Air pollution management aims at the elimination, or reduction to acceptable levels, of airborne gaseous pollutants, suspended particulate matter and physical and, to a certain extent, biological agents whose presence in the atmosphere can cause adverse effects on human health (e.g., irritation, increase of incidence or prevalence of respiratory diseases, morbidity, cancer, excess mortality) or welfare (e.g., sensory effects, reduction of visibility), deleterious effects on animal or plant life, damage to materials of economic value to society and damage to the environment (e.g., climatic modifications). The serious hazards associated with radioactive pollutants, as well as the special procedures required for their control and disposal, also deserve careful attention.
The importance of efficient management of outdoor and indoor air pollution cannot be overemphasized. Unless there is adequate control, the multiplication of pollution sources in the modern world may lead to irreparable damage to the environment and mankind.
The objective of this article is to give a general overview of the possible approaches to the management of ambient air pollution from motor vehicle and industrial sources. However, it is to be emphasized from the very beginning that indoor air pollution (in particular, in developing countries) might play an even larger role than outdoor air pollution due to the observation that indoor air pollutant concentrations are often substantially higher than outdoor concentrations.
Beyond considerations of emissions from fixed or mobile sources, air pollution management involves consideration of additional factors (such as topography and meteorology, and community and government participation, among many others) all of which must be integrated into a comprehensive programme. For example, meteorological conditions can greatly affect the ground-level concentrations resulting from the same pollutant emission. Air pollution sources may be scattered over a community or a region and their effects may be felt by, or their control may involve, more than one administration. Furthermore, air pollution does not respect any boundaries, and emissions from one region may induce effects in another region by long-distance transport.
Air pollution management, therefore, requires a multidisciplinary approach as well as a joint effort by private and governmental entities.
Sources of Air Pollution
The sources of man-made air pollution (or emission sources) are of basically two types:
- stationary, which can be subdivided into area sources such as agricultural production, mining and quarrying, industrial, point and area sources such as manufacturing of chemicals, nonmetallic mineral products, basic metal industries, power generation and community sources (e.g., heating of homes and buildings, municipal waste and sewage sludge incinerators, fireplaces, cooking facilities, laundry services and cleaning plants)
- mobile, comprising any form of combustion-engine vehicles (e.g., light-duty gasoline powered cars, light- and heavy-duty diesel powered vehicles, motorcycles, aircraft, including line sources with emissions of gases and particulate matter from vehicle traffic).
In addition, there are also natural sources of pollution (e.g., eroded areas, volcanoes, certain plants which release great amounts of pollen, sources of bacteria, spores and viruses). Natural sources are not discussed in this article.
Types of Air Pollutants
Air pollutants are usually classified into suspended particulate matter (dusts, fumes, mists, smokes), gaseous pollutants (gases and vapours) and odours. Some examples of usual pollutants are presented below:
Suspended particulate matter (SPM, PM-10) includes diesel exhaust, coal fly-ash, mineral dusts (e.g., coal, asbestos, limestone, cement), metal dusts and fumes (e.g., zinc, copper, iron, lead) and acid mists (e.g., sulphuric acid), fluorides, paint pigments, pesticide mists, carbon black and oil smoke. Suspended particulate pollutants, besides their effects of provoking respiratory diseases, cancers, corrosion, destruction of plant life and so on, can also constitute a nuisance (e.g., accumulation of dirt), interfere with sunlight (e.g., formation of smog and haze due to light scattering) and act as catalytic surfaces for reaction of adsorbed chemicals.
Gaseous pollutants include sulphur compounds (e.g., sulphur dioxide (SO2) and sulphur trioxide (SO3)), carbon monoxide, nitrogen compounds (e.g., nitric oxide (NO), nitrogen dioxide (NO2), ammonia), organic compounds (e.g., hydrocarbons (HC), volatile organic compounds (VOC), polycyclic aromatic hydrocarbons (PAH), aldehydes), halogen compounds and halogen derivatives (e.g., HF and HCl), hydrogen sulphide, carbon disulphide and mercaptans (odours).
