We in Ireland pride ourselves on the pristine purity of our air. The prevailing winds come to us fresh from the Atlantic, and their turbulent strength is sufficient to sweep away into the upper atmosphere any noxious matter they may encounter on their way. Only rarely does a persistent low-level inversion prevent the upward transport of pollutants, and allow high concentrations to cause nuisance near the ground.
Pure air is composed entirely of nitrogen and oxygen. It contains minute quantities of other benign substances, together with water vapour and carbon dioxide, but humanity has added unwelcome ingredients.
Often the city dweller, like Casca in Shakespeare's Julius Caesar, "durst not laugh, for fear of opening my lips and receiving the bad air".
Any substance artificially added to the mixture originally provided by nature can be thought of as air pollution. It arises from modern industrial processes and from the combustion of fossil fuels, the most common polluting agents being sulphur dioxide, or S 2O and the solid particles of ordinary smoke. Although some industrial plants emit other pollutants, their production tends to be very localised.
The scale of the problem can be frightening. A modern electric power station may discharge many tonnes of S2O into the air every hour. S 2O is a toxic gas. Over long periods, low concentrations are harmful to human health and plant life, and cause physical deterioration to the stone and paintwork of buildings.
Another pervasive source of air pollution is the ubiquitous motorcar. It's exhaust gases contain carbon monoxide, oxides of nitrogen, a variety of hydrocarbons and some compounds of lead. Some of these pollutants, though not directly harmful, are rendered toxic by the photochemical action of sunshine.
The significance of the weather in these matters lies in its importance for the dispersion of the noxious substances - the process whereby the concentration of the pollutant is diluted to tolerable levels when mixed with a large volume of air. The main factors which affect dispersion are the strength of the wind, its turbulence or gustiness, and the stability of the atmosphere - the extent to which vertical currents in the atmosphere mix the air at ground level with cleaner regions high above.
Our normally changeable Irish weather is effective in this way; but on those rare occasions when we enjoy a quiet, virtually windless anticyclone, levels of pollution in the air are liable to rise dramatically.