One of the exciting new fields of chemical research in the past half century involves chemical changes that take place in the atmosphere . Scientists have learned that a number of 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 break up 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 the 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 solvent for a number of 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 (SO 2- 4 ) from sea spray; potassium (K+), calcium (Ca2+), and carbonate (CO 2- 3 ) from soil dust; and ammonium (NH + 4 ) from organic decay.
The nature of cloud chemistry is often changed as a result 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 original 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.
[David E. Newton ]
Harrison, R. M., ed. Pollution: Causes, Effects, and Control. Cambridge: Royal Society of Chemistry, 1990.