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The Role of Ultrafine Particles in the
Atmosphere
Alfred Wiedensohler, Wolfram Birmili, Birgit Wehner
Leibniz Institute for Tropospheric Research, Leipzig
Dresden, October 23, 2007

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Definition
What is an aerosol?
Aerosol:
Solid
and/or liquid particles suspended in a gas
Size range of aerosol particles:
1
nm bis 100 μm

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Coarse, Fein and Ultrafine Particles
Coarse particles: (coarse mode)
Aerosol particles > 1 μm
Fine particles: (accumulation mde)
Aerosol particles < 1 μm and > 0.1 μm
Ultrafine particle: (nucleation and Aitken mode)
Aerosol particles < 0.1 μm

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Sources of Aerosol Particle
There are three different source mechanisms:
1. Bulk‐to‐Particle Conversion (BPC)
New particles produced from bulk material
2. Gas‐to‐Particle Conversion (GPC)
New particle number or mass produced from
condensable gases
3. Combustion
Emission of new particles or condensable gases

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Coarse Particles
Coarse particles are mainly produced by BPC processes:
‐ BPC: dispersion, erosions, re‐suspension
‐ Compounds: e.g. sea salt ,mineral dust, pollen,
bacteria
‐ Live time is hours to days
‐ Cloud condensation nuclei
‐ Scatter and absorb solar light
‐ Removed by sedimentation, rain and wash out
‐ Significant subfraction of the aerosol mass

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Fine Particles
Fine particles are mainly originated from GCP and
combustion processes:
‐ GCP: condensation, coagulation, and liquid phase
chemical reactions in clouds
‐ Compounds: sulfate, nitrate, ammonium,
organic and elemental carbon
‐ Live time is days to weeks
regional background
‐ Cloud condensation nuclei
‐ Scatter and absorb solar light
‐ Removed by rain and wash out
‐ Often main fraction of total aerosol mass

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Ultrafine Particles
Ultrafine particles are produced either by GCP or
combustion processes:
‐ GCP: Homogeneous nucleation of condensable
gases
‐ Combustion: emission of soot particles (precursor
gases)
‐ Composition: Sulfate, nitrate, ammonium,
organic carbon, elemental carbon
‐ Live time is hours to days
local aerosol
‐ Removed either by coagulation or diffusion to
surfaces
‐ Minor fraction of total aerosol mass

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Urban Number Size Distribution
1
10
100
1000
10
2
10
3
10
4
dN/ dlog D
p
[cm
-3
]
D
p
[nm]
street canyon, weekdays
urban background, weekdays
street canyon, Sundays
urban background, Sundays
Regional background
Eisenbahnstr, Mo‐Fr
Eisenbahnstr, Sa‐Su
IfT, Mo‐Fr
IfT, Sa‐Su
Urban influence

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Number and Mass Concentration
Number
3 -100nm
(#/cm
3
)
Number
3 -800nm
(#/cm
3
)
Mass
3 -100nm
(μg/m
3
)
Mass
3 -800nm
(μg/m
3
)
IfT, Mo-Fr
17500
19800
1,3
26,9
IfT, Sa-Su
8300
10900
0,9
30,8
Eiba, Mo-
Fr
29900
32900
1,9
28.9
Eiba , Sa-
Su
10600
12900
1,0
27,0

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Homogeneous Nucleation
There are two different kind of processes for
homogeneous nucleation:
1.
Photochemical production of condensable gases
from precursors
These nucleation process is usually regional
2.
Emissions of condensable gases from high
temperature combustion
This nucleation is observed in urban environments or
close to motorways

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Vehicle Emissions
Diesel‐driven vehicles emit soot particles:
Modern diesel‐cars emit number‐wise mainly
ultrafine soot particles
The mean number diameter lies from 50‐70 nm
The mean mass diameter is around 300 nm
Soot contains mainly elemental carbon
Organic carbons such PAHs are adsorbed to the
particles
Fresh soot particles might be surface reactive

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Soot number distribution in a street canyon

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Are ultrafine particles relevant for the
climate?
Continuous source to maintain the particle number
concentration in the atmosphere
Due to homogeneous nucleation and
combustion
Cloud formation and light scattering
Due to growth of ultrafine particles into the
accumulation mode range (> 0.1 μm)
Light Absorption
By soot particles

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Are (Ultra)-Fine Particles Dangerous?
An increased aerosol mass concentration is generally
not dangerous in each case.
Inhaling of sea salt is actually healthy
Chemical composition and particles size decide
about their influence on health risk
Water and lipid soluble particle material can be
efficiently removed from the respiratory tract
Ultrafine insoluble particles such as soot are not be
efficiently removed

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Are Soot Particles Dangerous?
Fresh soot particles
are 30‐200 nm in diameter
contribute only up to 20% of the total aerosol
mass in an extreme case (street canyon)
can pass cell walls
are surface reactive (damage cells)
carry e.g. polycyclic aromatic hydrocarbons
(carcinogenic)
can be distributed in the whole human body

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Regulations
Epidemiological studies showed an increased health risk
with increasing particle mass
EU‐Commission allows an upper limit of 50μm/m
3
for
particles mass for particles < 10μm
The major fraction of the total aerosol mass lies in the
size range above 0.1 μm
The aerosol below 0.1 μm accounts only for few
percents of the total mass
There are no regulations for the particle number of
ultrafine particles
Ultrafine particles account for the major number
of aerosol particles

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Summary
The total particle mass (< 10 μm) is a good measure for
the particulate air pollution
It might not the best parameter to determine the health
risk of aerosol particles
A large fraction of the particle mass is not dangerous
The major fraction of toxic, carcinogenic, and reactive
substances is in the particle range < 1μm

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Summary
The ultrafine size range (<0.1 μm) dominates the number
concentration (~ 90%) in urban areas
A major contributor here are soot particles emitted by
diesel‐driven vehicles
Soot particles are believed to have adverse health effects
on human body
The health risk nucleation mode particles is not well
known yet (large fraction of organic compounds)

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Questions
• Is the determination of the particle mass (>10μm or
>2,5μm) sufficient to estimate the health risk of the
local aerosol?
• Is it necessary to introduce an additional parameter
such as the ultrafine number concentration?
• Or are concentrations of soot particles a better
indicator to estimate the health risk?