World Health Organization Regional Office for Europe
Scherfigsvej 8, DK-2100 Copenhagen Ø, Denmark
Tel.: +45 39 17 17 17. Fax: +45 39 17 18 18.
E-mail: postmaster@euro.who.int
Web site: www.euro.who.int
ISBN 92 890 2192 6
Particulate matter, ozone, nitrogen dioxide and sulfur dioxide
Global Update 2005
Air Quality
A ir Q
u ality G
u id
elin es
G lo
b al U
p d
ate 2005
Guidelines The WHO air quality guidelines offer guidance on reducing the
effects on health of air pollution. This book presents revised
guideline values for the four most common air pollutants –
particulate matter, ozone, nitrogen dioxide and sulfur dioxide –
based on a recent review of the accumulated scientific evidence.
The rationale for selection of the guideline value is supported by
a synthesis of information emerging from research on the health
effects of each pollutant.
The book gives a brief yet comprehensive review of the issues
affecting the application of the guidelines in risk assessment
and policy development. It summarizes information on pollution
sources and levels in various parts of the world, on population
exposure and characteristics affecting sensitivity to pollution, on
methods for quantifying the health burden of air pollution, and
on the use of guidelines in developing air quality standards and
other policy tools. The special case of indoor air pollution is also
explored.
Prepared by a large team of renowned international experts
who considered conditions in various parts of the globe,
these guidelines are applicable throughout the world. They
provide reliable guidance for policy-makers everywhere when
considering the various options for air quality management.
Air Quality Guidelines Global Update 2005
Keywords
AIR - standards
AIR POLLUTION - analysis
AIR POLLUTANTS - adverse eff ects
AIR POLLUTION, INDOOR
OZONE - adverse eff ects
NITROGEN DIOXIDE - adverse eff ects
SULFUR DIOXIDE - adverse eff ects
ENVIRONMENTAL MONITORING
RISK ASSESSMENT
GUIDELINES
ISBN 92 890 2192 6
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© World Health Organization 2006 All rights reserved. Th e Regional Offi ce for Europe of the World Health Organization welcomes requests for permission to reproduce or translate its publications, in part or in full. Th e designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of the World Health Organization concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its fron- tiers or boundaries. Where the designation “country or area” appears in the headings of tables, it covers countries, territories, cities, or areas. Dotted lines on maps represent approximate border lines for which there may not yet be full agreement. Th e mention of specifi c companies or of certain manufacturers’ products does not imply that they are endorsed or recommended by the World Health Organization in preference to others of a similar na- ture that are not mentioned. Errors and omissions excepted, the names of proprietary products are distin- guished by initial capital letters. Th e World Health Organization does not warrant that the information contained in this publication is complete and correct and shall not be liable for any damages incurred as a result of its use. Th e views expressed by authors or editors do not necessarily represent the decisions or the stated policy of the World Health Organization.
Printed in Germany by Druckpartner Moser
Abstract
Th e WHO air quality guidelines off er guidance to policy-makers on reducing the
eff ects on health of air pollution. Th is book presents revised guideline values for
the four most common air pollutants – particulate matter, ozone, nitrogen diox-
ide and sulfur dioxide. It also gives a comprehensive review of the issues aff ecting
the use of the guidelines, which now apply the world over, in risk assessment and
policy development.
Particulate matter, ozone, nitrogen dioxide and sulfur dioxide
Global Update 2005
Air Quality Guidelines
Th is work was supported by grants obtained by WHO from the German Fed-
eral Ministry for the Environment, Nature Conservation and Nuclear Safety, the
Energy Research Centre of the Netherlands, the Swiss Federal Offi ce for the En-
vironment, the Department of Health in the United Kingdom and the United
States Environmental Protection Agency.
Acknowledgements
V
Foreword ix
Introduction 1
Development of the update 1
Scope of the update 2
Key scientifi c issues in the development of the guidelines 4
Th e updated guidelines and air quality management 5
References 6
Part 1 | Application of air quality guidelines
for policy development and risk reduction
1. Sources of air pollution 9
Summary 9
Introduction 10
Primary pollutants 12
Secondary pollutants 24
References 29
2. Global ambient air pollution concentrations and trends 31
Summary 31
Assessment of air quality based on available monitoring data 32
PM10 or respirable particulate matter 37
Ozone, a regional and global problem 41
Sulfur dioxide, traditionally from the burning of fossil fuel 45
Nitrogen dioxide, a problem related mainly to mobile sources 47
Trends in air quality 49
References 54
3. Human exposure to air pollution 61
Summary 61
Defi nition and concept of exposure 62
Where does human exposure occur? 63
Total exposure and time–activity patterns 64
Contents
V I
Th e infl uence of location on the relationship between sources
and exposures 66
Methods of exposure assessment 66
Assessing long-term exposure to air pollution 71
Critical time windows 72
Relationship between personal exposure, indoor concentration
and outdoor concentration 72
Key factors determining the relationship between indoor
and outdoor concentrations 72
Overall strength of the relationship between personal (or indoor)
and outdoor concentrations 76
Population characteristics 77
Impact of exposure measurement error 78
Policy implications of exposure assessment 79
References 81
4. Health eff ects of air pollution: an overview 87
Summary 87
Introduction 88
What is an adverse eff ect of air pollution? 89
Assessing the health eff ects of air pollution 93
Choice of study design 101
References 102
5. Determinants of susceptibility 111
Summary 111
Introduction 112
Who are most aff ected? 113
Chronic diseases as determinants of susceptibility 121
Future considerations 126
References 126
6. Environmental equity 135
Summary 135
Introduction to concepts of environmental equity and justice 136
Policy contexts relevant to environmental equity 137
Evidence of inequities in health eff ects of air pollution 139
Evidence on the links between pollution sources and inequity 141
Hot spots, episodes and cumulative impacts 145
Future research and policy implications 146
References 147
V I I
7. Health impact assessment 153
Summary 153
Introduction 154
Previous studies 155
Inputs for the analysis 156
Benefi ts of conducting impact assessment 161
A simple example of the methodology 164
Uncertainties 166
References 168
8. Application of guidelines in policy formulation 173
Summary 173
Introduction 173
Setting air quality standards 174
Implementation 182
References 186
9. Indoor air quality 189
Summary 189
Background 190
Characteristics of solid fuel smoke 194
Indoor air pollutant levels in households using solid fuel:
concentrations and exposures 194
Health eff ects associated with exposure to solid fuel smoke 197
Comparability of health impacts from indoor and outdoor
air pollution 201
Options for interventions 202
Framework for air quality guidelines 205
References 207
Part 2 | Risk assessment of selected pollutants
10. Particulate matter 217
Introduction 217
General description 218
Exposures 226
Mechanisms of toxicity 231
Health eff ects 247
Human exposure studies 251
Evaluation 272
Guidelines 275
References 280
A I R Q U A L I T Y G U I D E L I N E SV I I I
11. Ozone 307
General description 307
Route of exposure and toxicokinetics 311
Summary of the pathogenetic mechanisms of ozone toxicity 313
Health eff ects 314
Guidelines 324
References 326
12. Nitrogen dioxide 331
General description 331
Routes of exposure 333
Kinetics and metabolism 333
Health eff ects 333
Evaluation of human health risks 374
Guidelines 375
References 377
13. Sulfur dioxide 395
General description 395
Health eff ects 398
Evaluation 411
Guidelines 413
References 415
Part 3 | Annexes
Annex 1 423
Pathogenesis of ozone-dependent injury 423
Direct oxidation of cellular constituents 423
Induction of respiratory and systemic infl ammation 424
Eff ects on immunity 434
Factors defi ning susceptibility and tolerance to ozone 435
References 458
Annex 2 481
List of Working Group members present at the meeting in Bonn,
Germany, 18–20 October 2005 481
List of authors and reviewers not present at the
Working Group meeting 483
I X
Clean air is a basic requirement of human health and well-being. Air pollution,
however, continues to pose a signifi cant threat to health worldwide. According
to a WHO assessment of the burden of disease due to air pollution, more than
two million premature deaths each year can be attributed to the eff ects of urban
outdoor air pollution and indoor air pollution (from the burning of solid fuels).
More than half of this disease burden is borne by the populations of developing
countries. Th is update of the WHO air quality guidelines has been developed
in response to this real and global threat to public health. It continues the long
WHO tradition of supporting its Member States with the best available evidence
on health determinants, and on the risks of air pollution in particular. Previous
editions of the guidelines found wide application in environmental and public
health decision-making in various parts of the world.
Although these guidelines are neither standards nor legally binding criteria,
they are designed to off er guidance in reducing the health impacts of air pollu-
tion based on expert evaluation of current scientifi c evidence. Th ey are intended
to be relevant to the diverse conditions of all WHO’s regions, and to support a
broad range of policy options for air quality management. Knowledge about the
hazardous properties of the pollutants and indication of the risk related to expo-
sure, summarized by the guidelines, provide an essential scientifi c contribution
to the development of strategies for air quality management. Authorities prepar-
ing national strategies, especially in those countries that lack the necessary scien-
tifi c infrastructure and resources to conduct their own assessments in support of
public policy, will fi nd the guidelines an essential resource.
Th e synthesis of the research results that underlie the guidelines has been con-
ducted by outstanding scientists and was subject to scrupulous peer review. We
are grateful to these experts for their eff orts and believe that this work will con-
tribute to improving the health of people in all regions of the world.
Foreword
Margaret Chan
WHO Director-General
Marc Danzon
WHO Regional Director for Europe
1
Introduction
Th e fi rst edition of the WHO Air quality guidelines for Europe was published in
1987, since when scientifi c knowledge about the eff ects of exposure to air pollu-
tion and the magnitude of its public health impact has increased exponentially.
Th e fi rst edition summarized scientifi c knowledge on the health hazards related to
the 28 most common air pollutants, providing a uniform basis for risk assessment
for national authorities responsible for protecting populations from the adverse
eff ects of air pollution. In the early 1990s, the growing body of knowledge allowed
WHO to initiate a process for revising the guidelines, resulting in publication of
the second edition in 2000 both in hard copy, summarizing risk characterization
of 37 pollutants (1), and in an extended electronic version containing the full
background material of the review (www.euro.who.int/document/e71922.pdf).
