality, and several agencies of the Department of Commerce in the United States cooperated in developing a pollutant standard index (PSI) in order to integrate the several complex factors that make up "air quality". This index combines the ambient measures of the five major criteria pollutants into numbers ranging from 0 to 500. The pollutants rated are carbon monoxide, sulphur dioxide, total suspended particulates, photochemical oxidants or ozone, and nitrogen dioxide. If the concentration of any one of the five major and initial criteria pollutants rises to the level of its air quality standard at any monitoring station, the air quality in the area is deemed unhealthy for that particular day, even though concentrations of the other four criteria pollutants may be below the national standards.

Air quality is called "good" only when the ambient measures of all five criteria pollutants have an index value of 50 or less (that is, a value less than half that allowed by the standards).

GIDDINGS JC (1973) Chemistry, Man and Environmental Change. Canfield, San Francisco.

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

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