POLLUTION CONTROL TECHNOLOGIES – Vol. I - Dust-Particle Formation and Characteristics - A.

Buekens

DUST - PARTICLE FORMATION AND CHARACTERISTICS

A. Buekens
Department of Chemical Engineering – CHIS 2, Vrije Universiteit Brussel, Belgium

Keywords: Aerosols, Elutriation, Cascade Impactor, Particle Size Distribution, Sieving,

Source apportionment, Speciation.

Contents

1. Survey
2. Sources
3. Physical characteristics
3.1. Number and Number Distribution
3.2. Particle Sizes and Size Distribution

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3.3. Droplet Size

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3.4. Number Frequency Distribution
3.5. Mass Frequency Distribution
4. Chemical characteristics
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4.1. Elemental Composition
4.2. Speciation
4.3. Secondary Particles
4.4. Humidity
4.5. Carbonaceous Aerosols
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4.6. Particle Organic Composition

5. Source apportionment
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5.1. Atmospheric Aerosols

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5.2. Source Markers and Source Signatures

5.3. Spatial and Temporal Variation
5.4. Combustion Aerosols
5.4.1. Markers
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5.4.2. Heavy Metals

5.5. Association to Other Pollutants
5.6. Relevance for Health Related Issues.
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6. Dust Sample Analysis

6.1. Sieving
6.2. Sedimentation
6.3. Hydrometer Kit, Standard Set
6.4. Elutriation
6.5. Centrifugal techniques
6.6. Filtration
6.7. Inertial Impactors
6.8. Precipitation
7. Particle Evaluation
7.1. Optical Microscopic Examination
7.2. Optical Sizing
7.3. Automatic Coulter particle counters
7.4. Photic sensing

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POLLUTION CONTROL TECHNOLOGIES – Vol. I - Dust-Particle Formation and Characteristics - A. Buekens

7.5. Laser Aerosol Particle Size Spectrometer

7.5.1. Resolution
7.5.2. Particle Size Distribution
7.5.3. Characterising Submicrometer Particles
7.5.4. Backscatter Analysis
7.5.5. Polarization Intensity Differential Scattering
7.5.6. ISO 13320-1 Laser Diffraction Methods
7.6. Droplet Analysis
7.7. TEOM
8. Effluent Dust Analyzers
8.1. Scope
8.2. Isokinetic sampling
8.3. β-Radiation Attenuation Sensors
8.4. Flue gas opacity monitors
8.4.1. Transmissiometry

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8.4.2. Scattered-light Opacimetry

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8.4.3. Laser Dust Monitors
8.4.4. Triboelectric sensors
9. Dust Sampling Procedure
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9.1. Method
9.2. Data acquisition and reduction
9.3. Isokinetic Sampling Train
10. Conclusions
Glossary
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Bibliography
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Biographical Sketch
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Summary
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Dust is a very complex subject. Its origin may be natural, or a consequence of human
activity. Primary particles are as they were formed, secondary particles originate in
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atmospheric conversion processes.

Aitken particles are the largest in number, but represent little in mass units. They are
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important in cloud formation. Aerosols create haze and are relatively stable.

Total Suspended Particles represent the bulk of atmospheric dust and mass is still the
basis of dust emission codes. From a health point of view, however, it is important to
know whether dust is inhalable and how deeply it penetrates into the lungs. Today, there
is a shifting concern, from particulates smaller than 10 µm (PM10) to PM2.5 and even
much smaller.

The diameter of a particle may be defined in numerous ways, depending on the property
considered. Sieving addresses cross-sectional area, sedimentation, centrifugation and
elutriation an aerodynamic diameter. Particles can be examined using a microscope,
Coulter counter, laser diffraction, and cascade inertial impaction. Sampling and dust
monitoring methods are discussed.