Since the publication of the second edition there has been an increasing aware-
ness among scientists and policy-makers of the global nature and magnitude of
the public health problems posed by exposure to air pollution, based on hundreds
of new studies published in the scientifi c literature. Th e project “Systematic review
of health aspects of air pollution in Europe”, carried out by the WHO Regional Of-
fi ce for Europe to support the development of the European Union’s Clean Air
for Europe (CAFE) programme in 2002–2004, concluded that this new evidence
warranted revision of the air quality guidelines for particulate matter (PM), ozone
and nitrogen dioxide (2). Of particular importance in deciding that the guidelines
should apply worldwide was the substantial and growing evidence of the health
eff ects of air pollution in the low- and middle-income countries of Asia, where
air pollution levels are the highest (3). WHO’s comparative risk assessment (4–6)
quantifi ed the burden of disease due to air pollution worldwide and, as noted
above, found the largest burden in the developing countries of Asia.
Development of the update WHO established a steering group to advise and lead the guideline development
process.1 Th e steering group agreed on the scope and methodology of the update,
and identifi ed experts to contribute to the review of the scientifi c literature.
Th e steering group recommended to WHO experts in epidemiology, toxicol-
ogy, air quality exposure assessment, air quality management and public policy,
1 Steering group members: H. R. Anderson (United Kingdom), B. Brunekreef (Netherlands), B. Chen (China), A. Cohen (United States), R. Maynard (United Kingdom), I. Romieu (Mexico), K. R. Smith (United States) and S. Wangwongwatana (Th ailand).
A I R Q U A L I T Y G U I D E L I N E S2
who would draft the guideline document. Aft er review and approval by the steer-
ing group, initial draft s were distributed for external review to a wide group
of experts in all the relevant disciplines. WHO also sought the opinions of air
quality managers and policy-makers concerning the rationale and format of the
guidelines, seeking to improve their applicability in various parts of the world.
An eff ort was made to ensure representation of a wide selection of Member States
from all WHO regions.
WHO convened the Working Group on Air Quality Guidelines in Bonn, Ger-
many, on 18–20 October 2005 to fi nalize the updated guidelines. Th e tasks of
the meeting were to formulate guidelines for four specifi c pollutants (PM, ozone,
nitrogen dioxide and sulfur dioxide) and to agree on a supporting text. Th e Work-
ing Group consisted of the authors of the draft chapters, the external reviewers of
the draft s and members of the steering group (see Annex 2). Dr Robert May-
nard chaired the meeting, and Dr Aaron Cohen acted as meeting Rapporteur.
Comments on the draft s of the background material, received from the review-
ers, were circulated to the steering group members, authors and all reviewers in
advance of the meeting. Since not all reviewers participated in the meeting, a list
of those who submitted written comments but who were not present at the meet-
ing is also presented in Annex 2.
In a series of plenary discussions and draft ing sessions, the Working Group
reviewed the general approach to the formulation of the guidelines, discussed
outstanding comments from the reviewers and agreed on the general content of
the background material. Th e draft ing groups discussed in detail the formulation
of the updated guidelines and the text supporting them. Final decisions concern-
ing the recommended guidelines were arrived at in plenary by consensus. Fol-
lowing the Working Group meeting, a report was prepared presenting recom-
mendations for updated guidelines for PM, ozone, nitrogen dioxide and sulfur
dioxide and summarizing the Working Group’s discussions (7). Th e Working
Group’s recommendations were reviewed and cleared by WHO and announced
as updated air quality guidelines (8). Th e comments from the review and from
the Working Group meeting were considered in preparing the next draft s of the
background material. Th e steering group supervised the fi nalization of the text
and assisted WHO in its discussions with the main authors, in the scientifi c ed-
iting of the chapters and in ensuring that the fi nal text was consistent with the
Working Group’s recommendations.
Scope of the update Th ese updated guidelines comprise 13 chapters. Chapters 1–9 consist of back-
ground material, providing a concise yet comprehensive review of the issues af-
fecting the application of the WHO air quality guidelines in risk assessment and
policy development.
3
• Chapters 1 and 2 address the sources and emissions of the main pollutants
presented in the guidelines and discuss their ambient concentrations in vari-
ous parts of the world. Th is review demonstrates wide diversity of air quality
in the world, posing quite diff erent challenges to air quality management. In
many areas with high levels of pollution, it is caused by the use of obsolete
technologies and lack of pollution control systems. Pollution reduction is
technically feasible, but political or socioeconomic conditions and lack of
organizational capacity may limit the eff ectiveness of air quality management.
Poverty may be an obstacle in achieving improvements in air quality. In many
developed countries, air quality has already improved in the last few decades,
owing largely to air quality regulation. Further progress, necessary to reduce
the adverse health impacts of pollution observed even at those low levels,
requires the development and use of new technologies and, oft en, a change in
population lifestyle and the introduction of new approaches to urban devel-
opment.
• Chapters 3–5 present important concepts and methods concerning the quan-
tifi cation of human exposure to air pollution and the assessment of its eff ects
on health. Factors that determine individual susceptibility to air pollution are
also reviewed.
• Chapter 6 discusses the issue of environmental equity, and documents the un-
equal distribution of health risks due to air pollution both within and among
nations.
• Chapter 7 discusses methods for quantifying the health burden of air pollu-
tion that trigger policy reactions, and may be used to analyse the cost–eff ec-
tiveness of various policy options.
• Chapter 8 discusses the use of the guidelines in developing air quality stand-
ards and other policy tools.
• Chapter 9 focuses on indoor air pollution, especially on the conditions preva-
lent in developing countries owing to the indoor combustion of solid fuels.
Owing to the magnitude of the health impacts of this pollution and the need
to use risk reduction approaches that possibly diff er from those developed for
urban air quality management, this chapter makes preliminary recommenda-
tions for future WHO work on this specifi c problem.
• Chapters 10–13 comprise reviews of the health eff ects of PM, ozone, nitrogen
dioxide and sulfur dioxide, respectively. Health-based guidelines are pre-
sented for each pollutant, based on those reviews, together with the rationale
for the decision to revise the guideline value or to retain the existing value.
As noted above, the epidemiological evidence indicates that the possibility of
adverse health eff ects remains even if the guideline value is achieved. For this
reason, some countries might decide to adopt lower concentrations than the
WHO guideline values as national air quality standards.
I N T R O D U C T I O N
A I R Q U A L I T Y G U I D E L I N E S4
In addition to guideline values, interim targets are given for levels of PM, ozone
and sulfur dioxide. Th ese are proposed as incremental steps in a progressive re-
duction of air pollution, and are intended for use in areas where pollution is high.
Th ese targets aim to promote a shift from high air pollutant concentrations, with
acute and serious health consequences, to lower concentrations. If these targets
were to be achieved, one could expect signifi cant reductions in risks for acute and
chronic health eff ects from air pollution. Progress towards the guideline values
should, however, be the ultimate objective of air quality management and health
risk reduction in all areas.
Th e fact that other pollutants, such as carbon monoxide, were not included
in the present review refl ects the limited resources available to the project. As a
result, the 2000 WHO guidelines (1) for pollutants not considered in the current
update remain in eff ect. Th e steering group recommends that the update of the
guidelines be expanded to include additional pollutants as soon as possible, as
resources become available.
Key scientific issues in the development of the guidelines Th e guidelines are based on the extensive scientifi c evidence on air pollution and
its health consequences. Although this information has gaps and uncertainties,
it off ers a strong foundation for the guidelines. Several overall research fi ndings
need to be emphasized with regard to the guidelines.
First, the evidence for ozone and PM shows risks to health at concentrations
currently found in many cities in developed countries; these epidemiological
fi ndings imply that guidelines cannot provide full protection, since thresholds
below which adverse eff ects do not occur have not been identifi ed.
Second, an increasing range of adverse health eff ects has been linked to air
pollution, and at ever-lower pollutant concentrations. Th is is especially true of
airborne PM. New studies use more refi ned methods and more subtle but sen-
sitive indicators of eff ects such as physiological measures (e.g. changes in lung
function, infl ammation markers). Th erefore, the updated guidelines could be
based both on these sensitive indicators and on the most critical and traditional
population health indicators, such as mortality and unscheduled hospital admis-
sions.
Th ird, the complexity of the air pollution mixture has been better charac-
terized, making more clear the limitations of controlling air pollution through
guidelines for single pollutants. Nitrogen dioxide, for example, is a product of
combustion and is generally found in the atmosphere in close association with
other primary pollutants, including ultrafi ne particles. It is also a precursor of
ozone and therefore co-exists in photochemically generated oxidant pollution.
Nitrogen dioxide is itself toxic, and its concentrations are oft en strongly corre-
lated with those of other toxic pollutants. As it is easier to measure, it is oft en used
as a surrogate for the mixture as a whole. Achieving the guidelines for individual
5I N T R O D U C T I O N
pollutants such as nitrogen dioxide may therefore bring benefi ts for public health
that exceed those anticipated based on estimates of the pollutant’s specifi c toxic-
ity.
Th e updated guidelines provide new values for three of the four pollutants ex-
amined. For two of them (PM and ozone), it is possible to derive a quantitative
relationship between the concentration of the pollutant as monitored in ambient
air and specifi c health outcomes (usually mortality). Th ese relationships are in-
valuable for health impact assessment and allow insights into the mortality and
morbidity burdens from current levels of air pollution, as well as the improve-
ments in health that could be expected under diff erent air pollution reduction
scenarios. Th e estimates of disease burden can also be used for the purpose of
estimating the costs and benefi ts of interventions that reduce air pollution.
It is worth noting that the second edition of the WHO guidelines (1) did not
set a guideline value for PM, and instead off ered guidance for risk managers in
the form of a statistical model relating exposure to risk, suggesting that they
quantify the risk at locally relevant exposure levels and use those local estimates
to guide policy-making. Th is approach to no-threshold pollutants has been ap-
plied widely in risk management of environmental chemicals (e.g. in risk assess-
ment of genotoxic carcinogens). Although WHO has not evaluated formally how
this guidance has been used in air quality management, it was the view of Work-
ing Group members from developing countries that the approach taken for PM
in the 2000 guidelines had not been well-accepted by air quality managers and
policy-makers. Th erefore the updated guidelines defi ne concentrations for the
considered pollutants, which, if achieved, would be expected to result in signif-
icantly reduced rates of adverse health eff ects. Th ese concentrations should be
based on the available scientifi c evidence and would provide an explicit objective
for air quality managers and policy-makers to consider when setting national air
quality standards and management strategies. Given that air pollution levels in
some countries oft en far exceed the recommended guideline levels, interim tar-
get levels are proposed, in excess of the guideline levels themselves, to promote
steady progress towards meeting the WHO guidelines.
