Air Quality Management, Urban Air Pollution and Vehicle Traffic Emissions

THANKS TO PROF. EUGENE

Aims and Objectives

· A description of the main outdoor (ambient) air pollutants in urban areas, and their health impacts

A brief survey of the most common sources of air pollution: point sources, mobile (vehicular) sources and domestic fuel sources
An overview of the methods of assessing air quality (monitoring and modelling) and of estimating emission rates from each of the main categories of air pollutant sources.
A description of the South African Air Quality Management System.

Outcomes

This module includes a description of road transport as a major source of air pollution in large urban areas. Students should acquire an understanding of the factors that contribute to total vehicle emissions, methods used to control and reduce vehicle emissions, and the limitations of emission control methods.

Outline of Core Materials

Introduction
The common air pollutants and their impacts
Principle sources of air pollution
Methods of assessing air quality
Air Quality Management Systems
Case Study

1. Introduction

Economic development and industrialisation result in increasing concentrations of people in towns and cities (urban areas), and the increasing industrial, commercial and domestic activities associated with the historical process of industrial development. Industrial development may have undesirable environmental consequences, particularly an increase in air pollution in these growing urban areas. An increase in air pollution is frequently considered to be an undesirable but unavoidable result of ‘development’. Therefore the strict control and minimisation of environmental impacts is frequently seen as being in conflict with ‘development’ and ‘progress’. But an understanding of the activities and factors that generate air pollution, the human health and environmental consequences of exposure to air pollutants and the available alternative approaches for the reduction or elimination of air pollution enables a different development scenario. The alternative is the management of and avoidance of air pollution impacts within the development process, choosing a different way to do ‘development’.

Air pollution cannot be confined. So the general public, including those who do not in any way benefit from the activity causing the pollution (such as a factory or a passing bus) may suffer the discomfort or disease burden of the air pollution. Section 24 of our constitution says that we have the right to an environment that is not detrimental to our health and wellbeing. But the protection or enforcement of environmental rights in relation to air quality requires an insight into the relationship between the pollution source and the exposure of people or the environment to air pollutants, and the legal and regulatory framework that enables the enforcement of those environmental rights.

Sources of air pollution in an urban area may be characterized by factors such as the emission rates of specific pollutants, whether the source is stationary or mobile (cars and trucks), the elevation of the source in relation to environmental receptors (people, crops, buildings etc.), and the exit velocity and temperature of the gas if the source is emitted from a stack. Stationary sources may be further characterized as point sources such as chimneys or stacks, and area sources such as landfill sites, or agricultural areas.

Several factors not directly related to pollution source characteristics influence the pollutant concentrations in the air we breathe. These include the meteorological conditions, distance from the source and the nature of the intervening terrain – whether urban, rural or a water body. The concentrations are averaged over a given time period, usually 15 minutes, 1 hour, 3 hours, 24 hours or a year, at a particular location.

Certain air pollutants, such as heavy metals (compounds of lead, chromium, etc.) and dioxins/ furans, the primary exposure path for people is not direct inhalation of the polluted air, but through ingestion of contaminated food or dust. These persistent air pollutants (they do not break down into less toxic substances naturally, or break down very slowly) settle on crops or grass that are in turn eaten by livestock and subsequently by people. Water and sediments contaminated by persistent toxic substances result in the contamination of aquatic species and the food web, with attendant environmental and health risk consequences. Children may ingest contaminated dust – this is the main exposure pathway for leaded petrol emissions.

The World Health Organisation (WHO) has reviewed summarised and published information on the exposure-response relationships for the most commonly encountered urban air pollutants as well as Air Quality Guidelines Values.

2. The common air pollutants and their impacts

Exposure-response relationships (frequently called dose-response relationships) may be used to estimate potential impacts on people and/or the environment. The World Health Organisation (WHO) has published information on the exposure-response relationships for the most common urban air pollutants as well as Air Quality Guidelines Values.

Table 1 contains some of the more common air pollutants.

