fossil fuel by human activities. This has given rise to a continuing increase in the
CO 2 concentration in the atmosphere. The predicted rise in temperature, due to
the greenhouse effect, is explored in detail in Chapter 7.
There are many sources of trace components in the atmosphere, which can
be divided into different categories, such as geochemical, biological and human
or anthropogenic sources. Some of these sources are hard to categorize. Is a forest
fire a geochemical, biological or human source—particularly if the forest was
planted or the fire started through human activities? Although our definitions can
become a little blurred, it is nevertheless useful to categorize sources.
3.4.1 Geochemical sources
Perhaps the largest geochemical sources are wind-blown dusts and sea sprays,
which put huge amounts of solid material into the atmosphere (see also Chapter
6). The dust is largely soil from arid regions of the Earth. If this dust is fine
enough, it can spread over large areas of the globe and is important in redistrib-
uting material. Often, however, the chemical effects of the dust in the atmosphere
are not particularly evident, because dusts are not chemically very reactive. By
contrast, wind-blown sea spray places a more reactive entity into the atmosphere
as salt particles.
The salt particles from the oceans are hygroscopic and under humid condi-
tions these tiny NaCl crystals attract water and form a concentrated solution
droplet or aerosol. Ultimately, this process can take part in cloud formation. The
droplets can also be a site for important chemical reactions in the atmosphere. If
strong acids (Box 3.3) in the atmosphere, perhaps nitric acid (HNO 3 ) or sulphuric
acid (H 2 SO 4 ), dissolve in these small droplets, hydrogen chloride (HCl) can be
formed. It is thought that this process is an important source of HCl in the
atmosphere:
eqn. 3.6Incoming meteors also inject particles into the atmosphere. This is a very small
source compared with wind-blown dust or forest fires, but meteors make their
contribution to the upper parts of the atmosphere where the gas is at a low
density. Here, a small contribution can be particularly significant and the metals
ablated from incoming meteors enter a series of chemical reactions.
Volcanoes are a large source of dust and particularly powerful eruptions can
push dust into the stratosphere. It has long been known that volcanic particles
can change global temperature by blocking out sunlight. They can also perturb
the chemistry at high altitudes. Along with the dust, volcanoes are huge sources
of gases such as sulphur dioxide (SO 2 ), CO 2 , HCl and hydrogen fluoride (HF).
These gases can react in the stratosphere to provide a further source of par-
ticles, with H 2 SO 4 being the most important particle produced indirectly from
volcanoes.
It is important to realize that the volcanic source is a very discontinuous one,
both in time and space. Large volcanic eruptions are infrequent. It may be that
years pass without any really major eruptions and then suddenly more material
H SO 24 ()in aerosol+Æ+NaCl()in aerosol HCl()g NaHSO 4 ()in aerosolThe Atmosphere 39