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POLLUTION CONTROL TECHNOLOGIES – Vol. I - Dust-Particle Formation and Characteristics - A. Buekens

1. Survey

Particulate matter (PM) is very variable in origin, nature, concentration, and size
distribution. Natural particulate arises from erosion, sand storms, and surf, from forest
fires and volcanic activity. Bio-aerosols comprehend pollen, a seasonal allergen, as well
as transformation products of natural semi-volatiles, such as terpenes and isoprene.

Anthropogenic particles originate from combustion, metallurgy, bulk and filter dust
handling, and are a carrier of adsorbed chemicals, bio-contaminants or condensed gases,
which can trigger various health effects.

Primary particles are introduced into the air in solid or liquid form, while secondary
particles are formed in the air by gas-to-particle conversion of oxidation products of
emitted precursors.

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Atmospheric particles are evaluated very differently, depending on whether mass or

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number is handled as a main criterion.

The amount of Total Suspended Particles (TSP) in the atmosphere is expresses in mass
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units. Strong winds will enhance TSP, by remobilizing settled dust.

Inhalable particles, presented as PM10, are considered more important from a health
viewpoint. Size strongly influences in which part of the respiratory tract the particles are
deposited. Larger particles are deposited in the nasal area and in the upper parts of the
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respiratory tract. Smaller particles follow the airflow to the deeper parts and have a high
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probability of depositing by diffusion.

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Aitken particles, smaller than 0.1 µm in diameter, represent by far the largest number of
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particles and play a role in atmospheric processes, such as cloud formation, rainout and
washout.
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Aerosols are defined differently in various disciplines. They are relatively stable
suspensions of particles, and may also be termed mist, fog, fumes, and smoke.
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The size of particles determines dynamic properties, behavior and fate during transport.
Large particles are mainly of crystal origin, and from natural sources. The highest level
of concentration of trace elements and toxins from anthropogenic sources and
radioactivity from natural sources is related to the very small particles.

Particle physical and chemical properties bear a relation to the sources generating the
particles. This helps to identify the parameters or their ranges that should be specifically
targeted for various types of emission sources operating in the environment under
investigation.

Atmospheric aerosol concentrations range from:

▪ 2 µg m-3 in polar regions;

▪ 10 µg m-3 as background value;

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POLLUTION CONTROL TECHNOLOGIES – Vol. I - Dust-Particle Formation and Characteristics - A. Buekens

▪ 30 µg m-3 for remote and rural locations;

▪ 170 µg m-3 for polluted urban areas up to
▪ 100 000 µg m-3 in sand storms!

2. Sources

Coarse particles are mainly from mechanical processes, including grinding, breaking
and wear of material, transport, and dust re-suspension, and contain largely earth crust
compounds.

Fine particles are mainly from combustion, photochemical processes, and gas to particle
conversion. Typically they contain soot, organic compounds, and acid condensates, i.e.
sulfates and nitrates, as well as trace metals and other toxins.

Some processes generate particles with broad size distributions, covering both fine and

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coarse ranges, e.g. in forest fires there are airborne combustion products, as well as

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large diameter particles that are entrained into the smoke column as a result of the
turbulence and buoyancy generated by the fire. In most cases, however, fine and large
particles result from totally different generation processes. Thus fine and coarse
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airborne particles, or particle number and mass are not necessarily correlated. It is
evident that from the measurement of total particle mass only limited information can
be obtained.

Primary biological aerosol particles are discussed in Indoor Air Quality Monitoring and
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Control.
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▪ Condensed water aerosols comprise haze, fog, and clouds.

▪ Raindrops, snow and hail are very short-lived.
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▪ Hydrometeors are atmospheric particles with water as a dominant component.

▪ Clouds can be subdivided into:
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o cumulus, clouds with considerable vertical development;

o stratus: layer clouds;
o nimbus: rain clouds and
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o cirrus: fibrous ice clouds.

These are further combined to subgroups, e.g. strato-cumulus.

In haze horizontal visibility is between 1 and 10 km; in fog it descends below 1 km.