Secondary pollutants may be formed by thermal, chemical or photochemical reactions. For example, by thermal action sulphur dioxide can oxidize to sulphur trioxide which, dissolved in water, gives rise to the formation of sulphuric acid mist (catalysed by manganese and iron oxides). Photochemical reactions between nitrogen oxides and reactive hydrocarbons can produce ozone (O3), formaldehyde and peroxyacetyl nitrate (PAN); reactions between HCl and formaldehyde can form bis-chloromethyl ether.
While some odours are known to be caused by specific chemical agents such as hydrogen sulphide (H2S), carbon disulphide (CS2) and mercaptans (R-SH or R1-S-R2) others are difficult to define chemically.
Table 1. Common atmospheric pollutants and their sources
SPM, CO, VOC
Non-ferrous ore mining
SPM, SO2, NOx, VOC
Food, beverages and tobacco
Textiles and leather industries
Paper products, printing
SPM, CO, VOC, H2S
SPM, SO2, CO, VOC, H2S, R-SH
Lead oxide and pigments
Maleic anhydride and
Synthetic resins, plastic
Paints, varnishes, lacquers
Carbon black and printing ink
SPM, SO2, CO, VOC
SPM, SO2, NOx, CO, VOC, NH3
SPM, NOx, CO, VOC
SPM, NH3, HNO3
SPM, VOC, H2S, CS2
SPM, SO2, NOx, CO, VOC, H2S
SPM, SO2, NOx, SO3, HNO3
SPM, SO2, NOx, CO, VOC
Structural clay products
Cement, lime and plaster
SPM, SO2, NOx, CO, VOC, F
SPM, SO2, NOx, CO, VOC, F2
SPM, SO2, NOx, CO
Basic metal industries
Iron and steel
SPM, SO2, NOx, CO, VOC, Pb
SPM, SO2, F, Pb
Electricity, gas and steam
SPM, SO2, NOx, CO, VOC, SO3, Pb
Fuel storage, filling operations
SPM, SO2, NOx, CO, VOC, Pb
SPM, SO2, NOx, CO, VOC, Pb
Source: Economopoulos 1993
Clean Air Implementation Plans
Air quality management aims at the preservation of environmental quality by prescribing the tolerated degree of pollution, leaving it to the local authorities and polluters to devise and implement actions to ensure that this degree of pollution will not be exceeded. An example of legislation within this approach is the adoption of ambient air quality standards based, very often, on air quality guidelines (WHO 1987) for different pollutants; these are accepted maximum levels of pollutants (or indicators) in the target area (e.g., at ground level at a specified point in a community) and can be either primary or secondary standards. Primary standards (WHO 1980) are the maximum levels consistent with an adequate safety margin and with the preservation of public health, and must be complied with within a specific time limit; secondary standards are those judged to be necessary for protection against known or anticipated adverse effects other than health hazards (mainly on vegetation) and must be complied “within a reasonable time”. Air quality standards are short-, medium- or long-term values valid for 24 hours per day, 7 days per week, and for monthly, seasonal or annual exposure of all living subjects (including sensitive subgroups such as children, the elderly and the sick) as well as non-living objects; this is in contrast to maximum permissible levels for occupational exposure, which are for a partial weekly exposure (e.g., 8 hours per day, 5 days per week) of adult and supposedly healthy workers.
Typical measures in air quality management are control measures at the source, for example, enforcement of the use of catalytic converters in vehicles or of emission standards in incinerators, land-use planning and shut-down of factories or reduction of traffic during unfavourable weather conditions. The best air quality management stresses that the air pollutant emissions should be kept to a minimum; this is basically defined through emission standards for single sources of air pollution and could be achieved for industrial sources, for example, through closed systems and high-efficiency collectors. An emission standard is a limit on the amount or concentration of a pollutant emitted from a source. This type of legislation requires a decision, for each industry, on the best means of controlling its emissions (i.e., fixing emission standards).