Table 1: Common Urban Air Pollutants and their Effects

Pollutant	Primary(P) or Secondary (S)	Effects
Sulphur dioxide (SO₂)	P	Health, vegetation
Particulate Matter (PM10, PM2.5)	P and S	Health, visibility impairment
Nitrogen oxides (NOx) (NOx = NO + NO₂)	P and S	Health, vegetation, Global Warming
Volatile Organic Compounds (VOCs)	P and S	Health, ozone formation, smog
Ozone (O₃)	S	Health, vegetation
Compounds of heavy metals, including those of chromium, nickel, vanadium and lead	P	Health
Carbon monoxide (CO)	P	Health

Primary and secondary air pollutants:

SO₂, CO₂, CO, NO, NO₂, Particulate Matter (PM) and VOCs are primary pollutants; they are released directly into the atmosphere. Vehicle emissions and emissions from other combustion sources may be significant primary sources of fine fraction (PM2.5) particulate emissions. The combustion of the hydrocarbons - fuel oil, diesel and petrol - produces elemental carbon as a primary particulate. Windblown dust (coarse fraction) is a source of primary particulate matter.

Secondary pollutants are formed in the atmosphere through chemical reactions and physical processes. For example, SO₂ and NO₂ react with ammonia or other alkaline species, atmospheric oxygen and water vapour to form sulphates (ammonium bisulphate and /or sulphuric acid) and nitrates (ammonium nitrate, peroxyacetylnitrate (PAN) and/ or nitric acid). The nuclei that form when these substances condense may grow through the physical processes of deposition and agglomeration.

Text Box: Primary pollutants are released directly (from the source) into the atmosphere
Secondary pollutants are formed through chemical transformations and physical processes.

Secondary particulate matter formation:

SO₂ and NO₂ may react to form sulphuric and nitric acid mists – fine droplets. These acids may in turn react with ammonia to form ammonium bisulphate (NH₄HSO₄) and ammonium nitrate (NH₄NO₃). Further reactions with atmospheric hydrochloric acid (HCl)(gas) and salt (NaCl) may occur to form ammonium chloride (NH₄Cl) and sodium nitrate (NaNO₃) particulates.

These secondary pollutants form particles in the size range 0.1 to 1 micron and contribute to the total PM2.5 fraction (the ‘fine’ fraction), the most damaging to health.

Thus the main fine fraction (<2.5mm) constituents are sulphates, nitrates, ammonium chloride and elemental carbon. Of these, only elemental carbon is a primary pollutant.

Figure 1 illustrates the characteristic size distribution of particulate matter.

Figure 1: Size distribution of airborne particulate matter

(adapted from Holgate et al, Figure 5.2)

Nitric oxide (NO) is mainly produced by through combustion processes. NO is thus present in motor vehicle exhaust gases, stack emissions from stationary combustion sources such as coal, oil and diesel fired boilers and coal fired power stations, and waste incinerators. The negative environmental impacts of NO are not attributed to direct exposure to NO but to the atmospheric transformation products of NO.

Reactions of NO and NO₂and O₃:

NO + O₃ à NO₂ + O₂ …..(2.1)

This reaction is rapid (occurs within a few seconds) and is the dominant reaction. It takes place essentially to completion. That is, if there is an excess of O₃ (ozone) present in the atmosphere, all the NO is converted to NO₂; if there is an excess of NO present , all the O₃ is converted to O₂.

But the extent of the overall conversion of NO to NO₂ depends not only on the presence of ozone but on the degree of mixing between regions of high NO concentration, such as the vehicle exhaust or the combustion source stack, and the ozone contaminated atmosphere.

Under cold conditions (e.g. during winter), with poor atmospheric mixing, the much slower reaction of NO with O₂ may occur:

NO + NO + O₂ à 2NO₂ ….. (2.2)

Under the latter conditions, high concentrations of both NO and NO2 may be observed.

Ozone is a secondary pollutant formed through a complex series of reactions between NOx (NO₂ and NO), volatile organic compounds and ultraviolet sunlight.

Ozone formation (simplified scheme), using ethene (ethylene) as an example:

OH + C₂H₂ (ethane) + M à HOC₂H₄ + M

(M is an oxygen or nitrogen molecule)

HOC₂H₄ à HOC₂H₄O₂

NO₂ + UV radiation (200-420nm) à NO + O

O + O₂ + M à O₃ (ozone) + M

Different organic species have different chemical reaction rates and therefore different relative contributions to ozone formation.

These reactions require 6 to 9 hours of intense sunlight, and may therefore take place over 1 to 2 days. Over a two day period a polluted air mass may be transported several hundred kilometres. (A low to moderate wind speed of 10 km/ hour is equivalent to 240 km/day; under theses conditions the air mass would travel 480 kms in two days.)

(NETCEN or Aquis ozone trajectory maps.)

