Geoscience Reference
In-Depth Information
Peninsula (see Plate 5). Most of this is deposited down-
wind over the Atlantic. There is similar transport from
western China and Mongolia eastward over the North
Pacific Ocean. Large particles originate from mineral
dust, sea salt spray, fires and plant spores (Figure 2.1A);
these sink rapidly back to the surface or are washed out
(scavenged) by rain after a few days. Fine particles from
volcanic eruptions may reside in the upper stratosphere
for one to three years.
Small (Aitken) particles form by the condensation of
gas-phase reaction products and from organic molecules
and polymers (natural and synthetic fibres, plastics,
rubber and vinyl). There are 500 to 1000 Aitken particles
per cm
3
in air over Europe. Intermediate-sized (accu-
mulation mode) particles originate from natural sources
such as soil surfaces, from combustion, or they accu-
mulate by random coagulation and by repeated cycles
of condensation and evaporation (Figure 2.1A). Over
Europe, 2000 to 3500 such particles per cm
3
are
measured. Particles with diameters <10 µm (PM
10
), origi-
nating especially from mechanical breakdown processes,
are now often documented separately. Particles with
diameters of 0.1 to 1.0 µm are highly effective in scat-
tering solar radiation (Chapter 3B.2), and those of about
0.1 µm diameter are important in cloud condensation.
Having made these generalizations about the atmos-
phere, we now examine the variations that occur in
composition with height, latitude and time.
5 Variations with height
Figure 2.1
Atmospheric particles. (A) Mass distribution, 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).
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 100 km above the surface. The height variations
that do occur are related to the source locations of the
two major non-permanent gases - water vapour and
ozone. Since both absorb some solar and terrestrial
radiation, the heat budget and vertical temperature
structure of the atmosphere are affected considerably
by the distribution of these two gases.
Water vapour comprises up to 4 per cent of the
atmosphere by volume (about 3 per cent by weight) near
the surface, but only 3 to 6 ppmv (parts per million
by volume) above 10 to 12 km. It is supplied to the
atmosphere by evaporation from surface water or by
transpiration from plants and is transferred upwards
by atmospheric turbulence. Turbulence is most effective
reflecting incoming solar radiation (see Chapter 13).
Other aerosol sources are sea-salt and organic matter
(plant hydrocarbons and anthropogenically derived).
Natural sources are several times larger than anthro-
pogenic ones on a global scale, but the estimates
are wide-ranging. Mineral dust is particularly hard to
estimate due to the episodic nature of wind events and
the considerable spatial variability. For example, the
wind picks up some 1500 Tg (10
12
g) of crustal material
annually, about half from the Sahara and the Arabian