The basic aim of air pollution management is to derive a clean air implementation plan (or air pollution abatement plan) (Schwela and Köth-Jahr 1994) which consists of the following elements:
- description of area with respect to topography, meteorology and socioeconomy
- emissions inventory
- comparison with emission standards
- air pollutant concentrations inventory
- simulated air pollutant concentrations
- comparison with air quality standards
- inventory of effects on public health and the environment
- causal analysis
- control measures
- cost of control measures
- cost of public health and environmental effects
- cost-benefit analysis (costs of control vs. costs of efforts)
- transportation and land-use planning
- enforcement plan; resource commitment
- projections for the future on population, traffic, industries and fuel consumption
- strategies for follow-up.
Some of these issues will be described below.
Emissions Inventory; Comparison with Emission Standards
The emissions inventory is a most complete listing of sources in a given area and of their individual emissions, estimated as accurately as possible from all emitting point, line and area (diffuse) sources. When these emissions are compared with emission standards set for a particular source, first hints on possible control measures are given if emission standards are not complied with. The emissions inventory also serves to assess a priority list of important sources according to the amount of pollutants emitted, and indicates the relative influence of different sources—for example, traffic as compared to industrial or residential sources. The emissions inventory also allows an estimate of air pollutant concentrations for those pollutants for which ambient concentration measurements are difficult or too expensive to perform.
Air Pollutant Concentrations Inventory; Comparison with Air Quality Standards
The air pollutant concentrations inventory summarizes the results of the monitoring of ambient air pollutants in terms of annual means, percentiles and trends of these quantities. Compounds measured for such an inventory include the following:
- sulphur dioxide
- nitrogen oxides
- suspended particulate matter
- carbon monoxide
- heavy metals (Pb, Cd, Ni, Cu, Fe, As, Be)
- polycyclic aromatic hydrocarbons: benzo(a)pyrene, benzo(e)pyrene, benzo(a)anthracene, dibenzo(a,h)anthracene, benzoghi)perylene, coronen
- volatile organic compounds: n-hexane, benzene, 3-methyl-hexane, n-heptane, toluene, octane, ethyl-benzene xylene (o-,m-,p-), n-nonane, isopropylbenzene, propylbenezene, n-2-/3-/4-ethyltoluene, 1,2,4-/1,3,5-trimethylbenzene, trichloromethane, 1,1,1 trichloroethane, tetrachloromethane, tri-/tetrachloroethene.
Comparison of air pollutant concentrations with air quality standards or guidelines, if they exist, indicates problem areas for which a causal analysis has to be performed in order to find out which sources are responsible for the non-compliance. Dispersion modelling has to be used in performing this causal analysis (see “Air pollution: Modelling of air pollutant dispersion”). Devices and procedures used in today’s ambient air pollution monitoring are described in “Air quality monitoring”.
Simulated Air Pollutant Concentrations; Comparison with Air Quality Standards
Starting from the emissions inventory, with its thousands of compounds which cannot all be monitored in the ambient air for economy reasons, use of dispersion modelling can help to estimate the concentrations of more “exotic” compounds. Using appropriate meteorology parameters in a suitable dispersion model, annual averages and percentiles can be estimated and compared to air quality standards or guidelines, if they exist.
Inventory of Effects on Public Health and the Environment; Causal Analysis
Another important source of information is the effects inventory (Ministerium für Umwelt 1993), which consists of results of epidemiological studies in the given area and of effects of air pollution observed in biological and material receptors such as, for example, plants, animals and construction metals and building stones. Observed effects attributed to air pollution have to be causally analysed with respect to the component responsible for a particular effect—for example, increased prevalence of chronic bronchitis in a polluted area. If the compound or compounds have been fixed in a causal analysis (compound-causal analysis), a second analysis has to be performed to find out the responsible sources (source-causal analysis).