Text Box: Anthropogenic: Originating from human activities.
Non-anthropogenic or ‘Natural’ air pollution sources are those that do not arise from human activities.
The distinction between the two source types is not always clear-cut.

Deposition of pollutants takes place onto buildings, vegetation and other surfaces, and rain tends to scrub out pollutants from the atmosphere – the acidic pollutants, sulphuric acid and

nitric acid may form ‘acid rain’. The relationship between measured ambient pollutant levels and source emissions is therefore complex.

All pollutants undergo dispersion, chemical transformation and deposition in the lowest layer of the atmosphere – the troposphere. The troposphere extends to an altitude of about 16 to 18 km over the tropics, reducing to about 10 km over the poles and contains about 80% of the total air mass. All weather phenomena occur in this layer. Mixing between the troposphere and higher levels of the atmosphere (stratosphere and above) is negligible, therefore the dispersion of pollutants occurs almost exclusively within the troposphere.

What is clean air?

At locations that are remote from pollutant sources, air concentrations reach ‘background’ levels. ‘Background level’ concentrations refer to measurements done far from pollution sources. Due to the mixing and dispersion processes in the atmosphere, background levels may represent concentrations that are low but significantly different from the unpolluted air of pre-industrial periods.

The Cape Point monitoring station is regarded as a background station even though it is only about 100km from the major pollution sources of the City of Cape Town; in contrast the pollution plumes from the large Eskom power stations are measurable more than 1000 km from the sources.

Table 2 gives some ‘background level’ (Clean Air) concentrations compared to polluted air levels.

Table 3: Clean Air and Polluted Air

		Concentration, ppb
Species	Units	Clean Troposphere	Polluted Air
SO₂	ppb	1 – 10	20 – 200
CO	ppb	120	1000 – 10 000
NO₂	ppb	0.01 – 0.05	50 – 250
O₃	ppb	20 - 80	100 – 500
PM10	mg/m³	0?	30 -600?
VOCs	ppb	?	500 - 1200
Lead	mg/m³	0.0005 – 0.03	0.4 – 2.0+

ppb: parts per billion mg/m³: microgrammes per metre³

The concentration of pollutants in urban air is one to three orders of magnitude (10x to 1000x) greater than levels in ‘background’ or unpolluted air.

For example, Cape Point ‘background’ concentrations for ozone (average 20-25ppb) and CO (average +-55ppb) may be compared with values within the City of Cape Town of up to 100ppb for ozone and 10ppm for CO.

By contrast, hourly average CO levels in at the Drill Hall monitoring stations may be as high as 18 000 ppb, compared with about 55 ppb at Cape Point (Figure 5a); hourly average ozone concentrations in the City are up to 80ppb. Peak ozone levels in the City are about four times greater than at Cape Point. In some cities, peak ozone values of more than 200 ppb are not uncommon.

3. The Health and Environmental Effects of the Common Air Pollutants

The adverse health effects of ambient air pollution on exposed communities, demonstrated through many epidemiological studies (WHOa, 2002) include:

reduced lung functioning
provoking asthma attacks
worsening of respiratory symptoms
restricted physical activity

· increased medication use

increased hospital admissions
increased emergency room visits
development of respiratory diseases
premature death.

The expected health effects depend on the type of pollution, the level (pollutant concentration) and duration of exposure, and the personal susceptibility of an individual.

Summaries of the sources and health and environmental effects of the common air pollutants are as follows

SO₂

Sulfur dioxide belongs to the family of gases called sulfur oxides (SOx ). These gases are formed when fuel containing sulfur (mainly coal and oil) is burned, and during metal smelting and other industrial processes. Vehicle fuels (petrol and diesel) contain significant levels of sulphur and hence contribute to the emission of SO2 and sulphate particulates. SO2 in the atmosphere is converted to sulphuric acid (H2SO4) and other sulphate particulates. Large scale emissions of SO2 from power stations contribute to acid rain.

Health and Environmental Effects: The major health concerns associated with exposure to high concentrations of SO₂ include effects on breathing (decreased lung function), respiratory illness, alterations in pulmonary defences, and aggravation of existing cardiovascular disease. Children, the elderly, and people with asthma, cardiovascular disease or chronic lung disease (such as bronchitis or emphysema), are most susceptible to adverse health effects associated with exposure to SO₂ .

