Geoscience Reference
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such as soil surfaces, from combustion, or they
accumulate by random coagulation and by
repeated cycles of condensation and evaporation
(
Figure 2.1A
). Over Europe, 2000-3500 such
particles per cm
3
are measured. Particles with
diameters < 2.5
documented separately. Particles with diameters
of 0.1-1.0
m are highly effective in scattering
solar radiation (Chapter 3B.2), and those of
about 0.1
μ
m diameter are important in cloud
condensation. The climatological effects of
aerosols on precipitation are complex and the
overall impact is uncertain (see p.117).
Having made these generalizations about the
atmosphere, we now examine the variations that
occur in composition with height, latitude and
time.
μ
m (PM
2.5
) - that can cause
adverse health problems - are now often
μ
(A)
10
5
Volcanic
plumes
Forest
fires
10
4
Sand
storms
5 Variations with height
The light gases (hydrogen and helium especially)
might be expected to become more abundant in
the upper atmosphere, but large-scale turbulent
mixing of the atmosphere prevents such diffusive
separation up to at least 100km above the surface.
The height variations that do occur are related to
the source locations of the two major non-
permanent gases - water vapor and ozone. Since
both absorb some solar and terrestrial radiation,
the heat budget and vertical temperature structure
of the atmosphere are considerably affected by
the distribution of these two gases.
Water vapor comprises up to 4 percent of the
atmosphere by volume (about 3 percent by
weight) near the surface, but only 3-6ppmv (parts
per million by volume) above 10 to 12km. It is
supplied to the atmosphere by evaporation from
surface water or by transpiration from plants and
is transferred upwards by atmospheric turbu-
lence. Turbulence is most effective below about
10-15km and, as the maximum possible water
vapor density of cold air is very low anyway (see
B.2, this chapter), there is little water vapor in the
upper layers of the atmosphere.
Ozone (O
3
) is concentrated mainly between
15 and 35km. The upper layers of the atmosphere
are irradiated by ultraviolet radiation from the
sun (see C.1, this chapter), which causes the
breakup of oxygen molecules at altitudes above
30km (i.e., O
2
→
10
3
Dust
storms
Urban
particles
10
2
Continental
background
10
1
Sea
salt
Oceanic
background
1
10
-1
10
-2
10
-1
10
1
10
2
10
3
1
Particle diameter (µm)
(B)
Rapid rising air (Aitkin)
condensation nucle
i
Slow thermal
condensation nucle
i
M
echanical grinding re
gion
Vapor
condensation
Polymerized aggregates,
organic molecules, etc.
Combustion
condensates
Wind-lofter dust
Evaporation from
sea spray
Plant debris:
spores, etc.
GAS
CONVERSION
MODE
GIANT
PARTICLE
MODE
mass
0.1 to 10 µgm
-3
mass
0.1 to 10 µgm
-3
mass
0.01 to 0.1 µgm
-3
ACCUMULATION
MODE
MASS TRANSFER
by coagulation and evaporating cloud cycles
Figure 2.1
Atmospheric particles. A: Mass distri-
bution, together with a depiction of the surface-
atmosphere processes that create and modify
atmospheric aerosols, illustrating the three size
modes. Aitken nuclei are solid and liquid particles
that act as condensation nuclei and capture ions,
thus playing a role in cloud electrification. B:
Distribution of surface area per unit volume.
Sources: A: After Glenn E. Shaw, University of Alaska,
Geophysics Institute. B: After Slinn (1983).
O + O). These separated atoms
(O + O) may then combine individually with
