Clouds & Particles
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2. Particles
Solution Worksheet 1
1.
Factors that determine whether an aerosol can act as a cloud condensation
nuclei (CCN) include:
-
particle size,
chemical composition
supersaturation.
To act as CCN, particles must be hygroscopic enough, i.e. they must contain
sufficient amounts of water-soluble material. This is why the chemical
composition of particles also controls cloud droplet formation.
For example, soil dust particles don’t dissolve easily when they are first
emitted from the ground and so they can’t act as CCN. Seasalt particles, in
contrast, absorb water easily and dissolve readily (on a humid day it can be
difficult to pour salt from a salt shaker because the water vapour has
condensed onto the salt crystals, sticking them together). So seasalt
aerosols are important CCN. and are extremely soluble in water
2.
If the amount of aerosols in the atmosphere increases, cloud characteristics
and properties are likely to change. Aerosols are very important to cloud
formation. They modify cloud microphysics (the number and size of water
droplets in the cloud), the precipitation potential and the optical properties of
the cloud.
If the number of particles in the atmosphere increases, clouds tend to have
more but smaller droplets. The number and size of the water droplets in a
cloud is important as this controls how much rain falls from the cloud and
how much solar radiation the cloud reflects back into space or absorbs. As
human activities are important sources of aerosols, humans do modify the
number and characteristics of clouds.
3.
a)
Clouds form behind aeroplanes because the engines releases small particles
and water vapour in their exhaust. The particles act as CCN’s, allowing the
water vapour to condense onto them forming water droplets. This is why we
see long, narrow clouds from aeroplanes.
When we see contrails in the sky we know that the clouds are flying very
high in the sky. The air is saturated with moisture, but there are few CCNs
for droplets to form on. Thus as soon as CCNs are introduced, clouds form.
b)
The gap forms because the air and vapour just behind the aeroplane is very
hot. It takes some time before the air is sufficiently cooled for condensation
to take place. During this time interval, the aeroplane has moved on, and we
see a gap between the plane and the contrails.