Control Measures; Cost of Control Measures
Control measures for industrial facilities include adequate, well-designed, well-installed, efficiently operated and maintained air cleaning devices, also called separators or collectors. A separator or collector can be defined as an “apparatus for separating any one or more of the following from a gaseous medium in which they are suspended or mixed: solid particles (filter and dust separators), liquid particles (filter and droplet separator) and gases (gas purifier)”. The basic types of air pollution control equipment (discussed further in “Air pollution control”) are the following:
- for particulate matter: inertial separators (e.g., cyclones); fabric filters (baghouses); electrostatic precipitators; wet collectors (scrubbers)
- for gaseous pollutants: wet collectors (scrubbers); adsorption units (e.g., adsorption beds); afterburners, which can be direct-fired (thermal incineration) or catalytic (catalytic combustion).
Wet collectors (scrubbers) can be used to collect, at the same time, gaseous pollutants and particulate matter. Also, certain types of combustion devices can burn combustible gases and vapours as well as certain combustible aerosols. Depending on the type of effluent, one or a combination of more than one collector can be used.
The control of odours that are chemically identifiable relies on the control of the chemical agent(s) from which they emanate (e.g., by absorption, by incineration). However, when an odour is not defined chemically or the producing agent is found at extremely low levels, other techniques may be used, such as masking (by a stronger, more agreeable and harmless agent) or counteraction (by an additive which counteracts or partially neutralizes the offensive odour).
It should be kept in mind that adequate operation and maintenance are indispensable to ensure the expected efficiency from a collector. This should be ensured at the planning stage, both from the know-how and financial points of view. Energy requirements must not be overlooked. Whenever selecting an air cleaning device, not only the initial cost but also operational and maintenance costs should be considered. Whenever dealing with high-toxicity pollutants, high efficiency should be ensured, as well as special procedures for maintenance and disposal of waste materials.
The fundamental control measures in industrial facilities are the following:
Substitution of materials. Examples: substitution of less toxic solvents for highly toxic ones used in certain industrial processes; use of fuels with lower sulphur content (e.g., washed coal), therefore giving rise to less sulphur compounds and so on.
Modification or change of the industrial process or equipment. Examples: in the steel industry, a change from raw ore to pelleted sintered ore (to reduce the dust released during ore handling); use of closed systems instead of open ones; change of fuel heating systems to steam, hot water or electrical systems; use of catalysers at the exhaust air outlets (combustion processes) and so on.
Modifications in processes, as well as in plant layout, may also facilitate and/or improve the conditions for dispersion and collection of pollutants. For example, a different plant layout may facilitate the installation of a local exhaust system; the performance of a process at a lower rate may allow the use of a certain collector (with volume limitations but otherwise adequate). Process modifications that concentrate different effluent sources are closely related to the volume of effluent handled, and the efficiency of some air-cleaning equipment increases with the concentration of pollutants in the effluent. Both the substitution of materials and the modification of processes may have technical and/or economic limitations, and these should be considered.
Adequate housekeeping and storage. Examples: strict sanitation in food and animal product processing; avoidance of open storage of chemicals (e.g., sulphur piles) or dusty materials (e.g., sand), or, failing this, spraying of the piles of loose particulate with water (if possible) or application of surface coatings (e.g., wetting agents, plastic) to piles of materials likely to give off pollutants.
Adequate disposal of wastes. Examples: avoidance of simply piling up chemical wastes (such as scraps from polymerization reactors), as well as of dumping pollutant materials (solid or liquid) in water streams. The latter practice not only causes water pollution but can also create a secondary source of air pollution, as in the case of liquid wastes from sulphite process pulp mills, which release offensive odorous gaseous pollutants.
Maintenance. Example: well maintained and well-tuned internal combustion engines produce less carbon monoxide and hydrocarbons.
Work practices. Example: taking into account meteorological conditions, particularly winds, when spraying pesticides.