Particle properties strongly depend on both particle formation and post formation
processes. Fine and coarse mode particles have different chemical composition and
origins, are transported and removed by different mechanisms, and require different
detection techniques. Many efforts have been made to correlate emission sources and
immission of particulate, on a basis of composition, elemental and isotopic ratio, and
other characteristics, termed source markers.

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POLLUTION CONTROL TECHNOLOGIES – Vol. I - Dust-Particle Formation and Characteristics - A. Buekens

3. Physical Characteristics

Some important physical properties of particles (individually, or as a group) include:

▪ Number and number size distribution;

▪ Mass and mass size distribution;
▪ Specific surface area;
▪ Shape;
▪ Volatility;
▪ Hygroscopicity;
▪ Electrical charge.

There are no instruments that can measure the entire particle size range, from
nanometers to tens of micrometers and usually there is a size range selected for
investigations that depends on the objectives of the investigations. Various

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classifications and terminologies have been used to define particle size ranges.

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3.1. Number and Number Distribution
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Large numbers (up to millions per cubic meter) are associated with Aitken particles and
Cloud Condensation Nuclei. A second mode (i.e. a maximum in the distribution curves)
is generally generated by fine or coarser aerosol particles, caused by emissions and by
atmospheric chemical reactions often involving ammonia and sulfuric acid.
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Coarse particles are generally blown up by the wind and are of crystal origin.
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3.2. Particle Sizes and Size Distribution

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Size is characterized by particle diameter, or for irregular shapes - equivalent particle

diameter, i.e. the diameter of a sphere having the same value of a physical property as
the particle being measured. Equivalent diameter relates to any property, e.g. inertia,
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electrical or magnetic mobility, light scattering, radioactivity, Brownian motion, or to

chemical or elemental concentration, cross-sectional area, and volume to surface ratio.
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Particles from most atmospheric sources have a lognormal size distribution: a particle
concentration versus particle size curve is normal (bell shaped), when the particles are
plotted on a logarithmic scale. Particle size distributions often contain several distinct
peaks, called modes.

A wide-ranging aerosol collector (WRAC) provides an estimate of the full coarse mode
distribution. Inlet restrictions of the high volume sampler for TSP, the PM10 sampler,
and the PM2.5 sampler reduce the total mass reaching the sampling filter.

3.3. Droplet Size

Droplets are approximately spherical in shape, although elongation occurs under

industrial flow conditions. Depending on the application at hand the representative
droplet size is selected differently.

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POLLUTION CONTROL TECHNOLOGIES – Vol. I - Dust-Particle Formation and Characteristics - A. Buekens

Droplet size is frequently denoted by a mean or median diameter. Each nomenclature

for describing droplet sizes leads to different numerical values. Mass Volume Median
Diameter is the largest of all and number median diameter is the smallest. The
differences between the various mean and median diameters provide a way of
conveniently specifying the spread of droplet sizes produced. If all droplets had been
uniform in size, the mean and median diameters would have been identical with the
uniform size. Droplet size has been measured with a PDPA (Phase Doppler Particle
Analyzer) system.

Mean Value Definition

Arithmetic Weighted average of the diameters of all individual
droplets in the spray sample
Surface Diameter of a droplet whose surface area, if multiplied by
the total number of droplets, will equal the total surface
area of all droplets in the spray sample

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Volume Surface or Sauter Diameter of a droplet whose ratio of volume to surface

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area is equal to that of the entire spray sample
[Source: Dr. Alan Rawle, “Basic Principles of Particle Size Analysis”, Technical Paper, Malvern
Instruments Ltd.]
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Table 1: Usual Methods of Describing Droplet Size

Mass (Volume) Median Diameter: the diameter, which divides the mass (volume) of
spray into two equal halves.
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Vm = Vg + Vp volume of mixture (1)

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α = Vg Vm = 1 − Vp Vm void fraction (2)

where
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N = number of particles
Vg = volume of gas phase
Vp = volume of particulate phase
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3.4. Number Frequency Distribution

