Geoscience Reference
In-Depth Information
particles through photochemical reactions, particularly
over urban areas. Nuclei range in size from 0.001 µm
radius, which are ineffective because of the high super-
saturation required for their activation, to
giants
of over
10 µm, which do not remain airborne for very long
(see pp. 12-13). On average, oceanic air contains 1
million condensation nuclei per litre (i.e. dm
3
), and
land air holds some 5 or 6 million. In the marine tro-
posphere there are fine particles, mainly ammonium
sulphate. A photochemical origin associated with
anthropogenic activities accounts for about half of these
in the nor thern hemisphere. Dimethyl sulphide (DMS),
associated with algal decomposition, also undergoes
oxidation to sulphate. Over the tropical continents,
aerosols are produced by forest vegetation and surface
litter, and through biomass burning; particulate organic
carbon predominates. In mid-latitudes, remote from
anthropogenic sources, coarse particles are mostly of
crustal origin (calcium, iron, potassium and aluminium)
whereas crustal, organic and sulphate particles are
represented almost equally in the fine aerosol load.
Hygroscopic aerosols are soluble. This is very
important since the saturation vapour pressure is less
over a solution droplet (for example, sodium chloride or
sulphuric acid) than over a pure water drop of the same
size and temperature (Figure 5.8). Indeed, condensation
begins on hygroscopic particles before the air is satu-
rated; in the case of sodium chloride nuclei at 78 per
cent relative humidity. Figure 5.8 illustrates Kohler
curves showing droplet radii for three sets of solution
droplets of sodium chloride (a common sea-salt) in
relation to their equilibrium relative humidity. Droplets
in an environment where values are below/above the
appropriate curve will evaporate/grow. Each curve has
a maximum beyond which the droplet can grow in air
with less supersaturation.
Once formed, the growth of water droplets is far
from simple. In the early stages the solution effect
is predominant and small drops grow more quickly
than large ones, but as the size of a droplet increases,
its growth rate by condensation decreases (Figure 5.9).
Radial growth rate slows down as the drop size
increases, because there is a greater surface area to cover
with every increment of radius. However, the condensa-
tion rate is limited by the speed with which the released
latent heat can be lost from the drop by conduction to the
air; this heat reduces the vapour gradient. In addition,
competition between droplets for the available moisture
acts to reduce the degree of supersaturation.
Figure 5.7
Graph illustrating the effects of vertical mixing in an
airmass. The horizontal lines are pressure surfaces (
P
2
,
P
1
). The
initial temperature (
T
1
) and dew-point temperature (
T
d
1
) gradients
are modified by turbulent mixing to
T
2
and
T
d
2
. The condensation
level occurs where the dry adiabat (
) through
T
1
intersects the
saturation humidity mixing ratio line (
X
s
) through
T
d
2
.
θ
average-value lines cross the initial environment curves
are equal.
D CLOUD FORMATION
The formation of clouds depends on atmospheric insta-
bility and vertical motion but it also involves microscale
processes. These are discussed before we examine cloud
development and cloud types.
1 Condensation nuclei
Remarkably, condensation occurs with utmost difficulty
in
clean
air; moisture needs a suitable surface upon
which it can condense. If clean air is cooled below its
dew-point it becomes
supersaturated
(i.e. relative
humidity exceeding 100 per cent). To maintain a pure
water drop of radius 10
-7
cm (0.001 mm) requires a
relative humidity of 320 per cent, and for one of radius
10
-5
cm (0.1 mm) only 101 per cent.
Usually, condensation occurs on a foreign surface;
this can be a land or plant surface in the case of dew
or frost, while in the free air condensation begins on
hygroscopic nuclei
. These are microscopic particles
-
aerosols
- the surfaces of which (like the weather
enthusiast's seaweed!) have the property of
wettability
.
Aerosols include dust, smoke, salts and chemical com-
pounds. Sea-salts, which are particularly hygroscopic,
enter the atmosphere by the bursting of air bubbles in
foam. They are a major component of the aerosol load
near the ocean surface but tend to be removed rapidly
due to their size. Other contributions are from fine soil
particles and various natural, industrial and domestic
combustion products raised by the wind. A further
source is the conversion of atmospheric trace gas to