The updated guidelines and air quality management Th e guidelines are written for worldwide use, and are intended to support actions
aiming for the optimal achievable level of air quality in order to protect public
health in diff erent contexts. Air quality standards are an important instrument
of risk management and environmental policy, and should be set by each coun-
try to protect the health of its citizens. Th e standards set in each country will
vary according to specifi c approaches to balancing risks to health, technological
feasibility, economic considerations and other political and social factors. Th is
variability will depend on the country’s level of development, capability in air
quality management and other factors. Th e guidelines recommended by WHO
6 A I R Q U A L I T Y G U I D E L I N E S
acknowledge this heterogeneity and recognize in particular that, in formulating
policy targets, governments should consider their own local circumstances care-
fully before using the guidelines directly as legal standards.
References 1. Air quality guidelines for Europe, 2nd ed. Copenhagen, WHO Regional
Offi ce for Europe, 2000 (WHO Regional Publications, European Series,
No. 91).
2. Health aspects of air quality in Europe. Results from the WHO project
“Systematic review of health aspects of air pollution in Europe”. Copenhagen,
WHO Regional Offi ce for Europe, 2004 (http://www.euro.who.int/
document/E83080.pdf, accessed 25 November 2006).
3. Health eff ects of outdoor air pollution in developing countries of Asia: a
literature review. Boston, MA, Health Eff ects Institute, 2004 (Special
Report 15).
4. Th e world health report 2002 – reducing risks, promoting healthy life. Geneva,
World Health Organization, 2002.
5. Cohen A et al. Mortality impacts of urban air pollution. In: Ezzati M et al.,
eds. Comparative quantifi cation of health risks: global and regional burden
of disease attributable to selected major risk factors. Geneva, World Health
Organization, 2004:1353–1434.
6. Smith KR, Mehta S, Maeusezahl-Fuez M. Indoor air pollution from
household use of solid fuels. In: Ezzati M et al., eds. Comparative
quantifi cation of health risks: global and regional burden of disease
attributable to selected major risk factors. Geneva, World Health
Organization, 2004:1436–1493.
7. WHO air quality guidelines: global update 2005. Report on a Working Group
meeting, Bonn, Germany, 18–20 October 2005. Copenhagen, WHO Regional
offi ce for Europe, 2005 (http://www.euro.who.int/Document/E87950.pdf,
accessed 25 November 2006).
8. WHO air quality guidelines for particulate matter, ozone, nitrogen dioxide
and sulfur dioxide: global update 2005. Summary of risk assessment.
Geneva, World Health Organization, 2006 (http://www.who.int/phe/air/
aqg2006execsum.pdf, accessed 25 November 2006).
Part 1
Application
of air quality
guidelines
for policy
development
and risk
reduction
9
1. Sources of air pollution
Air pollutants may be either emitted into the atmosphere (primary air pol-
lutants) or formed within the atmosphere itself (secondary air pollutants).
Apart from the physical state of pollutants (such as gaseous or particulate
matter) it is important to consider the geographical location and distribu-
tion of sources. The local, urban, regional and global scale of air pollution
can be distinguished, depending primarily on the atmospheric lifetime of
specific air components.
Primary air pollutants include sulfur dioxide, oxides of nitrogen, car-
bon monoxide, volatile organic compounds, and carbonaceous and non-
carbonaceous primary particles. Some sources can be categorized on a
geographical scale (point, line or area sources). Properties of a variety of
sources such as road transport, stationary combustion sources and natural
sources are described. Secondary air pollutants arise from chemical reac-
tions of primary pollutants in the atmosphere, often involving natural com-
ponents of the environment such as oxygen and water. Prominent second-
ary pollutants in the air include ozone, oxides of nitrogen and secondary
PM.
For primary air pollutants, emission inventories are (often in combina-
tion with dispersion models) a powerful tool for predicting air quality. They
can, for example, be used to model local, regional and global conditions
and observe spatial and temporal trends in emissions. Receptor modelling
is an alternative method that uses measurements of air quality, frequently in
combination with simultaneously measured meteorological data, to recog-
nize and quantify the contributions of specific characteristic source types to
air pollutant concentrations. For secondary air pollutants, the mode of their
formation makes it difficult to readily include them in emissions invento-
ries or receptor modelling. Nevertheless, it is possible to estimate formation
rates of secondary pollutants per unit volume of atmosphere per unit time.
Summary
Roy M. Harrison
A I R Q U A L I T Y G U I D E L I N E S1 0
Introduction
Basic definitions
Before discussing in detail the sources of air pollutants it is necessary to establish
a few basic principles that will place the information on sources in context. Air
pollutants may be either emitted into the atmosphere or formed within the at-
mosphere itself.
Primary air pollutants
Primary air pollutants are those that are emitted into the atmosphere from a
source such as a factory chimney or exhaust pipe, or through suspension of con-
taminated dusts by the wind. In principle, therefore, it is possible to measure the
amounts emitted at the source itself. This is relatively straightforward in terms of
the factory chimney or vehicle exhaust pipe; it becomes very much more difficult
when considering diffuse sources such as wind-blown dusts. When such sources
are added together they comprise an emissions inventory of primary sources, as
described below.
Secondary air pollutants
Secondary air pollutants are those formed within the atmosphere itself. They
arise from chemical reactions of primary pollutants, possibly involving the natu-
ral components of the atmosphere, especially oxygen and water. The most fa-
miliar example is ozone, which arises almost entirely from chemical reactions
that differ with altitude within the atmosphere. Because of this mode of forma-
tion, secondary pollutants cannot readily be included in emissions inventories,
although it is possible to estimate formation rates per unit volume of atmosphere
per unit time (1).
Another important distinction must be made in relation to the physical state
of a pollutant.
Gaseous air pollutants
Gaseous air pollutants are those present as gases or vapours, i.e. as individual
small molecules capable of passing through filters provided they do not adsorb
to or chemically react with the filter medium. Gaseous air pollutants are read-
ily taken into the human respiratory system, although if water-soluble they may
very quickly be deposited in the upper respiratory tract and not penetrate to the
deep lung.
Particulate air pollutants
Particulate air pollutants comprise material in solid or liquid phase suspended in
the atmosphere. Such particles can be either primary or secondary and cover a
wide range of sizes. Newly formed secondary particles can be as small as 1–2 nm
1 1S O U R C E S O F A I R P O L L U T I O N
in diameter (1 nm = 10–9 m), while coarse dust and sea salt particles can be as
large as 100 μm (1 μm = 10–6 m) or 0.1 mm in diameter. However, the very large
particles have a short atmospheric existence, tending to fall out rapidly through
gravity and wind-driven impaction processes. Thus in practice there are few par-
ticles in the atmosphere exceeding 20 μm in diameter, except in areas very close
to sources of emission. Particulate matter can be separated from atmospheric
gases by drawing air through a filter fine enough to retain the particles, or by
accelerating air through a jet that fires them at a fixed plate, onto which the parti-
cles impact and are collected. Particulate air pollutants have very diverse chemi-
cal compositions that are highly dependent on their source. They are also di-
verse in terms of particle size. Fig. 1 illustrates the range of sizes (on a logarithmic
scale) together with the ranges where certain important components are typically
encountered. It shows also the PM10, PM2.5 and ultrafine particle fractions, which
are typically those measured within the atmosphere for the purposes of health ef-
fects studies; the first two fractions are also used for compliance monitoring.
In the context of discussing sources of air pollution, it is important to consider
the geographical location and distribution of sources. Air pollution occurs on a
range of spatial scales linked primarily to the atmospheric lifetime of the specific
pollutants. Typical spatial scales are the following.
Local scale
Some pollutants, by virtue of their source or of having a very short atmospheric
lifetime, are only encountered in appreciable concentrations close to where
they are emitted. Examples are mainly rather esoteric chemicals, emitted from
Fig. 1. Size range of airborne particles, showing the health-related ultrafine, PM2.5 and
PM10 fractions and the typical size range of some major components
0.001 0.01 0.1 1.0 10 100
Soil, road dust
Diesel smoke
Particle diameter (μm)
Ultrafine fraction
PM2.5
PM10
PM2.5−10
Sulfate
Nitrate
A I R Q U A L I T Y G U I D E L I N E S1 2
specific industrial processes, that are not present at significant concentrations in
the atmospheric background. Hydrogen fluoride is a pollutant with a relatively
low general background concentration in the atmosphere, which can be encoun-
tered in high concentrations close to brickworks and other industrial sources.
1,3-Butadiene is an example of a pollutant with a very short atmospheric lifetime
(typically of the order of an hour in daytime) that is encountered at elevated con-
centrations only rather close to its source, in this case mainly road traffic. In less
developed countries, poorly controlled household and neighbourhood sources,
often involving the burning of biomass fuels, cause serious local pollution.
Urban scale
Pollutants from urban sources, such as nitrogen oxides and carbon monoxide
generated by road traffic, tend to be present at high concentrations throughout
the city and at significantly reduced concentrations in adjacent rural areas. Their
atmospheric lifetimes are not long (typically hours) and therefore concentrations
in the remote background atmosphere tend to be very low (except in the case of
carbon monoxide, which is more persistent). In countries such as China, coal
burning may cause severe urban pollution with smoke and sulfur dioxide. Urban
processes are discussed in detail by Fenger (2).
Regional scale
Pollutants in the form of fine particles (<2.5 μm diameter, but not ultrafine par-
ticles) and some gas-phase pollutants such as ozone have atmospheric lifetimes
of days or even weeks, which permit them to be transported on a regional scale.
Pollutants such as sulfate particles and ozone readily travel thousands of kilome-
tres in a process known as long-range transport, crossing national boundaries
in doing so. Fine aerosols (particles) of black carbon arising from the burning of
fossil fuels and biomass are also capable of long-range transport.
Hemispheric and global scales
Some pollutants, and especially those associated with greenhouse warming effects
(carbon dioxide, nitrous oxide and methane) have atmospheric lifetimes of years
and are therefore capable of distribution throughout a hemisphere and ultimately
globally. In these cases, concentrations are often only marginally higher close to
sources compared to the regional background, unless the sources emit very large
quantities.
Primary pollutants
Primary pollutant formation mechanisms
To understand the sources and source inventories for air pollutants it is helpful to
know the mechanisms of formation and release of the common air pollutants.