NOx
NOx consists of nitric oxide (NO) and nitrogen dioxide (NO2). Combustion processes are the main sources of NOx; about 90% of the NOx is released in the form of NO which is converted to NO2. Major sources are power stations (particularly coal fired power stations), vehicles (particularly if not fitted with catalytic converters) and certain industrial processes – mainly nitric acid manufacture. NO2 is a suffocating, brownish gas; nitrogen dioxide is a strong oxidizing agent that reacts in the air to form corrosive nitric acid, as well as toxic organic nitrates. It also plays a major role in the atmospheric reactions that produce ground-level ozone (or smog) and fine particulate matter (PM2.5) in the form of nitrates.
Health and Environmental Effects: Nitrogen dioxide can irritate the lungs and lower resistance to respiratory infections such as influenza. The effects of short-term exposure are still unclear, but continued or frequent exposure to concentrations that are typically much higher than those normally found in the ambient air may cause increased incidence of acute respiratory illness in children. EPA's health-based national air quality standard for NO₂ is 0.053 ppm (measured as an annual arithmetic mean concentration). Nitrogen oxides contribute to ozone formation and can have adverse effects on both terrestrial and aquatic ecosystems. Nitrogen oxides in the air can significantly contribute to a number of environmental effects such as acid rain and eutrophication in coastal waters like the Chesapeake Bay (USA). Eutrophication occurs when a body of water suffers an increase in nutrients that leads to a reduction in the amount of oxygen in the water, producing an environment that is destructive to fish and other animal life.

Carbon monoxide is a colorless, odorless, poisonous gas formed when carbon in fuels is not burned completely. It is a byproduct of highway vehicle exhaust, which contributes about 60 percent of all CO emissions. In cities, vehicle exhaust can cause as much as 95 percent of all CO emissions. These emissions can result in high concentrations of CO, particularly in local areas with heavy traffic congestion. Other sources of CO emissions include industrial processes and fuel combustion in sources such as boilers and incinerators.
Health and Environmental Effects:
Carbon monoxide enters the bloodstream and reduces oxygen delivery to the body's organs and tissues. The health threat from exposure to CO is most serious for those who suffer from cardiovascular disease. Healthy individuals are also affected, but only at higher levels of exposure. Exposure to elevated CO levels is associated with visual impairment, reduced work capacity, reduced manual dexterity, poor learning ability, and difficulty in performing complex tasks.

Lead (Pb)

The main source of environmental lead is emissions from cars using leaded petrol. Smelters and battery plants are major sources of lead in the air in their immediate vicinity. The highest concentrations of lead may be found in the vicinity of nonferrous smelters and other stationary sources of lead emissions.
Health Effects: Exposure to lead mainly occurs through inhalation of air and ingestion of lead in food, paint, water, soil, or dust. Lead accumulates in the body in blood, bone, and soft tissue. Because it is not readily excreted, lead can also affect the kidneys, liver, nervous system, and other organs. Excessive exposure to lead may cause anemia, kidney disease, reproductive disorders, and neurological impairments such as seizures, mental retardation, and/or behavioral disorders. Even at low doses, lead exposure is associated with changes in fundamental enzymatic, energy transfer, and other processes in the body. Fetuses and children are especially susceptible to low doses of lead, often suffering central nervous system damage or slowed growth. Recent studies show that lead may be a factor in high blood pressure and subsequent heart disease in middle-aged males. Lead may also contribute to osteoporosis in post-menopausal women.

Ozone

Nature and Sources of the Pollutant: Ground-level ozone (the primary constituent of smog) is the most complex, difficult to control, and pervasive of the six principal air pollutants. Unlike other pollutants, ozone is not emitted directly into the air by specific sources. Ozone is created by sunlight acting on NOx and VOC in the air. There are thousands of types of sources of these gases. Some of the common sources include gasoline vapors, chemical solvents, combustion products of fuels, and consumer products. Emissions of NOx and VOC from motor vehicles and stationary sources can be carried hundreds of miles from their origins, and result in high ozone concentrations over very large regions.
Health and Environmental Effects: Scientific evidence indicates that ground-level ozone not only affects people with impaired respiratory systems (such as asthmatics), but healthy adults and children as well. Exposure to ozone for 6 to 7 hours, even at relatively low concentrations, significantly reduces lung function and induces respiratory inflammation in normal, healthy people during periods of moderate exercise. It can be accompanied by symptoms such as chest pain, coughing, nausea, and pulmonary congestion. Recent studies provide evidence of an association between elevated ozone levels and increases in hospital admissions for respiratory problems in several U.S. cities. Results from animal studies indicate that repeated exposure to high levels of ozone for several months or more can produce permanent structural damage in the lungs. Ozone damages crops and forest ecosystems.