By analogy with adequate practices at the workplace, good practices at the community level can contribute to air pollution control - for example, changes in the use of motor vehicles (more collective transportation, small cars and so on) and control of heating facilities (better insulation of buildings in order to require less heating, better fuels and so on).
Control measures in vehicle emissions are adequate and efficient mandatory inspection and maintenance programmes which are enforced for the existing car fleet, programmes of enforcement of the use of catalytic converters in new cars, aggressive substitution of solar/battery-powered cars for fuel-powered ones, regulation of road traffic, and transportation and land use planning concepts.
Motor vehicle emissions are controlled by controlling emissions per vehicle mile travelled (VMT) and by controlling VMT itself (Walsh 1992). Emissions per VMT can be reduced by controlling vehicle performance - hardware, maintenance - for both new and in-use cars. Fuel composition of leaded gasoline may be controlled by reducing lead or sulphur content, which also has a beneficial effect on decreasing HC emissions from vehicles. Lowering the levels of sulphur in diesel fuel as a means to lower diesel particulate emission has the additional beneficial effect of increasing the potential for catalytic control of diesel particulate and organic HC emissions.
Another important management tool for reducing vehicle evaporative and refuelling emissions is the control of gasoline volatility. Control of fuel volatility can greatly lower vehicle evaporative HC emissions. Use of oxygenated additives in gasoline lowers HC and CO exhaust as long as fuel volatility is not increased.
Reduction of VMT is an additional means of controlling vehicle emissions by control strategies such as
- use of more efficient transportation modes
- increasing the average number of passengers per car
- spreading congested peak traffic loads
- reducing travel demand.
While such approaches promote fuel conservation, they are not yet accepted by the general population, and governments have not seriously tried to implement them.
All these technological and political solutions to the motor vehicle problem except substitution of electrical cars are increasingly offset by growth in the vehicle population. The vehicle problem can be solved only if the growth problem is addressed in an appropriate way.
Cost of Public Health and Environmental Effects; Cost-Benefit Analysis
The estimation of the costs of public health and environmental effects is the most difficult part of a clean air implementation plan, as it is very difficult to estimate the value of lifetime reduction of disabling illnesses, hospital admission rates and hours of work lost. However, this estimation and a comparison with the cost of control measures is absolutely necessary in order to balance the costs of control measures versus the costs of no such measure undertaken, in terms of public health and environmental effects.
Transportation and Land-Use Planning
The pollution problem is intimately connected to land-use and transportation, including issues such as community planning, road design, traffic control and mass transportation; to concerns of demography, topography and economy; and to social concerns (Venzia 1977). In general, the rapidly growing urban aggregations have severe pollution problems due to poor land-use and transportation practices. Transportation planning for air pollution control includes transportation controls, transportation policies, mass transit and highway congestion costs. Transportation controls have an important impact on the general public in terms of equity, repressiveness and social and economic disruption - in particular, direct transportation controls such as motor vehicle constraints, gasoline limitations and motor vehicle emission reductions. Emission reductions due to direct controls can be reliably estimated and verified. Indirect transportation controls such as reduction of vehicle miles travelled by improvement of mass transit systems, traffic flow improvement regulations, regulations on parking lots, road and gasoline taxes, car-use permissions and incentives for voluntary approaches are mostly based on past trial-and-error experience, and include many uncertainties when trying to develop a viable transportation plan.
National action plans incurring indirect transportation controls can affect transportation and land-use planning with regard to highways, parking lots and shopping centres. Long-term planning for the transportation system and the area influenced by it will prevent significant deterioration of air quality and provide for compliance with air quality standards. Mass transit is consistently considered as a potential solution for urban air pollution problems. Selection of a mass transit system to serve an area and different modal splits between highway use and bus or rail service will ultimately alter land-use patterns. There is an optimum split that will minimize air pollution; however, this may not be acceptable when non-environmental factors are considered.