1 3
Sulfur dioxide
By far the main source of sulfur dioxide is the combustion of fuels containing
sulfur. Fossil fuels, most notably coal and oil, contain varying amounts of sulfur
according to their source but typically between 1% and 5%. On combustion, the
sulfur in the fuel is converted almost quantitatively to sulfur dioxide. Nowadays
in developed countries, much of the sulfur is removed from motor fuels in the
refining process and from stack gases prior to emission. Sulfur is most abundant
in the less volatile fractions of crude oil and hence shipping, which burns residual
fuel oil, can be a very high emitter of sulfur dioxide. The sintering process used in
metal smelting, which involves roasting metal sulfide ores in a stream of air, can
also be a major mechanism of sulfur dioxide production. In less developed coun-
tries, however, unabated burning of coal and the use of fuel oils and automotive
diesel with a higher sulfur content are major sources of sulfur dioxide.
Oxides of nitrogen
In a process parallel to that of sulfur dioxide production during fuel combustion,
nitrogen in fuels is converted to oxides of nitrogen in the combustion process.
Coal is the most important fuel in this context, as oil and gas contain much lower
levels of nitrogen. However, there is a further process in which atmospheric ni-
trogen and oxygen are combined during high-temperature combustion to form
oxides of nitrogen. This occurs in all high-temperature combustion processes and
explains why road traffic and electricity generation tend to be among the predomi-
nant sources of these gases. The majority of nitrogen oxides formed through this
route are emitted as nitric oxide. A smaller amount, typically 5% of the total, is
emitted as primary nitrogen dioxide, while the major proportion of atmospheric
nitrogen dioxide is a secondary product of atmospheric chemistry (see page 25).
Carbon monoxide
This is a gas formed during the incomplete combustion of carbon-containing fu-
els. While complete combustion leads to the formation of carbon dioxide, most
combustion systems involve some fuel-rich regions in which a proportion of car-
bon is oxidized only to carbon monoxide. The most important example is the
combustion of petrol in road vehicles.
Volatile organic compounds (VOC)
VOC comprise a very wide range of hydrocarbons, oxygenates, halogenates and
other carbon compounds existing in the atmosphere in the vapour phase. The
predominant source is typically through leakage from pressurized systems (e.g.
natural gas, methane) or evaporation of a liquid fuel such as benzene from the
fuel tank of a vehicle. However, combustion of fossil fuels and incineration proc-
esses also give rise to combustion emissions containing some unburned or par-
tially burned fuel fragments that are emitted in the form of VOC. The exhaust
S O U R C E S O F A I R P O L L U T I O N
A I R Q U A L I T Y G U I D E L I N E S1 4
pipe of a vehicle may therefore be as important as the fuel tank as a source of
VOC emissions. Organic solvents, used for example in paints and adhesives, are
designed to disperse in the atmosphere to allow the active ingredients to dry.
Carbonaceous particles
The particles emitted from burning fossil fuels and biomass, for example in diesel
and petrol engines, are typically composed largely of carbon, both in the elemen-
tal form and as organic compounds of low volatility. The elemental carbon is in
the form of microcrystalline graphite formed from the build-up of carbon-con-
taining free radicals into polycyclic aromatic structures within the flame. When
these build only relatively small molecules they are emitted as polycyclic aro-
matic hydrocarbons, an important pollutant in their own right often associated
with airborne particles (3). If larger graphite structures are created in the flame,
these will be emitted as particles of elemental carbon. Such combustion systems
also tend to emit hydrocarbons of low volatility, deriving for example from lubri-
cating oils, and these will typically condense on to the carbon particles. Carbon
present within organic compounds, rather than as elemental carbon, is referred
to as organic carbon.
Non-carbonaceous primary particles
An important source of non-carbonaceous particles is fly ash, which comprises
particles largely of mineral material freed from a fuel such as coal in a combus-
tion source and carried into the atmosphere with the flue gases. Purely mechani-
cal processes such as quarrying can also create fragments of rock small enough
to become suspended in the atmosphere and, as mentioned above, the action
of the wind can suspend particles of soil and dust from land surfaces into the
atmosphere. Construction and demolition activity can be an important source
of coarser particles (4), even in a street canyon where traffic emissions typically
dominate.
Types of source
Before considering how inventories of emissions are constructed, it is valuable
to consider the types of source responsible for air pollutant emissions. There are
many ways of categorizing sources, and this section explores some of the main
subdivisions and their characteristics. One of the main distinctions frequently
drawn is between stationary and mobile sources.
This is a fairly obvious distinction whereby road vehicles, railway trains, ships,
etc. comprise the mobile sources while industrial and household emissions, etc.
comprise the stationary sources. In practice, however, air pollution science is
rarely concerned with individual mobile sources but rather with the aggregated
effects, such as that of all the vehicles travelling on a road within a defined period
of time.
1 5
Consequently, a more useful categorization of sources is:
• point sources
• line sources
• area sources.
The term point source refers to sources that appear as individual points in the
context of a gridded emissions inventory, which may resolve emissions spatially
down to a 1 × 1-km scale or lower. Thus, for example, a power station might be
considered as a point source even though it has more than one chimney. Individ-
ual industrial sites are typically considered as point sources of pollution unless
the emissions occur from multiple sources within the site, probably at different
heights. In such a case, each individual point of emission may need to be con-
sidered as a separate point source, particularly for dispersion modelling. Nev-
ertheless, for the purposes of constructing emissions inventories, which tend to
be concerned mostly with the masses of pollutant emitted as opposed to other
characteristics of the release, individual company sites are often considered as
point sources.
As stated above, air pollution science is rarely concerned with emissions from
individual vehicles. Since road vehicles and railway trains typically travel along
common routes, from the perspective of a source of emissions they form a line
source. In the case of road vehicles, road networks are typically broken up into
individual sections between junctions referred to as road links, and emissions
from individual road links are added together to compile an inventory of total
traffic emissions. Dispersion modelling uses formulae to calculate downwind
concentrations from line sources that are modified from those used for point
sources.
Many sources of emissions fit neither the point source nor the line source
model. Rather, they are more diffuse and therefore spread over a significant spa-
tial region. An example would be emissions from boilers used for space heating,
whereby most homes would possess their own boiler, each of which is a small
source of emissions. Rather than treating each as an individual point source,
however, they are typically aggregated over an area, such as a grid square in an
inventory or over a city, and treated as a uniform source within that area. This
is justified provided they are distributed relatively homogeneously and no indi-
vidual boiler makes such a large contribution that it requires modelling as an
individual point source.
Properties of some types of source
Road transport
One of the major sources within any emissions inventory is road transport. The
term is used to describe all road traffic emissions, irrespective of the size or usage
of the vehicle. Emissions from road vehicles are typically thought of in terms
S O U R C E S O F A I R P O L L U T I O N
A I R Q U A L I T Y G U I D E L I N E S1 6
of the exhaust, though this is only part of the story (see below). Combustion of
petrol or diesel fuel leads to the production of exhaust gas containing a range
of potentially harmful pollutants. In many modern vehicles this passes through
a control device, such as a three-way catalytic converter, before emission to the
atmosphere. Pollutants emitted from the combustion of petrol or diesel fuels
typically include carbon monoxide, oxides of nitrogen, VOC and suspended par-
ticles. Some countries still use lead additives in petrol and this generates an im-
portant air pollutant emission.
The amounts of carbon monoxide, nitrogen oxides, VOC and PM from road
vehicles are closely regulated. There are essentially four sets of regulatory limits,
i.e. those set by the US Environmental Protection Agency (USEPA), the State of
California (which are more strict than those set by USEPA), the European Union
(EU) and Japan. While these emission limits are set for vehicles sold within the
boundaries of the standard-setting organization, they are typically also adopted
by adjacent countries such as Canada (for American standards) and non-EU
countries within Europe (for EU standards). The exhaust emission standards are
set as limits in grams per kilometre or grams per mile of pollutant emitted over a
standard driving cycle, within which a vehicle on a chassis dynamometer (rolling
road) goes through a standard set of speed and load variations reflecting those
experienced on the road. These are not normally evaluated for each individual
new vehicle. Rather, a type approval process is used by which a manufacturer
submits a small number of representative vehicles of a given type for testing. If
these comply with the regulatory limits, the manufacturer gains approval for the
sale of further vehicles of that type.
While exhaust emissions are often the most important emissions from a vehi-
cle, they are far from being the only ones. Evaporative fuel emissions can also be
important, especially from petrol vehicles, and these are measured and included
in inventories of emissions. Far more difficult to account for, however, are the
other non-exhaust emissions of PM from road vehicles that arise from sources
such as the wear of brake components and tyres and the attrition of the road sur-
face itself. Crude estimates have been made of the magnitude of these sources,
which are included in many emissions inventories. However, road vehicles also
cause the emission of particles by suspending particles from the road surface
into the air, either through the turbulence in the wake of the vehicle or by the
shear forces between the tyre and the road surface. These are far more difficult
to account for and are not widely included in emissions inventories.
Stationary combustion sources
The burning of fossil fuels in stationary combustion plants is a further major
source of pollutant emissions in most countries. High-temperature combustion
is a source of nitrogen oxides, and also of sulfur dioxide if sulfur is present in
the fuel. Fuel combustion also typically emits VOC, especially from coal and oil,
1 7
which are difficult to combust completely. While emissions from domestic heat-
ing and cooking can normally be considered as a ground-level source, those from
combustion plants can occur at a wide range of altitudes, ranging from ground
level for most domestic boilers to heights of more than 300 metres for large power
station chimneys. Consequently, per kilogram of pollutants emitted, the impact
on ground-level concentrations is very different: the ground-level source leads to
far higher local concentrations than the elevated point source, but the elevated
source influences areas much further afield because of the widespread dispersion
of emissions.
Power stations have typically been among the greatest sources of combustion
emissions but much is being done in the developed world to control the emis-
sions from such sources. Examples are the use of burners producing low levels of
nitrogen oxides and flue gas desulfurization of emissions from coal-fired power
plants, in addition to the electrostatic precipitators normally fitted to limit emis-
sions of PM.
Other industrial sources
A very wide range of industries and industrial processes lead to emissions of air
pollutants, both the classical pollutants but also more esoteric pollutants that may
be specific to a particular industrial process and may arise from leakage of an
intermediate or product from a chemical plant. The measurement of emissions
from chimneys and designed process vents is generally fairly straightforward,
although it may require many measurements to obtain representative data. As
well as the defined process emissions, however, many industrial operations also
generate fugitive emissions. These are those arising from other, less well-defined
sources, such as the wind blowing of raw materials from exposed stockpiles, and
these are very much more difficult to quantify. A good example is that of second-
ary metal smelters, many of which have been operational for a large number of
years and caused substantial contamination of the site and local area at a time
when emissions were far less closely regulated than at present. For such a smelter
it is likely that the impact of stack emissions on local air quality is very small,
whereas there is a much larger impact on local air quality from resuspension of
metal-containing soils and dusts by the action of the wind.