PM10

Nature and Sources of the Pollutant: Particulate matter is the term for solid or liquid particles found in the air. Some particles are large or dark enough to be seen as soot or smoke. Others are so small they can be detected only with an electron microscope. Because particles originate from a variety of mobile and stationary sources (diesel trucks, woodstoves, power plants, etc.), their chemical and physical compositions vary widely. Particulate matter can be directly emitted or can be formed in the atmosphere when gaseous pollutants such as SO₂ and NOx react to form fine particles. Figure 2.1 show the very wide size range of ambient air particles – the largest particles are due to windblown dust, the smallest particles are formed as secondary pollutants, directly emitted (primary) particles are predominantly from combustion sources.
Health and Environmental Effects: The smaller particles that are likely responsible for adverse health effects because of their ability to reach the lower regions of the respiratory tract. The PM-10 standard includes particles with a diameter of 10 micrometers or less (one-seventh the width of a human hair). Major concerns for human health from exposure to PM-10 include: effects on breathing and respiratory systems, damage to lung tissue, cancer, and premature death. The elderly, children, and people with chronic lung disease, influenza, or asthma, are especially sensitive to the effects of particulate matter. Acidic PM-10 can also damage human-made materials and is a major cause of reduced visibility in many parts of the U.S. New scientific studies suggest that fine particles (smaller than 2.5 micrometers in diameter) may cause serious adverse health effects.

PM2.5

PM10 may be considered to be composed of two size fractions – the ‘fine’ fraction, PM2.5, and the ‘coarse’ fraction, (PM10-PM2.5). The particulate matter size fraction less than 2.5 mm in diameter, is more harmful (per unit mass) than the coarse fraction because it penetrates deeper into the lungs (to the alveoli, region F in Figure 8b) and because it contains the more harmful chemical components – sulphates, nitrates and transition metals.

VOCs

Volatile Organic Compounds or VOCs are organic chemicals that easily vaporize at room temperature. They are called organic because they contain the element carbon in their molecular structures. VOCs have no colour, smell, or taste. VOCs include a very wide range of individual substances, such as hydrocarbons (for example benzene and toluene), halocarbons and oxygenates.

Hydrocarbon VOCs are usually grouped into methane and other non-methane VOCs. Methane is an important component of VOCs, its environmental impact principally related to its contribution to global warming and to the production of ground level or lower atmosphere ozone. Most methane is released to the atmosphere via the leakage of natural gas from distribution systems. Benzene, a non-methane hydrocarbon, is a colourless, clear liquid. It is fairly stable but highly volatile, readily evaporating at room temperature. Since 80% of man-made emissions come from petrol-fuelled vehicles, levels of benzene are higher in urban areas than rural areas. Benzene concentrations are highest along urban roadsides. Oxygenates arise in vehicle exhausts and via atmospheric chemical reactions. Evaporation of solvents, used for example in paints, cause a release of hydrocarbons, oxygenates and halocarbons to the atmosphere.

Some VOCs are extremely harmful, including the carcinogens benzene, polycyclic aromatic hydrocarbons (PAHs) and 1,3 butadiene. Benzene may increase susceptibility to leukaemia, if exposure is maintained over a period of time. There are several hundred different forms of PAH, and sources can be both natural and man-made processes. PAHs can cause cancer. Sources of 1,3 butadiene include the manufacturing of synthetic rubbers, petrol driven vehicles and cigarette smoke. There is an apparent correlation between butadiene exposure and a higher risk of cancer.

In comparison to other pollutants, the monitoring of VOCs is not yet well developed and the database of information is limited.

Other air pollutants

Dioxins and furans are two groups of extremely harmful substances emitted from waste incinerators that are not equipped with the most sophisticated operational and emission control systems. This is one of the main reasons for communities’ opposition to the use of incineration as a means fro ‘disposal’ of solid waste. (Other reasons are – incineration discourages recycling and reuse of the ‘waste’ materials, and is wasteful of energy and other natural resources.)

Dioxins and furans belong to a group of substances known as Persistent Organic Pollutants (POPs) or Persistent Toxic Substances (PTS). Particulate matter containing heavy metals (principally Pb) or persistent organic compounds (such as dioxins) contaminate soil and crops. Exposure to these types of contaminants occurs mainly through ingestion of contaminated food or soil rather than through inhalation of the polluted air.