AREA = FN (D ) ΔD = Fraction of particles between D and D + ΔD (3)

Number mean μ N = ∫ 0∞ D FN ( D) dD (4)

Number variance σ N2 = ∫ ∞
0 ( D − μ N ) FN ( D ) dD
2
(5)

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POLLUTION CONTROL TECHNOLOGIES – Vol. I - Dust-Particle Formation and Characteristics - A. Buekens

Figure 1: Number Frequency Distribution

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3.5. Mass Frequency Distribution

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Figure 2: Mass Frequency Distribution

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AREA = FM ( D ) ΔD (6)

= Fraction of particulate mass associated with particles between D and D + ΔD

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Mass mean μ M = ∫ 0∞ D FM ( D) dD (7)

Mass variance σ M
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= ∫ 0∞ ( D − μ M ) 2 FM ( D) dD (8)

4. Chemical Characteristics

Important chemical properties are:

▪ Elemental composition, including mineral dust and water.

▪ Secondary inorganic ions, mainly sulfate, nitrate and ammonium.
▪ Carbonaceous compounds (organic and elemental carbon).

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POLLUTION CONTROL TECHNOLOGIES – Vol. I - Dust-Particle Formation and Characteristics - A. Buekens

▪ Organic composition.

4.1. Elemental Composition

Combustion results in emissions of a cluster of different trace elements, present in the

fuel or the lubricants used (motor vehicles). Since most trace elements are associated
with ultra fine particles and are less prone to chemical transformations, they undergo
long-range atmospheric transport.

Mechanical processes, such as mining, mineral processing, quarrying, breaking,

grinding, dust re-suspension, etc, generate particles predominantly containing crustal
elements. Table 2 relates common outdoor particle sources to common suites of
elements.

Emission Source Characteristic Elements Emitted

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Road transport

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Motor vehicle emissions Br, Pb, Ba, Mn, Cl, Zn, V, Ni, Se, Sb, As
Engine wear Fe, Al
Catalytic converters Rare earths, Pt
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Tyre wear ZnO, carbon black
Road side dusts EC, Al, Si, K, Ca, Ti, Fe, Zn
Industrial facilities
Oil fired power plants V, Ni
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Coal combustion Se, As, Cr, Co, Cu Al, S, P, Ga

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Oil Refineries V
Nonferrous metal smelters As, Sb, Cu, Zn, Pb, Cd, Hg
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Iron and steel mills Zn, Pb

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Copper refinery Cu, Zn

Refuse incineration Zn, Pb, Cu, Cd, Hg, K
Mineral and material processing Si, Al, Ca, Mg, K, Sc, Fe, Mn.
Sea spray Na, Cl, S, K
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Resuspended soil Si, Al, Ca, Mg, Fe, Ti, Sr, Mn, Sc
[Source: Adapted from “Guidelines for concentration and exposure-response measurement of fine and
ultra fine particulate matter for use in epidemiological studies”, Ed. D. Schwela et al. World Health
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Organization, 2002.]

Table 2: Elements emitted from various particle sources (according to WHO), EC = Earth
Crust

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Bibliography

[1] Baron, P.A. and Willeke, K. (1992) Aerosol Measurement: Principles, Techniques and Applications,
New York, John Wiley & Sons, ISBN: 0471284068 [Treatise on various aspects of aerosols, focusing on
their characterization].
[2] Cachier H. (1998), “Carbonaceous Combustion Aerosols”, Atmospheric Particles, Ed. R.M. Harrison
and R. Van Grieken, Chapter 9, pp. 295-348, John Wiley & Sons Ltd., ISBN 0 471 95935 9 [Book, with
chapter on the title topic,and figuring in an excellent compilation of texts on Atmospheric Particles, as
supplied by a variety of authors].
[3] Cooper J. (1998). Particle Size Analysis – The Laser Diffraction Technique. Materials World, 6 (1), 5-
7 [Paper on a major method of Particle Size Analysis].
[4] http://www.cleanair.com/Services/Analytical/Serv_Prices/bahco_size.html [Particle sizing with a
Bahco Centrifugal Classifier, including a determination of resistivity and particle size analysis].
[5] http://www.who.int/peh/ Guidelines for concentration and exposure-response measurement of fine and
ultra fine particulate matter for use in epidemiological studies. Ed. Schwela, D. et al. Published in 2002

