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.
|
Figure 1 illustrates the characteristic
size distribution of particulate matter.
|
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.
|
Ozone is a secondary pollutant formed
through a complex series of reactions between NOx (NO2 and NO),
volatile organic compounds and ultraviolet sunlight.
|
(NETCEN or Aquis ozone trajectory
maps.)
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 .
NOxNOx 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.
CO
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.
|
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):
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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.
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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.
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