3. Principle Sources of Air Pollution

Air Quality (air pollutant concentrations) is the result of the interaction of pollutant emissions, chemical and physical transformations, dispersion of pollutants and pollution sinks. There are natural and ‘anthropogenic’ (the result of human activities) sources of pollution. Dispersion of pollutants is greatly influenced by meteorology; the oceans and surfaces of plants, the earth’s surface or buildings act as ‘sinks’

The sources of air pollution may be classified as stationary point sources (generally industrial in origin), diffuse or area sources and mobile sources (mainly cars and trucks). The stationary industrial sources are usually classified by process type or sub-type. Thus an oil refining plant also includes large industrial boilers as a sub-type. Small and medium scale plants such as garment or food processing plants may include industrial boilers, a common source of air pollution. The quality and type of fuel used for energy production are important determinants of the air pollution potential of a plant. Each type of plant or activity generally emits more than one pollutant, and the pollutant emission rate depends on the fuel type and quality, the design of the plant (and whether fitted with air pollution control devices or not), and the activity rate or output of the plant.

Table 3 lists some stationary sources and typical pollutants emitted by these sources. Depending on the classification system, stationary sources may be classified into 50 to 100 different categories. South African regulations, for example, list of about 70 ‘Scheduled Industries’.

Table 4: Examples of stationary sources and the pollutants emitted

Source	Pollutants
Coal Fired Power Stations	SO2, NOx, PM, VOCs, …
Sulphuric Acid Plants	SO2, sulphuric acid mist, SO3
Boilers, Combustion Plants	SO2, NOx, PM, VOCs, …
Nitric Acid Plants	NOx
Fertiliser Plants	PM (ammonium nitrate, phosphate rock, etc.
Oil refineries	SO2, NOx, PM, VOCs, …
Glass manufacture	SO2, NOx, PM
Landfills	CH₄, H₂S, odourous gases
Incinerators	Dioxins, SO2, NOx, PM, VOCs, …
Open burning of solid waste	Dioxins, PM, VOCs, …
Mines and smelters	PM, NOx, SO₂, ..
VOC storage facilities, paintshops, dry cleaners etc….	VOCs

The list of pollutants in Table 4 is by no means complete. A single pollutant (SO2 for example) may have a number of sources.

In general, fossil fuels (coal, fuel oil or gas (LPG or LNG)) are a major source of pollutant emissions, and emissions of the greenhouse gas carbon dioxide (CO₂). However, the pollutant profile of each of these fuel sources is markedly different, with gas being by far the least polluting. The predominant use of fossil fuels as an energy source - (coal) for power (electricity) generation and oil (petrol and diesel) for transport - result therefore in both urban air pollution and climate change.

Fuels may be compared for pollution potential on the basis of their emissions per kg or per litre, as illustrated in the Table 5.

Table 5: Emission Factors for different fuel types (CT Brown Haze Study)

		Pollutant
Fuel	Units	SO2	NOx	PM10	VOCs
Coal	g/kg	19	1.5	4.1	5.0
Paraffin	g/l	8.5	1.5	0.2	0.09
LPG	g/l	0.01	1.4	0.07	0.5
wood	g/kg	0.75	5	17.3	22

Note that these emission factors are to some extent dependent on the individual fuel composition. For example, the SO₂ emissions from coal and fuel oil are a direct function of the sulphur content of these fuels. Good design and the installation of emission control devices such as precipitators or baghouses (to reduce PM10 emissions), or stack gas scrubbers (for SO2 emissions) may reduce emissions of these pollutants by 70 to 90%. The above emission factors may be compared to the US EPA AP-42 values.

Exercise: If 90% of the sulphur in coal is emitted through the boiler stack in the form of sulphur dioxide, calculate an approximate emission factor (kg SO2/ kg coal) for coal containing 0.9% sulphur.

Mobile sources refer mainly to emissions from cars, trucks, minibuses and buses. The fuel source may be petrol or diesel, and emissions include exhaust emissions and fugitive emissions. Vehicle (mobile) source emissions depend on a number of factors, including vehicle size, fuel type, speed and vehicle technology. Total vehicle emissions depends on the vehicle population on the road at a given time.