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by The World Health Organization (WHO) on behalf of The European Commission [Interesting treatise
on various aspects of aerosols, including their health effects].

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[6] http://www.rpco.com/products/ambprod/amb1400/. [Commercial site with description of the TEOM
Series 1400a Monitor].
[7] Horvath H. (1998), “Influence of Atmospheric Aerosols upon the Global Radiation Balance”,
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Atmospheric Particles, Ed. R.M. Harrison and R. Van Grieken, Chapter 16, pp. 543-596, John Wiley &
Sons Ltd., ISBN 0 471 95935 9 [Chapter on the effect of aerosols on Global Heating, published in a
major reference book].
[8] Jennings S. G. (1998), “Wet Processes Affecting Atmospheric Aerosols”, Atmospheric Particles, Ed.
R.M. Harrison and R. Van Grieken, Chapter 14, pp. 475-507, John Wiley & Sons Ltd., ISBN 0 471 95935
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9 [Chapter dealing with wet deposition and other interactions of aerosols with water, published in a major
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reference book].
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[9] van de Hulst H. C. (1981). Light Scattering by Small Particles. New York, Dover Publications, ISBN:
0486642283 [Treatise on the interaction of light with aerosols].
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Biographical Sketch
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Alfons Buekens was born in Aalst, Belgium; he obtained his M.Sc. (1964) and his Ph.D (1967) at Ghent
University (RUG) and received the K.V.I.V.-Award (1965), the Robert De Keyser Award (Belgian Shell
Co., 1968), the Körber Foundation Award (1988) and the Coca Cola Foundation Award (1989). Dr.
Buekens was full professor at the Vrije Universiteit Brussel (VUB), since 2002 emeritus. He lectured in
Ankara, Cochabamba, Delft, Essen, Sofia, Surabaya, and was in 2002 and 2003 Invited Professor at the
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Tohoku University of Sendai.

Since 1976 he acted as an Environmental Consultant for the European Union, for UNIDO and WHO and
as an Advisor to Forschungszentrum Karlsruhe, T.N.O. and VITO. For 25 years, he advised the major
industrial Belgian Bank and conducted more than 600 audits of enterprise.
Main activities are in thermal and catalytic processes, waste management, and flue gas cleaning, with
emphasis on heavy metals, dioxins, and other semi-volatiles. He coordinated diverse national and
international research projects (Acronyms Cycleplast, Upcycle, and Minidip). Dr. Buekens is author of
one book, edited several books and a Technical Encyclopedia and authored more than 90 scientific
publications in refereed journals and more than 150 presentations at international congresses. He is a
member of Editorial Boards for different journals and book series.
He played a role in the foundation of the Flemish Waste Management Authority O.V.A.M., of a
hazardous waste enterprise INDAVER, and the Environmental Protection Agency B.I.M./I.B.G.E. He
was principal ministerial advisor in Brussels for matters regarding Environment, Housing, and Classified
Enterprise (1989). Since 1970 he has been a Member of the Board of the Belgian Consumer Association
and of Conseur, grouping more than a million members in Belgium, Italy, Portugal, and Spain.

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POLLUTION CONTROL TECHNOLOGIES – Vol. I - Dust-Particle Formation and Characteristics - A. Buekens

He is licensed expert for conducting Environmental Impact Assessments (Air, Water, Soil) and Safety
Studies regarding large accidents (Seveso Directive).

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