The automobile has been called the greatest generator of economic externalities ever known. Some of these, such as jobs and mobility, are positive, but the negative ones, such as air pollution, accidents resulting in death and injury, property damage, noise, loss of time, and aggravation, lead to the conclusion that transportation is not a decreasing cost industry in urbanized areas. Highway congestion costs are another externality; lost time and congestion costs, however, are difficult to determine. A true evaluation of competing transportation modes, such as mass transportation, cannot be obtained if travel costs for work trips do not include congestion costs.
Land-use planning for air pollution control includes zoning codes and performance standards, land-use controls, housing and land development, and land-use planning policies. Land-use zoning was the initial attempt to accomplish protection of the people, their property and their economic opportunity. However, the ubiquitous nature of air pollutants required more than physical separation of industries and residential areas to protect the individual. For this reason, performance standards based initially on aesthetics or qualitative decisions were introduced into some zoning codes in an attempt to quantify criteria for identifying potential problems.
The limitations of the assimilative capacity of the environment must be identified for long-term land-use planning. Then, land-use controls can be developed that will prorate the capacity equitably among desired local activities. Land-use controls include permit systems for review of new stationary sources, zoning regulation between industrial and residential areas, restriction by easement or purchase of land, receptor location control, emission-density zoning and emission allocation regulations.
Housing policies aimed at making home ownership available to many who could otherwise not afford it (such as tax incentives and mortgage policies) stimulate urban sprawl and indirectly discourage higher-density residential development. These policies have now proven to be environmentally disastrous, as no consideration was given to the simultaneous development of efficient transportation systems to serve the needs of the multitude of new communities being developed. The lesson learnt from this development is that programmes impacting on the environment should be coordinated, and comprehensive planning undertaken at the level where the problem occurs and on a scale large enough to include the entire system.
Land-use planning must be examined at national, provincial or state, regional and local levels to adequately ensure long-term protection of the environment. Governmental programmes usually start with power plant siting, mineral extraction sites, coastal zoning and desert, mountain or other recreational development. As the multiplicity of local governments in a given region cannot adequately deal with regional environmental problems, regional governments or agencies should coordinate land development and density patterns by supervising the spatial arrangement and location of new construction and use, and transportation facilities. Land-use and transportation planning must be interrelated with enforcement of regulations to maintain the desired air quality. Ideally, air pollution control should be planned for by the same regional agency that does land-use planning because of the overlapping externalities associated with both issues.
Enforcement Plan, Resource Commitment
The clean air implementation plan should always contain an enforcement plan which indicates how the control measures can be enforced. This implies also a resource commitment which, according to a polluter pays principle, will state what the polluter has to implement and how the government will help the polluter in fulfilling the commitment.
Projections for the Future
In the sense of a precautionary plan, the clean air implementation plan should also include estimates of the trends in population, traffic, industries and fuel consumption in order to assess responses to future problems. This will avoid future stresses by enforcing measures well in advance of imagined problems.
Strategies for Follow-up
A strategy for follow-up of air quality management consists of plans and policies on how to implement future clean air implementation plans.
Role of Environmental Impact Assessment
Environmental impact assessment (EIA) is the process of providing a detailed statement by the responsible agency on the environmental impact of a proposed action significantly affecting the quality of the human environment (Lee 1993). EIA is an instrument of prevention aiming at consideration of the human environment at an early stage of the development of a programme or project.
EIA is particularly important for countries which develop projects in the framework of economic reorientation and restructuring. EIA has become legislation in many developed countries and is now increasingly applied in developing countries and economies in transition.
EIA is integrative in the sense of comprehensive environmental planning and management considering the interactions between different environmental media. On the other hand, EIA integrates the estimation of environmental consequences into the planning process and thereby becomes an instrument of sustainable development. EIA also combines technical and participative properties as it collects, analyses and applies scientific and technical data with consideration of quality control and quality assurance, and stresses the importance of consultations prior to licensing procedures between environmental agencies and the public which could be affected by particular projects. A clean air implementation plan can be considered as a part of the EIA procedure with reference to the air.