Intermittent and poorly defined sources
Globally, forest fires and deliberate biomass burning represent a major source of
combustion emissions, including nitrogen oxides, carbon monoxide, VOC and
PM. These are typically intermittent and often unplanned events that are very dif-
ficult to account for in any inventory. There are also smaller sources of this kind,
which may be quite significant in national inventories. Concern over the health
consequences of dioxin emission has led to a very tight regulation of known
sources, especially incineration of refuse. This has highlighted the importance
S O U R C E S O F A I R P O L L U T I O N
A I R Q U A L I T Y G U I D E L I N E S1 8
of other sources, however, such as accidental fires and even planned events such
as celebrations involving bonfires, as important sources of dioxin emissions. Be-
cause of their sporadic and very variable nature and the difficulty of measuring
emissions, the magnitude of such sources (although included within emissions
inventories) is rather uncertain. In less developed countries, burning of refuse
and biomass-based fuels in clusters of poor households represent poorly defined
and often intermittent sources that are very difficult to quantify.
Natural sources
Nature is an important source of many trace gases and particles within the at-
mosphere. One of the best known natural contributions to air pollution is the re-
lease of biogenic VOC from trees and other vegetation. These substances, which
comprise isoprene, terpenes and other constituents, contribute to the production
of both tropospheric ozone and secondary organic PM, and hence their impact
on air quality through secondary pollutant formation can be very important.
Globally, the natural production of sea spray and wind-blown soil is large, al-
though its relevance to air pollution phenomena and health is likely to be very
much smaller. In arid countries, dust storms can cause massive increases in PM
concentrations, and wind-blown soils and dusts are one of the major particulate
pollutants (5).
Inventories of primary emissions
Emissions inventories are important tools and are the result of summing emis-
sions from different sources across a geographical area, whether a grid square on
a map or the entire area of a city, country or continent (6). Emissions inventories
are applicable only to emissions of primary pollutants; there is no straightfor-
ward way of including secondary pollutants. Typical applications of emissions
inventories include local, regional and global air quality modelling and the sur-
veillance of trends in emissions, both spatially and temporally. They are one of
the key tools used by governments for air quality management.
Emissions inventories are compiled by summing the contributions of indi-
vidual sources or source categories. Typically, for individual large point sources,
measurement data will be available from which a direct estimate of the annual
emission can be made. For smaller sources and particularly for area sources,
however, it is more usual to calculate emissions using the following approach:
annual emissions = measure of activity × emission factor
In this context the measure of activity may vary considerably. In the case of space
heating with gas, the most readily available statistic may be the annual domestic
consumption of natural gas. For an industrial process, on the other hand, it may be
the number of tonnes of cement manufactured. These activity statistics are then
multiplied by an emission factor expressed in a compatible unit. Thus, following
1 9
the above examples, the emission factor for nitrogen oxides from domestic gas
consumption would be expressed as grams of nitrogen oxides per cubic metre of
gas consumed. In the case of cement manufacture, it would be as grams of PM
per tonne of cement produced. Such emission factors are widely published by
national and international agencies and are often specific to particular processes
and applied technologies, including abatement. Thus emissions per tonne of ce-
ment manufactured would be different for a dry than for a wet manufacturing
process, and different for an electrostatic precipitator than for a baghouse filter.
In the case of road vehicles, the vehicle fleet will need to be subdivided ac-
cording to the type of vehicle, the fuel it uses, and its age or any abatement tech-
nology fitted. Emission factors are developed specifically for each of these ele-
ments. In conducting calculations for an inventory, it will be necessary not only
to know the type of vehicle in each category but also the annual mileage of that
type of vehicle or the proportion of the total mileage that it represents on a given
road link. Inventories are becoming increasingly sophisticated in disaggregating
vehicles according to their age and mileage, and also in allowing for high-emis-
sion vehicles with faulty abatement devices. It is not feasible to take data directly
from type approval testing and assume that a vehicle that has been operating for,
say, 100 000 km produces the same emissions as a new vehicle on a dynamom-
eter test. Test cycles, although aiming to reflect the real world, do not always do
so very well.
In compiling emissions inventories it is usual first to define a domain or spatial
resolution, and second to define categories of activity into which emissions will
be subdivided. The largest domain for most emissions inventories is the nation
state, with many countries actively maintaining their own inventories of national
emissions. Such data are valuable in setting targets for reducing emissions and
in monitoring compliance with the requirements of international protocols such
as the EU’s National Emissions Ceilings Directive. International organizations
and programmes such as the European Environment Agency and the European
Monitoring and Evaluation Programme (EMEP) also maintain emissions inven-
tories, in this case resolved into 50 × 50-km grid squares both at national and in-
ternational levels. Many national inventories are more highly resolved; the United
Kingdom initially developed urban inventories on a 1 × 1-km scale, but now has
inventories at this resolution for many pollutants over the entire country.
There are different conventions for subdividing emissions, according to the
activity responsible for them. One example is the SNAP 97 activity system used
within the CORINAIR database by the European Environment Agency. The main
categories used in the SNAP 97 system are listed in Box 1. Within these are nu-
merous subcategories allowing the inventory user to investigate in greater depth
the relative importance of different source types. Such inventories have many
uses and are critical to the operation of global, regional and mesoscale (1–100
km) air quality models that require spatially disaggregated source input data.
S O U R C E S O F A I R P O L L U T I O N
A I R Q U A L I T Y G U I D E L I N E S2 0
Source: Air Quality Expert Group (7).
Fig. 2. Emissions of nitrogen oxides, sulfur dioxide, ammonia and non-methane VOC
from anthropogenic sources by sector, United Kingdom, 2001
Other transport
9.1%
Road transport
46.5%
Other 2.1%
Public power 22.6%
Industry 15.2%
Domestic 4.5%
Nitrogen oxides Sulfur dioxide
Non-methane VOCAmmonia
Processes 13%
Public power 64%
Other 4%
Other transport
8%
Road transport
36%
Industry combustion
31%
Domestic 1%
Waste 3%
Production processes
2%
Fuel combustion
6%
Agriculture (animal wastes)
77%
Agriculture (other)
12%
Fuel extraction
17%
Solvents 18%
Other 8%
Box 1.
Main
categories of
air pollutant
emissions
used in
SNAP 97
1 Combustion in energy and transformation industries
2 Non-industrial combustion plants
3 Combustion in manufacturing industry
4 Production processes
5 Extraction and distribution of fossil fuels and geothermal energy
6 Solvent and other product use
7 Road transport
8 Other mobile sources and machinery
9 Waste treatment and disposal
10 Agriculture
11 Other sources and sinks
2 1
United Kingdom
EU15
AC9
United States
Austria
Belgium
Denmark
Germany
Finland
France
Italy
Luxembourg
Netherlands
Spain
Sweden
Delhi, India
Carbon monoxide
Nitrogen oxides
NM VOC Sulfur dioxide
PM10 PM2.5 PM1
RT OT RT OT RT OT RT OT RT OT RT OT RT OT
5
6.1b
69
57
51
24.2
53.3
56.0
53.0
48.6
41.5
68.1
64.0
60.5
53.8
57.5
85.5
Area
11
7
26
42
45
37
34
40.8
48.8
36.8
50.9
45.9
51.4
50.3
43.8
41.8
39.9
44.9
82.4
11
18
12
22
11.9
9–19
6.5
15
24
31
37
29
9.9
30.3
34.2
20.4
31.7
25.5
43.6
37.5
36.0
15.2
21.8
84.1
4
6
5
18
2.6
5
5–12
1.6
1
3
2
2
7.1
3.3
1.9
3.1
4.9
1.0
25.0
4.9
1.5
1.9
39.0
3
4
1
5
0.9
1–3
2.6
18
28a
1.4b
12.8
12.2
13.0
16.1
11.7
11.3
14.7
8.8
14.7
16.1
13.9
15.6c
6
11a
2.2b
24
3.4b
30 7
Table 1. Contribution of road transport (RT) and other modes of transport (OT) to se-
lected pollutant emissions by percentage of total emissions for the United Kingdom
in 2000, the EU (EU15) in 1999, the EU accession countries (AC9) in 1999, the United
States in 1999, various European countries in 1999 and Delhi, India in 1995
a Emissions of particulates assigned as primary and secondary fine particulates, of which 12% are considered primary PM10. b Direct emissions only (i.e. does not include fugitive dust). c Based on inventory for total suspended particulates.
Sources: Thomas & Harrison (8); Goodwin et al. (9); Goodwin & Mareckova (10); US Environmental Protection Agency (11); European Environment Agency (12); Gurjar et al. (13).
National inventories are also valuable in the following ways.
• They illustrate the relative importance of different source categories. For
example, Fig. 2 shows national emission inventories for the United Kingdom
for nitrogen oxides, sulfur dioxide, ammonia and non-methane VOC for
2001, showing widely different source profiles. Such information is useful
in comparing the emissions between different countries. Table 1 shows, for
a range of countries, the contributions of road transport and other modes of
transport to emissions of selected pollutants by percentage of total emissions.
• They illustrate temporal trends in pollutant emissions. This is illustrated in
Fig. 3, which shows the percentage change in PM10 emissions by country
for Europe between 1990 and 2001, illustrating widely differing behaviour
between different economies. Fig. 4 shows a time series by source category of
sulfur dioxide emissions in Hong Kong, China. This shows successive rapid
rises and declines in emissions, followed by a gradual increase between 1999
and 2004. It also shows that the dominant contribution due to electricity gen-
eration drives the temporal trends.
S O U R C E S O F A I R P O L L U T I O N
A I R Q U A L I T Y G U I D E L I N E S2 2
Fig. 3. Percentage changes in PM10 emissions in selected European countries
between 1990 and 2001
Source: Air Quality Expert Group (7).
Fig. 4. Temporal trends in sulfur dioxide emissions by source category in Hong Kong,
China, 1990–2004
Source: Hong Kong Environmental Protection Department (14).
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004
200 000
180 000
160 000
140 000
120 000
100 000
80 000
60 000
40 000
20 000
0
Year
United Kingdom
Sweden
Netherlands
Germany
Luxembourg
France
Italy
Ireland
Denmark
Spain
Austria
Greece
Portugal
Belgium
Finland
Switzerland
Liechtenstein
Norway
Hungary
Poland
Czech Republic
–60 –40 –20 0 20 40 60 80
European Union
European Free Trade Area
Accession countries
−42%
−39%
−36%
−23%
−17%
−13%
−9%
−1%
−1%
−1%
3%
5%
5%
19%
71%
−26%
−18%
−8%
2%
3%
9%
Other fuel combustion
Civil aviation
Shipping
Road transport
Public power generation
To n
n e
s
0–10 tonnes
10–30 tonnes
30–50 tonnes
50–100 tonnes
100–200 tonnes
2 3
• They allow one to compare the emissions profiles of different geographical
areas. For example, when emissions inventories for major urban areas are
compared with national inventories, it is typical for road traffic to be seen to
play a much greater role in the emissions of cities than of whole countries.