Vehicle emission factors are generally measured by sampling the vehicle population and measuring emissions under controlled conditions. The following vehicle emission factors have been extracted from a large European Union database (COPERTIII, vergina.eng.auth.gr/mech/lat/copert/copert.htm, reports.eea.eu.int/Technical_report_No_50/en).

Table 6: Copert III Emission Factors [g/km]: Petrol Cars, 1986-92 technology (no controls)
Speed [km/h] ->		5	10	15	20	30	40	50	70	80	90
	Size(capacity)
CO	all capacities	60.29	32.08	22.18	17.07	11.81	9.09	7.42	4.96	4.50	4.28
VOCs	all capacities	6.25	3.87	2.92	2.39	1.81	1.48	1.27	0.90	0.79	0.73
NOx	< 1.4 l	1.45	1.47	1.49	1.52	1.60	1.69	1.80	2.09	2.26	2.45
	1.4--2.0 l	1.55	1.62	1.70	1.77	1.94	2.12	2.32	2.76	3.00	3.25
	> 2.0 l	2.36	2.31	2.28	2.25	2.25	2.29	2.39	2.75	3.01	3.32

Vehicle emissions at a give speed are a product of kms travelled at that speed and the appropriate emission factor. For example, using the above table, a medium sized car (1.4 to 2.01l) travelling 5kms at 20 km/h would emit CO: 85.4g, VOCs: 12.0g, NOx: 8.9g.

The influence of speed on emissions (per km) is illustrated by plotting the above data:

Total emissions in a given area may be estimated using the following general relationship:

Emission Rate = Emission Factor x Activity Rate

For stationary sources, emission rates may be directly measured using in-stack samplers and continuous or intermittent measurement.

4. Methods of assessing air quality - measuring, monitoring and modelling ambient pollutant concentrations.

(Main reference: Monitoring ambient air quality for health impact assessment, WHO Regional Publications, European Series No. 85 (1999))

Pollutant sources may be classified as Natural (e.g. volcanic eruptions) and ‘anthropogenic’ (the result of human activities).

The Air Quality (air pollutant concentrations) in a given area is the result of the interaction of pollutant emissions from sources, chemical and physical transformations of these pollutants, dispersion of the pollutants in the atmosphere and the action of ‘sinks’ such as the oceans, other water bodies and solid surfaces that absorb pollutants.

Air quality monitoring

An air quality monitoring system essentially measures ambient air concentrations at a number of fixed locations, for example across a city or within a region. In principle, the function of a monitoring station is to compare the measured values against a standard or a guideline and to take action if the measured values exceed the standard or guideline. (Unfortunately, in the absence of a regulated management system, too frequently no action is taken even if guidelines are exceeded.)

Continuous monitors are instruments capable of measuring pollutant concentrations (for example, SO₂, NO₂, CO, PM) continuously and more or less instantaneously (in reality, over very short periods of time). The ‘instantaneous’ values are not in themselves useful for assessing air quality. Thus these values may be averaged over time periods of 10 or 15 minutes, one hour, 3 hours, 8 hours, 24 hours or longer periods. The time-averaged values (time weighted averages) may be compared with air quality standards or guidelines, or may be used to estimate the potential health impacts of the air pollutant concentrations.

Instruments capable of measuring air pollutant concentrations continuously are comparatively expensive, and have to be housed in a protected and controlled environment, usually at a fixed site or in a mobile station or caravan. Thus a city with a monitoring system would have a limited number of monitoring sites, each measuring a limited number of pollutants. For example, the City of Cape Town has the following monitoring network:

The choice of locations for monitoring sites should consider: proximity to major pollution sources or suspected areas of high concentration, areas with high population density and an area remote from local pollution sources to assess ‘background’ pollution levels.

Modelling air quality

Monitoring sites provide detailed information on concentration of a particular set of pollutants at a specific site. For example, the Cape Town network provides data on 5 or 6 pollutants, at the 8 sites shown in Figure 1. However, the air quality data obtained at these sites cannot be assumed to represent conditions throughout the metropolitan area. The 8 (or 10) monitoring stations in the Cape Metropolitan Area cannot give a representative assessment of air quality of an area covering several hundred square kilometres, even if optimally located in relation to pollution sources and the exposed population. Very localised spatial and temporal variations in concentration may occur due to the proximity to point sources, major roads or the effect meteorology and/ or of building downwash. Monitoring on its own does not provide a coherent integrated picture of air quality.

