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 redistributing 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 conditions 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 (HNO3) or sulphuric acid (H2SO4), 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:

H2SO4(in aerosol) + NaCl(in aerosol) ^ HCl(g) + NaHSO4(in aerosol) eqn. 3.6

Incoming 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 (SO2), CO2, HCl and hydrogen fluoride (HF). These gases can react in the stratosphere to provide a further source of particles, with H2SO4 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

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