This behaviour is also reflected in the air quality measurements within cities.
Fig. 5 shows a map of emissions of nitrogen oxides from road transport in
London in 1999. The influence of road traffic is clearly seen, first in terms
of a substantial gradient in emissions between the heavily trafficked central
areas and the less trafficked suburban regions of the city, and second through
the clear delineation of major road links. The most obvious one is the M25
motorway, which takes an orbital route around the city and hence appears to
provide a boundary to the figure. Major arterial routes are also visible on the
map, despite the fact that they cover only a small proportion of the 1 × 1-km
grid squares into which the map is resolved.
Receptor modelling of pollutants
Emissions inventories, in combination with dispersion models, are a powerful
tool for predicting air quality. Receptor modelling is an alternative method, used
most frequently in relation to emissions of PM. This method uses the measure-
ments of air quality itself, often in combination with simultaneously measured
meteorological data, to recognize and quantify the contributions of specific
characteristic source types to air pollutant concentrations. In the case of PM,
multi-component chemical analyses of consecutively collected air samples al-
low recognition of components that co-vary in time and therefore have the same
source. Typically some 6–10 individual source types can be identified through
their chemical profiles. Table 2 illustrates the source apportionment of PM in
Fig. 5. Emissions of
nitrogen oxides from
road transport in
London, 1999
Source: Air Quality Expert Group (7).
S O U R C E S O F A I R P O L L U T I O N
A I R Q U A L I T Y G U I D E L I N E S2 4
Bangladesh as determined in a receptor modelling study (15), and Fig. 6 illus-
trates the results of a receptor modelling study of PM2.5 in three Indian cities:
Delhi, Calcutta and Mumbai. These contrast with results from most developed
countries in showing large contributions from biomass burning, coal combus-
tion and road dust. In developed countries, road dust is seen mainly in the coarse
particle fraction and makes little contribution to PM2.5.
Secondary pollutants
Introduction
As mentioned above, a number of important air pollutants arise predominantly
through formation within the atmosphere itself. These arise as a result of atmos-
pheric chemical reactions, and this section outlines the important reaction proc-
esses as well as giving some indication of their implications for pollutant forma-
tion.
Source profile
Coarse particles
Sea salt
Soil dust
Road dust
Two-stroke engine
Metal smelter
Motor vehicle
Resuspended/fugitive lead
Construction
Fine particles
Road dust
Soil dust
Biomass burning/brick kiln
Sea salt
Metal smelter
Two-stroke engine
Motor vehicle
Resuspended/fugitive lead
Unknown source
Dhaka (city centre)
9.41
48.7
–
12.9
–
23.4
2.29
3.20
–
1.00
37.5
–
–
2.41
43.0
3.32
12.7
Dhaka (semi-residential area)
4.45
43.0
7.30
3.78
1.12
40.2
–
–
19.4
10.2
11.9
1.00
9.96
9.36
38.2
–
–
Rajshahi
12.7
44.1
14.2
–
–
23.2
–
5.87
5.29
1.88
50.4
13.9
–
–
28.5
–
–
Table 2. Average percentage contributions of the sources of coarse and fine particles at
three sites in Bangladesh
Source: Begum et al. (15).
2 5
The oxides of nitrogen/ozone system
As mentioned above, emissions of oxides of nitrogen occur predominantly in
the form of nitric oxide, which typically comprises around 95% of nitrogen ox-
ides from a combustion source. The pollutant of far greater concern in relation
to human health is nitrogen dioxide (NO2). The main pathway of conversion is
via reaction with atmospheric ozone (O3), which is present in the background
atmosphere from a range of sources, including atmospheric transport from the
stratosphere. There is also a pathway by which this chemistry can be reversed,
with nitrogen dioxide breaking down as a result of absorption of sunlight to form
nitric oxide (NO) and an oxygen atom (O), which reacts with an oxygen mol-
ecule (O2) to re-form ozone. The three reactions are as follows:
NO + O3 → NO2 + O2 (1)
NO2 + hυ (sunlight) → NO + O (2)
O + O2 → O3 (3)
Since each of these reactions is relatively fast, an equilibrium (the photostationary
state), containing amounts of all three components, is quite rapidly established.
Since reaction 2 depends on sunshine it becomes ineffective at night, and if there
is sufficient ozone present reaction 1 can convert all nitric oxide to nitrogen diox-
ide, which often occurs in rural areas. In a highly polluted environment there is
Fig. 6. Results of receptor modelling of PM2.5 in three Indian cities
Source: World Bank (16).
Unidentified
Secondary ammonium
Secondary nitrates
Secondary sulfates
Biomass
Coal
Road dust
Petrol
Diesel
D e
lh i,
sp ri
n g
D e
lh i,
su m
m e
r
D e
lh i,
a u
tu m
n
D e
lh i,
w in
te r
C a
lc u
tt a
, w in
te r
M u
m b
a i,
sp ri
n g
M u
m b
a i,
a u
tu m
n
M u
m b
a i,
w in
te r
350
300
250
200
150
100
50
0
C o
n ce
n tr
a ti
o n
(μ g
/m 3 )
S O U R C E S O F A I R P O L L U T I O N
A I R Q U A L I T Y G U I D E L I N E S2 6
unlikely to be sufficient ozone to complete the conversion and ozone concentra-
tions, particularly at night in winter, may well fall to zero.
This chemistry has obvious implications for the control of nitrogen dioxide
concentrations. Fig. 7 shows a typical relationship between annual mean con-
centrations of nitrogen dioxide and nitrogen oxides at an urban location in the
United Kingdom. The relationship is curvilinear and reduction of high concen-
trations of nitrogen oxides leads to a far less than commensurate reduction in
nitrogen dioxide. Consequently, very large reductions in emissions of nitrogen
oxides may be needed to achieve compliance with air quality objectives for nitro-
gen dioxide.
Sources of ground-level ozone
Ozone is a secondary pollutant with three rather distinct sources within the lower
atmosphere (see Fig. 8). There is a low background, probably less than half of the
concentration currently encountered in the northern hemisphere, which arises
from downward transport of ozone formed in the stratosphere by the photolytic
breakdown of oxygen, where its presence is essential to filtering harmful ultra-
violet light before it reaches the lower atmosphere (troposphere).
Ozone also forms in the troposphere as a result of atmospheric chemical re-
actions. The simple cycle shown in reactions 1–3 will not produce appreciable
amounts of ozone, since as soon as it forms in reaction 3 it can readily be broken
Source: Air Quality Expert Group (17).
Fig. 7. Relationship between annual mean concentrations of nitrogen oxides and
nitrogen dioxide measured at background sites in the United Kingdom, 1998–2001,
showing central London separately from other cities
0 50 100 150 200
100
80
60
40
20
0
N it
ro g
e n
d io
xi d
e (μ
g /m
3 )
Nitrogen oxides (μg/m3 as nitrogen dioxide)
y = 2.28 × 0.6887
y = 1.9301 × 0.6997
Central London
Elsewhere
Local and regional ozone formation
Hemispheric background
chemistry
Stratospheric air
2 7
down in reaction 1. The situation is different, however, when there is bright sun-
shine and chemically reactive hydrocarbons are present. In this situation, oxida-
tion of the hydrocarbons can lead to the formation of transient, highly reactive
species known as peroxy radicals. In a polluted atmosphere, peroxy radicals react
with nitric oxide and oxidize it to nitrogen dioxide:
NO + RO2 (alkyl peroxide) → NO2 + RO (4)
In doing so, the peroxy radical has converted nitric oxide to nitrogen dioxide
without consuming an ozone molecule. Therefore, when reaction 4 is added to
reactions 1−3 it can readily be appreciated that high concentrations of ozone can
build up as long as peroxy radicals continue to be created. For a fuller account of
the chemistry, the reader is referred to Harrison (18) or Monks (19).
The creation of ozone can occur on both short and longer timescales. The
longer timescales involve reactions in the remote atmosphere, for example over
the oceans, where low concentrations of nitrogen oxides interact with methane
and carbon monoxide (peroxy radical sources) to increase the hemispheric back-
ground of ozone, probably to about double that which prevailed in the pre-in-
dustrial era. The reactions are relatively slow but are important because of the
large availability of the long-lived methane and carbon monoxide in the remote
atmosphere. In more polluted atmospheres, as typified by Los Angeles but ap-
plying to many parts of the world, there is an abundance of more reactive hy-
drocarbons arising from anthropogenic emissions. In the presence of high con-
centrations of nitrogen dioxide and bright sunshine, high ozone concentrations
can form relatively rapidly, leading to substantial pollution. While this is best
known in the context of large cities such as Los Angeles and Mexico City, it also
Fig. 8. Typical source contribution to ozone concentrations measured on a polluted day
at a mid-latitude location in the northern hemisphere C
o n
ce n
tr a
ti o
n (μ
g /m
3 )
120
100
80
60
40
20
0
S O U R C E S O F A I R P O L L U T I O N
A I R Q U A L I T Y G U I D E L I N E S2 8
occurs on a larger regional scale in areas such as western Europe, where the emis-
sions from many cities combine together to form a highly polluted atmosphere
in which ozone forms and is transported over long distances. The presence of
peroxy radicals and their involvement in reaction 4 will also allow the creation of
high concentrations of nitrogen dioxide, which far exceed those predicted by the
photostationary state (reactions 1–3). One consequence of reaction 1 in polluted
cities is that ozone concentrations within the city itself are often lower than those
in the surrounding countryside, because fresh nitric oxide emissions from traf-
fic suppress high concentrations of ozone entering in air from the surrounding
countryside.
Secondary particulate matter
In some parts of the world, secondary particles can represent up to 50% of the
total concentration of particles in the air. They comprise three main components.
