Clouds & Particles
More
Unit 2: Particles
Aerosol particles have both natural and human (anthropogenic) sources. They
can be emitted directly into the air as primary aerosols or formed in the
atmosphere as secondary aerosols. Different aerosols have different chemical
and physical properties, some play a key role in the formation and behaviour of
clouds and on the climate system and others have an impact on human health.



Properties of particles
Transformation and removal
Particles and health
Part 1: Properties of particles
Atmospheric aerosol particles have many different sources. Once in the
air they undergo lots of different reactions which change their
properties. This means that aerosols have a wide range of chemical and
physical characteristics and these control how they behave in the
atmosphere.
One of the reasons why aerosols are so important is because they are
essential for the formation of clouds.
The chemical composition of aerosols
The chemical composition of particulate matter is strongly related to its
origin. The major chemical components of aerosols are sea-salt, sulphate,
nitrate, ammonium, organic material, crustal species, trace metals and water.
The largest aerosols (which have a diameter greater than 1 µm) are known
as coarse mode aerosols. These particles consist of chemicals found in the
Earth's crust (such as silicon, aluminium, iron and calcium), those found in seaspray (primarily sodium and chloride), biological elements (pollen, spores, insect
debris) and coal fly ash.
Particles with diameters smaller than 1 µm are known as fine mode aerosols.
These generally form either as a result of secondary reactions in the atmosphere
such as gas to particle conversion (for example, nitrate, sulphate and some
organic carbon compounds) or are emitted during high temperature combustion
processes (such as lead, zinc and nickel and elemental carbon).
Chemical mixing
ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 1
English offline version
supported by the International Max Planck Research School on Atmospheric Chemistry and Physics
As the atmosphere is continuously moving and changing, the chemical
composition of a particle often alters during its atmospheric residence time,
which is typically a few days. Two mixing states occur: internal and external
mixtures.
In an external mixture, particles from
In an internal mixture, the various
different sources remain separated, i.e. chemical components are mixed within
not attached to each other.
a single particle. The older the air mass
is, the greater the degree of internal
mixing.
1. TEM image of mineral dust collected from the marine troposphere. Copyright © 1999, The National
Academy of Sciences
Cloud Condensation Nuclei
Aerosols are essential for the formation of clouds, providing a site for water
vapour to condense onto. However, not all aerosols can serve as a nucleus for
water drop formation. Those which can are called Cloud Condensation Nuclei
(CCN). This ability depends on size, chemical composition and supersaturation
(see formation processes).
About half the aerosol particles over the oceans can act as CCN, whereas only
1% of the aerosols in polluted environments can. However, the total
concentration of aerosol particles in polluted areas is much higher than over the
oceans. CCN concentrations of around 100 per cm3 are typically found in
marine air masses whereas concentrations of many thousands of CCN per cm3
are found in polluted air.
To act as a CCN, particles must be hygroscopic, i.e. they must contain sufficient
amounts of water-soluble material. This is why the chemical composition of
aerosols affects cloud droplet formation. For example, soil dust particles are
not very soluble and can't act as CCN, whereas sea-salt particles are efficient
CCN (on humid days, it can be difficult to pour salt from a salt shaker because
water vapor has condensed on the salt crystals, sticking them together).
ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 2
English offline version
supported by the International Max Planck Research School on Atmospheric Chemistry and Physics
2. Ship tracks: particles emitted from the exhausts of large ships act as CCN and form the clouds
which are shown in this image. France is on the right and Spain at the bottom. As the ships move
about the East Atlantic, these clouds form and leave a visible record of where the ships have recently
been. Ship tracks can last for hours and give clues to the relative speed of the ships. The faster the
ship, the narrower and longer the ship track will be. Slower ships leave shorter and wider ship tracks.
Source: NASA.
Effects of aerosols on clouds
Aerosol particles are necessary for cloud formation and the size and number of
particles changes the characteristics of the clouds. In fact, aerosols are essential
players in the cloud system; they change the microphysics of the cloud (the
number and size of the water droplets).
One of the fundamental observations is that increasing the number of particles in
the atmosphere on which cloud droplets can form, leads to clouds with more, but
smaller, droplets. The number and size of droplets is important in governing the
rain potential of the cloud and its optical properties. In climate, the influence of
aerosols on clouds is called the indirect effect.
Therefore, as anthropogenic activities
are an important source of particulate
matter, humans modify the number
and characteristics of clouds.
The photograph on the left
shows condensation trails (also called
"contrails") over the Rhône Valley in
France. These artificial clouds are
formed as a result of aeroplane
exhausts and are made of ice crystals.
It is estimated that these “artificial
clouds” cover 0.1% of the planet's
surface.
3. Condensation trails over the Rhône Valley
Source: NASA.
ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 3
English offline version
supported by the International Max Planck Research School on Atmospheric Chemistry and Physics
4. a+b) Contrails are often seen in the sky
on sunny days.
4. b) Authors: C. Gourbeyre, J. Gourdeau.
Part 2: Transformation and removal
Aerosol particles typically stay one week or less in the
troposphere. Sooner or later these particles are lost from the air and
there are two ultimate exits: dry and wet deposition.
However, before particles return naturally to the Earth's surface, their
size, concentration, and chemical composition may change.
Evolution of particles in the
atmosphere
1. Coagulation processes. Author: J
Gourdeau.
