One of the exciting new fields of chemical research in the past half-century, which introduces Cloud Chemistry, involves chemical changes that take place in the atmosphere. Scientists have learned that several reactions are taking place in the atmosphere at all times. For example, oxygen (O2) molecules in the upper stratosphere absorb solar energy and are converted to ozone (O3).
This ozone forms a layer that protects life on Earth by filtering out the harmful ultraviolet radiation in sunlight. Chlorofluorocarbons and other chlorinated solvents (e.g., carbon tetrachloride and methyl chloroform) generated by human activities also trigger chemical reactions in the upper atmosphere, including the breakup of ozone into the two-atom form of oxygen. This reaction depletes the earth’s protective ozone layer.
Clouds are often an important locus for atmospheric chemical reactions. They provide an abundant supply of water molecules that act as the solvent required for many reactions.
An example is a reaction between carbon dioxide and water, resulting in the formation of carbonic acid. The abundance of both carbon dioxide and water in the atmosphere means that natural rain will frequently be somewhat acidic.
Although conditions vary from time to time and place to place, the pH of natural, unpolluted rain is normally about 5.6. (The pH of pure water is 7.0). Other naturally occurring components of the atmosphere also react with water in clouds.
In regions of volcanic activity, for example, sulfur dioxide released by outgassing and eruptions is oxidized to sulfur trioxide, which then reacts with water to form sulfuric acid.
The water of which clouds are composed also acts as a solvent for many other chemical species blown into the atmosphere from the earth’s surface.
Among the most common ions found in solution in clouds are sodium (Na+), magnesium (Mg2+), chloride (Cl-), and sulfate (SO2- 4 ) from sea spray; potassium (K+), calcium (Ca2+),
and carbonate (CO2-3) from soil dust; and ammonium (NH4+) from organic decay.
The nature of cloud chemistry is often changed because of human activities. Perhaps the best-known and most thoroughly studied example of this involves acid rain.
When fossil fuels are burned, sulfur dioxide and nitrogen oxides (among other products) are released into the atmosphere. Prevailing winds often carry these products for hundreds or thousands of miles from their source.
Once deposited in the atmosphere, these oxides tend to be absorbed by water molecules and undergo a series of reactions by which they are converted to acids.
Once formed in clouds by these reactions, sulfuric and nitric acids remain in solution in water droplets and are carried to earth as fog, rain, snow, or other forms of precipitation.
In general, clouds are dispersions in the air of solids or liquids in the form of microscopic particles.
Among the various types of clouds, the most common are atmospheric ones; droplets of water dispersed in the air – which are formed in the upper atmosphere – in thickness and density such as to prevent more or less the view of the sky, constitute the atmospheric clouds.
Atmospheric clouds form when humid air, that is, containing water vapour, becomes saturated and condenses into droplets.
The condensation of water into droplets does not occur spontaneously once saturation is reached.
In perfectly “clean” air (i.e. made up of gas only and devoid of other solid or liquid particles suspended in it) the water vapour can be brought to high degrees of super saturation without condensation occurring.
The condensation of water vapour in the atmosphere occurs thanks to the presence of tiny solid or liquid particles, which are always found in suspension in the air.
On these particles, called condensation nuclei, the water condenses into droplets each of which has “grown” onto a core particle.
Even in the presence of nuclei, however, the water vapour must reach a certain degree of super saturation before condensing into cloud droplets.
The degree of super saturation that water vapour must reach to be nucleated by a particle depends on the size of the particle itself in the sense that the larger the nucleus, the lower the degree of super saturation that the water vapour must reach in order to form a droplet.
The concentration in the atmosphere of active condensation nuclei varies from the place to place and depends on the greater or lesser distance from the sources of particles (friable soils, vegetation, industrial fumes, etc.).
In the air of large cities, 4 x 10 nuclei per cubic centimeter has been found, while
Zero concentrations have been observed in some marine locations.