The first is sulfate, which arises from the atmospheric oxidation of sulfur dioxide
and leads initially to the formation of sulfur trioxide, which rapidly condenses
with water to form sulfuric acid. In regions with low ammonia emissions, sulfu-
ric acid comprises the major form of sulfate. In many places, however, there are
ample emissions of ammonia, which neutralizes the sulfuric acid to form solid
particles of ammonium sulfate. Nitrogen dioxide is also oxidized in the atmos-
phere (typically faster than sulfur dioxide) to form nitric acid, which is present
in the air as a vapour. Nitric acid, however, tends to react either with ammonia
or with materials such as calcium carbonate or sodium chloride, leading to the
formation of solid particles of nitrate. When these are in the form of ammonium
nitrate, the formation process is appreciably reversible:
HNO3 (nitric acid) + NH3 (ammonia) ↔ NH4NO3 (ammonium nitrate) (5)
Ammonium nitrate can dissociate back to nitric acid and ammonia, a process
favoured by high temperature and low relative humidity. There can therefore be
important diurnal and seasonal fluctuations in the amounts of ammonium ni-
trate in the air.
The third major form of secondary PM is secondary organic aerosol (SOA).
This comprises oxidized organic compounds formed in the atmosphere by reac-
tions of VOC. Biogenic VOC such as α-pinene emitted by trees are highly reac-
tive in this context, and in some areas provide a very significant source of SOA.
Anthropogenic VOC emissions are also capable of atmospheric oxidation, form-
ing species of lower volatility that condense to form SOA.
Typically, the formation of secondary aerosol is relatively slow, taking a day or
more. Consequently, the airborne concentrations of species such as sulfate tend
to be rather uniform over quite large distances. In the case of nitrates and SOA,
the formation processes are more rapid, and in the case of ammonium nitrate
may be reversible, and therefore higher spatial gradients are to be expected.
2 9
References 1. Seinfeld JH, Pandis SS. Atmospheric chemistry and physics. New York, Wiley
Interscience, 1998.
2. Fenger J. Urban scale processes. In: Hewitt CN, Jackson A, eds. Handbook of
atmospheric science, principles and applications. Oxford, Blackwell, 2003.
3. Smith DJT, Harrison RM. Polycyclic aromatic hydrocarbons in atmospheric
particles. In: Harrison RM, Van Grieken R, eds. Atmospheric particles. John
Wiley & Sons, 1998:253–294 (IUPAC Series on Analytical and Physical
Chemistry of Environmental Systems, Vol. 5).
4. Charron A, Harrison RM. Fine (PM2.5) and coarse (PM2.5–10) particulate
matter on a heavily trafficked London highway: sources and processes.
Environmental Science & Technology, 2005, 39:7768–7776.
5. Chueinta W, Hopke PK, Paatero P. Investigation of sources of atmospheric
aerosol at urban and suburban residential areas of Thailand by positive
matrix factorization. Atmospheric Environment, 2000, 34:3319–3320.
6. Hutchinson D. Emission inventories. In: Hewitt CN, Jackson A, eds.
Handbook of atmospheric science, principles and applications. Oxford,
Blackwell, 2003.
7. Air Quality Expert Group. Particulate matter in the United Kingdom.
London, Department of Environment, Food and Rural Affairs, 2005.
8. Thomas SB, Harrison RM. Human health impacts of air pollution emission
from transport. In: Hester RE, Harrison RM, eds. Issues in environmental
science & technology, Vol. 20. Cambridge, Royal Society of Chemistry, 2004.
9. Goodwin JWL et al. UK emissions of air pollutants 1970 to 2000. National
Atmospheric Emissions Inventory, 2002.
10. Goodwin J, Mareckova K. Emissions of atmospheric pollutants in Europe,
1990–1999. Copenhagen, European Environment Agency, 2002 (Topic
Report 5/2002).
11. National air quality and emissions trends report, 1999. Washington, DC,
US Environmental Protection Agency, 1999 (www.epa.gov/oar/aqtrnd99,
accessed 12 July 2006).
12. Emission inventory guidebook. Copenhagen, European Environment
Agency, 2002.
13. Gurjar BR et al. Emission estimates and trends (1990–2000) for megacity
Delhi and implications. Atmospheric Environment, 2004, 38:5663–5681.
14. Environmental Protection Department, Government of the Hong Kong
Special Administrative Region, 2006 [web site] (http://www.epd.gov.hk/epd/
english/environmentinhk/air/data/emission_inve.html, accessed 12 July
2006).
15. Begum BA et al. Investigation of sources of atmospheric aerosol at a hot
spot area in Dhaka, Bangladesh. Journal of the Air & Waste Management
Association, 2005, 55:227–240.
S O U R C E S O F A I R P O L L U T I O N
3 0 A I R Q U A L I T Y G U I D E L I N E S
16. Urban air pollution. Washington, DC, World Bank, 2004 (South Asia Urban
Air Quality Management Briefing Note No. 14).
17. Air Quality Expert Group. Nitrogen dioxide in the United Kingdom. London,
Department of Environment, Food and Rural Affairs, 2004.
18. Harrison RM. Chemistry and climate change in the troposphere. In:
Harrison RM, ed. Pollution: causes, effects and control. Cambridge, Royal
Society of Chemistry, 2001.
19. Monks P. Tropospheric photochemistry. In: Hewitt CN, Jackson A, eds.
Handbook of atmospheric science, principles and applications. Oxford,
Blackwell, 2003.
3 1
2. Global ambient air pollution concentrations and trends
Air quality measurements over the last decade have revealed air pollution
problems in many of the major urban areas of the world, with some cities in
developing countries currently facing the greatest challenges. Some typical
ranges of concentrations of the four indicator pollutants found in a selec-
tion of cities around the world are summarized in Table 1.
The highest concentrations of the “classical” indicators such as PM10 and
sulfur dioxide are found in Africa, Asia and Latin America. The highest
levels of secondary pollutants such as ozone and nitrogen dioxide are meas-
ured in Latin America and in some larger cities and urban airsheds in the
developed countries.
Trends in air quality development differ in respect of the four indicator
pollutants. In Europe, PM10 levels had decreased by the end of last century
but have tended to rise again, which may be partially explained by changing
weather conditions. Even though large Asian cities have seen a slight reduc-
tion in PM10 levels over the last few decades, PM (PM10 and PM2.5) is still
the major air pollutant in Asia. Many of the large cities in Latin America, as
well as Mexico City, still experience high levels of PM.
Sulfur dioxide levels have fallen in most parts of the world, including
Region
Africa
Asia
Australia/New Zealand
Canada/United States
Europe
Latin America
PM10
40–150
35–220
28–127
20–60
20–70
30–129
Nitrogen dioxide
35–65
20–75
11–28
35–70
18–57
30–82
Sulfur dioxide
10–100
6–65
3–17
9–35
8–36
40–70
Ozone (1-hour maximum
concentration)
120–300
100–250
120–310
150–380
150–350
200–600
Annual average concentration
Table 1. Ranges of annual average concentrations (μg/m3) of PM10, nitrogen
dioxide and sulfur dioxide and one-hour average maximum concentrations of
ozone for different regions, based on a selection of urban data
Bjarne Sivertsen
Summary
A I R Q U A L I T Y G U I D E L I N E S3 2
Assessment of air quality based on available monitoring data
Which pollutants are addressed?
Ambient air pollution consists of a highly variable and complex mixture of dif-
ferent substances, which may occur in the gas, liquid or solid phase. Several hun-
dred different components have been found in the troposphere, many of them
potentially harmful to human health and the environment. Institutions such as
environmental agencies around the world have developed a core set of air pollu-
tion indicators and criteria pollutants, which have been widely used to character-
ize air quality.
The assessment in this chapter is based mostly on data from monitoring of
health-related air pollutants and focuses on four indicators of air pollution:
• PM, measured as particles with an aerodynamic diameter <10 μm (PM10) and
<2.5 μm (PM2.5)
• nitrogen dioxide
• sulfur dioxide
• ozone.
substantial declines in Europe, China and the United States and to a more
moderate extent in larger cities in Asia and Latin America.
Average national nitrogen dioxide concentrations have generally not
declined, except in the United States. Since the principal sources of some
secondary pollutants such as nitrogen dioxide and ozone are traffic-related,
there is growing concern about rising levels in fast-growing cities with large
numbers of vehicles. Data from Asian cities show a high year-to-year varia-
tion in nitrogen dioxide and ozone levels that currently renders trend anal-
yses inconclusive.
As with nitrogen dioxide, ozone concentrations generally do not show
a tendency to fall. Overall, the hemispheric background concentration of
tropospheric ozone is increasing. Rising concentrations have been recorded
for North American and European cities, and levels exceeding WHO’s 2000
guideline values have been reported from cities in Mexico, Latin America,
Africa, Australia and Europe.
One of the trends predicted to lead to increasing air pollution levels is
the high rate of urbanization in countries where most of the population is
on low income. It is expected that the rapid growth in urban populations
will lead to a dramatic increase in vehicle numbers combined with inex-
pensive solutions for daily commuting, more frequent use of older and two-
wheeled vehicles, poor car maintenance and other developments that in-
crease air pollution.
3 3G L O B A L A M B I E N T A I R P O L L U T I O N C O N C E N T R A T I O N S A N D T R E N D S
These indicators have been selected for the purpose of identifying typical air pol-
lution concentrations. This selection, however, does not imply that other sub-
stances do not pose a considerable threat to human health and the environment
at levels present in urban and industrialized areas around the world.
These four indicator pollutants are linked by complex atmospheric chemistry.
Air pollution exists as a complex mixture and effects attributed to ozone, nitro-
gen dioxide, sulfur dioxide or PM may be influenced by the underlying toxicity
of the full mixture of air pollutants. Also, various sources such as cars or power
plants emit mixtures. Processes in the atmosphere further transform these pol-
lutants to new compounds. For example, ground-level ozone is a secondary pol-
lutant produced by the interaction of sunlight with nitrogen dioxide and VOC, as
described in detail in Chapters 1 and 11.
Only a small number of parameters are usually measured in order to char-
acterize the mixture; these parameters are then used as indicators in epidemio-
logical studies. The lack of monitoring data sometimes impairs the possibility of
identifying the most relevant indicator for different health endpoints (1).
Assessment of air quality is conducted by national or local authorities in many
countries. In some regions, such as Europe, international harmonization both of
monitoring and of data exchange takes place. However, there is currently no such
data exchange system covering other regions. Thus the information presented
later in this chapter is based on selected data only, published independently and
not in a harmonized way. The data illustrate patterns and trends in pollution but
cannot be considered to provide a comprehensive global overview of air quality.
Measurement methods, and quality assurance and quality control
Instruments for measuring air pollutants may vary greatly in complexity and
price, from the simplest passive sampler to the most advanced and expensive
automatic remote monitoring system based on light absorption spectroscopy of
various kinds.