In general, ambient air monitoring does not give an indication of the source of pollution. For example, sulphur dioxide and nitrogen oxides are both emitted from stationary combustion sources and vehicles, both petrol and diesel driven. Measurements at a particular location cannot be apportioned to one or other source on the basis of monitoring data alone. Thus if (health based) standards are exceeded, action cannot easily be taken to manage and control pollution sources.

Air quality modelling, particularly dispersion modelling, is used to predict air pollution concentrations in the modelled region using data on pollutant emission sources and meteorology as inputs. Broadly speaking, atmospheric dispersion models are mathematic procedures that result in an estimation of ambient air quality as a function of time and location. Dispersion models combine emission data from one or several sources (up to several thousand sources, including stationary and mobile sources) and meteorological data to predict ambient concentrations of pollutants. Dispersion models are therefore able to interpolate and extrapolate measured data, providing a coherent integrated picture of the link between the sources of pollution and the ambient air quality. Models are therefore essential to air quality assessment, but their limitations must understood and accounted for. The calibration and validation of models is essential.

Model results may be used to make air quality management decisions.

The influence of meteorology (dispersion potential):

Height

The dry adiabatic lapse rate is the rate of temperature decrease with height that would occur if a dry parcel of air rises adiabatically, that is without loosing or gain heat (energy). The dry adiabatic lapse rate is 9.8 ^oC per 1000m or about 1 ^oC per 100m.

Influence on air stability and dispersion:

· Super-adiabatic or strong lapse rate – unstable air, good dispersion conditions

· Sub-adiabatic or weak lapse rate – poor dispersion conditions

· Temperature inversion – stable atmosphere, very poor dispersion conditions

Figure 13 illustrates the different pollution plumes, with different dispersion potentials, that may be observed under different meteorological conditions.

Air pollution modelling may also be used to study the potential impact of a single emission source.

The following modelling example illustrates the value of this tool.

Consider the impact of a large industrial boiler using coal as a fuel source. For modelling purposes, assume a stack height of 40m, a stack exit velocity of 6m/s and an exit gas temperature of 300 ^oC. A dispersion model (in this case a general Gaussian model was used) is able to predict concentrations in a specified area, for given meteorological conditions.

Figures 4 and 5 illustrate the substantial differences in ambient concentrations, and the location of the point of highest concentration that may occur due to differences in wind speed and atmospheric stability. (Stable or very stable conditions may occur at night, particular during winter, under conditions of low wind speed. Unstable conditions may occur during the day with strong insolation (heating by the sun) and moderate wind speed.) Under stable or poor dispersion conditions (Figure 4), maximum concentrations are high (about 28 ug/m3) and occur over a comparatively large area; under unstable conditions (good dispersion, Figure 5), maximum concentrations are lower (about 20 ug/m3) and occur over a smaller area. Note also that the point of maximum concentration is closer to the source in Case 2.

Figure 14: Case 1: Emission rate 2g/s, 40m stack, 10km/h wind, atmospheric stability class f

Figure 15: Case 2: Emission rate 2g/s, 40m stack, 20 km/h wind, atmospheric stability class a

5. Air Quality Management Systems

An air quality management system needs to address the complexity of the relationship between sources and exposure. The setting of Ambient Air Quality Standards, an Emission Inventory, Ambient Air Quality Monitoring, gathering appropriate Meteorological Data, Air Quality Modelling, Source Emission Limits and an integrated regulatory system are essential components of such a system.

Interpreting the WHO Guidelines

The WHO Guidelines have to be interpreted with care. Air pollutant concentrations that are below the guideline values may not be assumed to be 100% ‘safe’. The guideline values are periodically reviewed against ongoing research into the relationship between air pollution and health impacts.

Air Quality or Air Pollution Indices (AQI or API)

These indices are attempts to compare overall air quality in an area against a standard or guideline value by calculating an index that is indicative of the degree to which the air concentration values meet or exceed the guideline/ standards value(s). The index value may be used to advise vulnerable groups (such as asthmatics) to avoid exposure that might result in adverse health effects. A model (DAPPS – Dynamic Air Pollution Prediction System) is currently under development, by a consortium consisting of Pentech, CSIR, SA Weather Service and SRK Consulting, using Cape Town as a pilot site.

Since we are simultaneously exposed to a number of air pollutants, particularly to the common air pollutants, there is a need for an Air Pollution Index that reflects this simultaneous exposure.

Friday, 6 April 2012