Several mechanisms can change
the physical and chemical characteristics of
a particle.
As they are continuously moving, particles
may collide with other particles. Sometimes
collisions make particles stick together to
form new larger particles. This process is
known as coagulation and leads to a
continual removal of small particles
replaced by larger ones.
Water vapour can condense on particles and
thus the particles grow as agglomeration
proceeds.
The cloud: a big laboratory
Aerosols play an important role in the formation of clouds. Clouds form when
water vapour condenses onto certain types of aerosols known as cloud
condensation nuclei (CCN). So during cloud formation some aerosols become
cloud water droplets. In a water droplet, aqueous chemical reactions can take
place, in the same way as in a test tube during your chemistry experiments.
ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 4
English offline version
supported by the International Max Planck Research School on Atmospheric Chemistry and Physics
2. The particle inside the droplet may
change during the condensation (1) and
evaporation (2) processes. Author: J.
Gourdeau.
Only a very few clouds lead to rain. Most
cloud droplets simply evaporate rather
than grow into raindrops. The aerosol
particle left after the water has evaporated
is often rather different from the original
aerosol, both chemically and physically (its
size may have changed). A single aerosol
particle undergoes many condensationevaporation cycles before it is removed
from the air either in rain or through dry
deposition. Transformation of aerosols in
clouds is known as cloud processing.
Deposition - how particles are removed from the atmosphere
When particles fall out of the atmosphere onto surfaces and the process doesn't
involve water, the removal mechanism is called dry deposition. When the
particles are scavenged by atmospheric water (like rain or snow), the
removal process is called wet deposition.
The simplest dry deposition
mechanism is sedimentation
and here the particles
fall under the influence
of gravity. The heaviest
particles are removed from the
atmosphere by
sedimentation and, because
they are heavy, they are found
close to their source.
In a dry still atmosphere,
sedimentation processes
control how big the largest
particles in the atmosphere
are. Dry deposition is,
however, strongly related to
atmospheric movements, like
wind. Wind can keep large
particles in the air and can
carry them long distances
away from their source.
3. Dust storm
Source: NOAA Library
ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 5
English offline version
supported by the International Max Planck Research School on Atmospheric Chemistry and Physics
Wet deposition occurs when there is water in the air,
either in the clouds themselves or in the air below the
clouds.
4. It's raining.... Source:
www.freefoto.com
In the clouds a fraction of the aerosols act as CCN.
Water vapour condenses on these particles to
form cloud droplets. When droplets get so large that
the air cannot hold them, the droplets fall as rain and
the aerosols within the droplets are deposited to the
surface. In addition, droplets in the cloud can
“scavenge or catch” the other particles in the
surrounding air. These particles are then also removed
from the air in the falling rain. Aerosol particles below
the cloud can also be scavenged by the falling raindrops
or snowflakes and removed from the atmosphere.
Wet deposition is very efficient at cleaning the atmosphere: look at car
windscreens after rain, they often have a layer of dust on them and the air we
breathe often feels cleaner after a big rain storm.
Part 3: Particles and respiratory tract
Particles are dangerous for our health. The toxicity of particles depends
on their chemical composition and their size: the finer the particle is,
the deeper it penetrates into our lungs. Scientists classify particles into
two different size ranges which are known as PM10 and PM2.5.
The respiratory system
Look at the diagram of our respiratory
tract and the penetration of particles
according to their size.
1: Pharynx
2: Larynx
3: Trachea
4: Bronchus
5: Bronchioles
6: Pulmonary Alveoli
1. Respiratory tract.
You can see that the coarsest particles
(from 3 to 10 micrometers in diameter)
tend to be deposited in the upper parts of
the respiratory system. These particles
can generally be expelled back into the
throat.
PM2.5 are responsible for causing the greatest harm to human health because they are
so small. These fine particles can be inhaled deep into the lungs, reaching the 600
million pulmonary alveoli. They can cause breathing and respiratory problems,
irritation, inflammation and cancer.
ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 6
English offline version
supported by the International Max Planck Research School on Atmospheric Chemistry and Physics
Smallest particles
Scientists are becoming more
and more concerned about
the impact of the smallest
particles, those which are
around just 100 nanometers
in size. These particles can
get into the deepest parts of
the lung and can even enter
our blood circulation, leading
to cardio-vascular disease.
2. Red corpuscles.
Source:http://www.ulb.ac.be/sciences/biodic
Chemical composition
The chemical composition of a particle strongly governs its toxicity. The
composition determines either how the respiratory tract reacts of how the body
responds. Toxic air pollutants attach themselves to airbourne particulate
matter. This particulate matter is then breathed into the lungs and the toxic
species are absorbed into our blood and body tissues.
3. Aerosols are sampled by pumping air through a filter onto
which the particles deposit. On the left is a new filter. On the
right is a filter on which particles have been collected. Look at
the colour difference! Source: I. Cousteix.
Pollution is a strong source
of particles, especially the
very small ones which pose
the greatest hazard to
health. A very important
source of very fine particles
in the urban atmosphere
is diesel vehicles as these
produce much finer particles
than petrol cars. Work
is underway to try and
reduce particulate emissions
from diesel engines by, for
example, adding pipe filters.
ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 7
English offline version
supported by the International Max Planck Research School on Atmospheric Chemistry and Physics