Relatively simple equipment is usually adequate for determining background
levels, estimating long-term average concentrations and observing trends. Pas-
sive samplers may also be adequate for undertaking simple screening studies.
For the complete determination of air pollution distributions and relative source
impacts and the operation of warning systems, however, more complex and ad-
vanced monitoring systems are needed. Also, when data are needed for verifica-
tion of model performance, expensive monitoring systems are usually needed.
The accuracy of the air quality data and their representativeness in space and
time are important for the quality of the assessments produced from the data.
Data quality objectives are set, so that when they are fulfilled one can use the data
confidently for the purposes for which the monitoring objectives have been set
(2).
A I R Q U A L I T Y G U I D E L I N E S3 4
In Europe the criteria that guide the quantification of data quality objectives
are defined such as to permit:
• comparison of air quality across Europe;
• detection, over a reasonable period, of the trends in air quality in Europe as
well as in each area where stations are located; and
• assessment of exposure.
Data quality objectives may also include specifications of accuracy, precision,
area of representativeness and temporal coverage. Setting quality objectives and
following a well-defined quality assurance and quality control (QA/QC) pro-
gramme are essential for obtaining good quality data and comparable informa-
tion. The measurement methods used and information concerning data quality
requirements have not always been reported and controlled. Data quality may
thus vary from one city to another and from region to region.
Representativeness of data
Information about the ambient air pollution levels have been based on measure-
ments representative for different types of area and microenvironment, such as:
• traffic, near roads and in streets;
• urban areas, representative for the kilometre scale inside the urban airshed;
and
• rural areas, away from local sources representative for residential areas.
The selection of representative measurement sites, as well as the use of different
measurement methods, has made the interpretation and comparisons of the data
difficult. Concentration levels presented in this book must thus be looked on as
indicative of the air quality to be expected in different urban areas, in different
regions and on different continents. Fig. 1 illustrates typical average concentra-
tions of nitrogen dioxide, PM10 and ozone in rural, urban and traffic environ-
ments in Europe.
The ranges of concentration shown in Fig. 1 illustrate the variability in concen-
trations between different measurement sites, different environments and differ-
ent cities. Sources of PM10, nitrogen dioxide and other pollutants are normally of
local origin, and concentrations often increase the closer one is to the source. For
ozone, the picture is more complicated owing to chemical reactions involved in
the build-up of ground-level ozone concentrations. Various factors independent
of the air quality may affect the data, and therefore it is important to have infor-
mation about site locations, measurement methods and data quality assurance
procedures when evaluating air pollution levels in different cities and countries.
Megacities of the world
Air pollution in megacities around the world has been an issue for several years.
At the turn of the century there were 24 megacities each with more than 10 million
3 5
inhabitants (Fig. 2). Four of these had more than 20 million inhabitants in 2002.
Twelve of these megacities are located in Asia, four in Latin America and two in
Africa: Cairo in Egypt and Lagos in Nigeria. According to the World Bank some
African cities are growing by more than 10% annually.
Fig. 2. The 24 megacities in the world with populations (including suburbs) exceeding
10 million in 2002
Source: United Nations (4).
Fig. 1. Ranges of concentrations at rural, urban and traffic stations in Europe,
based on data for 2001
Max 19: 19th highest one-hour average concentration of nitrogen dioxide measured during the year. Max 36: 36th highest daily PM10 concentration during the year. Max 26: 26th highest 8-hour mean within one day, calculated from hourly running 8-hour average concentrations of carbon monoxide.
Source: Larssen S (3).
To k
yo
C h
o n
g q
in g
N e
w Y
o rk
S e
o u
l
S ã
o P
a u
lo
M e
xi co
C it
y
Lo s
A n
g e
le s
O sa
k a
M o
sc o
w
Ja k
a rt
a
M u
m b
a i
C a
ir o
K a
ra ch
i
C a
lc u
tt a
D e
lh i
S h
a n
g h
a i
D h
a k
a
La g
o s
B u
e n
o s
A ir
e s
Lo n
d o
n
B e
iji n
g
R io
d e
J a
n e
ir o
P a
ri s
M a
n ila
P o
p u
la ti
o n
(m ill
io n
s)
35
30
25
20
15
10
5
0
A n
n u
a l m
e a
n (1
0 th
– 9
0 th
p e
rc e
n ti
le s)
(μ g
/m 3 )
S h
o rt
-t e
rm m
e a
n (1
0 th
– 9
0 th
p e
rc e
n ti
le s)
(μ g
/m 3 )
Nitrogen dioxide
PM10 Ozone
80
60
40
20
0
R u
ra l
U rb
a n
Tr a
ff ic
R u
ra l
U rb
a n
Tr a
ff ic
R u
ra l
U rb
a n
Tr a
ff ic
160
120
80
40
0
Nitrogen dioxide max. 19
PM10 max. 36
Ozone max. 26
R u
ra l
U rb
a n
Tr a
ff ic
R u
ra l
U rb
a n
Tr a
ff ic
R u
ra l
U rb
a n
Tr a
ff ic
1990
2002
G L O B A L A M B I E N T A I R P O L L U T I O N C O N C E N T R A T I O N S A N D T R E N D S
A I R Q U A L I T Y G U I D E L I N E S3 6
In many megacities, such as Beijing, Calcutta, Mexico City, Rio de Janeiro and
Cairo, high levels of PM constitute a major problem. Los Angeles and Mexico
City are still recording high concentrations of ozone and nitrogen dioxide. Sev-
eral other cities, such as Karachi, Mumbai and Lagos, are fighting multiple air
pollutants that exceed national standards.
Air pollution levels are normally higher in developing countries than in highly
developed industrialized countries. This is illustrated by the typical annual aver-
age concentrations of nitrogen dioxide, sulfur dioxide and suspended particles
presented in Fig. 3.
Fig. 3. Typical annual average concentrations of nitrogen dioxide, sulfur dioxide
and suspended particles in different parts of the world
Source: United Nations Human Settlements Programme (5).
PM levels indeed present serious problems in the developing countries. PM10
concentrations have been reported from countries such as India and Pakistan to
be 4–5 times international air quality limit values.
Urban air quality problems
The serious consequences of exposure to high levels of urban ambient air pol-
lution were made clear in the mid-twentieth century, when cities in Europe and
the United States experienced air pollution episodes (such as the infamous 1952
London Fog) that resulted in many deaths and hospital admissions. Subsequent
clean air legislation and actions reduced ambient air pollution in many regions.
The winter smog problems associated with coal combustion that were common
in some cities during the 1980s and early 1990s have been eradicated, and it is
C o
n ce
n tr
a ti
o n
(μ g
/m 3 )
200
180
160
140
120
100
80
60
40
20
0
Developing countries
Countries in transition
Highly industrialized countries
Nitrogen dioxide
Sulfur dioxide
Suspended particles
3 7
now mainly emissions from traffic that pose the main threat to good air quality.
The main sources for the present air pollution levels in western cities are traffic-
related.
The previously frequent winter smogs comprising a mixture of sulfurous com-
pounds and particles (soot) have in this way changed over the years. Suspended
particles, and especially submicron particles, combined with secondary pollut-
ants such as oxides of nitrogen and ozone, have become a major problem in the
large urban areas around the world. At the same time, the populations of the
rapidly expanding megacities of Asia, Africa and Latin America are increasingly
exposed to levels of ambient air pollution that rival and often exceed those expe-
rienced in industrialized countries in the first half of the twentieth century (6).
PM10 or respirable particulate matter Up to now, the most frequently used indicator for suspended particles in the air
has been PM10 (particles with an aerodynamic diameter <10 μm). An overview of
typical annual average PM10 concentrations in selected cities around the world is
presented in Fig. 4. The data selected for this presentation demonstrate that the
general levels of suspended particles in Asia and Latin America are higher than
those in Europe and North America. The annual average PM10 concentrations in
the selected Asian cities ranged from about 35 μg/m3 to 220 μg/m3 and in Latin
America from about 30 μg/m3 to 129 μg/m3, while in Europe and North America
the typical range of annual average PM10 concentrations was 15–60 μg/m3. About
70% of the cities selected from these regions had annual average PM10 concentra-
tions above 50 μg/m3.
In general, the highest concentrations of PM10 were reported from Asia. This
region also experiences relatively high background concentrations owing to for-
est fires and local emissions of particles from the use of poor-quality fuels. A
well-known springtime meteorological phenomenon throughout East Asia,
causing the Asian dust cloud, originates from windblown dust from the deserts
of Mongolia and China and adds to the general level of PM in the region.
Chinese cities experience very high airborne particle concentrations due to
primary particles emitted from coal and biomass combustion and motor vehi-
cle exhaust, as well as secondary sulfates formed by atmospheric chemical reac-
tion from the sulfur dioxide emitted when coal is burned. Typical annual average
PM10 concentrations were reported to be as high as 140 μg/m3 in Beijing (16).
In many areas of the world, massive and prolonged forest fires have caused
significant increases in PM concentrations. During the approximately 2½ weeks
of the large forest fires in California in 1987, PM10 concentrations as high as
237 μg/m3 were measured (17). In another fire in the United States, lasting 10
weeks, PM10 levels exceeded 150 μg/m3 (24-hour average) 15 times, and on 2 days
the levels exceeded 500 μg/m3 (18). In the 1997 fires in south-east Asia, PM10 lev-
els as high as 930 and 421 μg/m3 were measured in Sarawak and Kuala Lumpur,
G L O B A L A M B I E N T A I R P O L L U T I O N C O N C E N T R A T I O N S A N D T R E N D S
A I R Q U A L I T Y G U I D E L I N E S3 8
respectively, while levels in Singapore and southern Thailand were somewhat
lower (19) (see Chapter 10 for more examples).
PM10 levels in Europe have been presented for 2002 based on data from more
than 1100 monitoring stations in 24 countries, including some 550 urban areas
(3).
In urban areas the average concentrations were:
• annual average 26.3 μg/m3 in urban background and 32.0 μg/m3 in streets;
and
• daily average (36th highest value) 43.2 μg/m3 in urban background and
51.8 μg/m3 in streets.
In rural areas the concentrations were:
• annual average 21.7 μg/m3; and
• daily average (36th highest value) 38.1 μg/m3 (141 stations).
The highest annual average concentrations measured exceeded 80 μg/m3, while
the highest daily average concentrations exceeded 150 μg/m3.
Fig. 4. Annual average PM10 concentrations observed in selected cities worldwide
Sources: Bourotte et al. (7); US Environmental Protection Agency (8); Sivertsen & El Seoud (9); Sivertsen et al. (10); State Environmental Protection Agency (11); CAFE (12); Department of