David Rowley, of Ilam school, asks :-

How does acid rain form?

Reimer Herrmann, an environmental chemist and hydrologist at the University of Bayreuth, Germany, who regularly visits New Zealand to work with scientists at our Forest Research Institute, responded.

The burning of fossil fuels (coal, oil and petrol) in power-plants, metal-smelters, homes, cars etc produce vast quantities of waste gases such as sulphur dioxide and nitrogen oxides. When these mix with oxygen and water in the atmosphere they produce sulphuric acid and nitric acid respectively. Similarly, ammonia emitted by animals also produces nitric acid.

In heavily industrialised areas, such as in Europe and the Northeast part of America, the acid concentrations are very high. Winds can carry these acids over long distances, perhaps to other countries, before they reach the ground as acid rain, acid snow or acid fog. Thus lakes and rivers in southern Norway become acidified from sulphur dioxide and nitrogen oxide from British power-plants and traffic, and Dutch piggeries emit ammonia which produces nitric acid rain which falls over northwest Germany.

These acid levels are sufficiently high in many places to be harmful to plants, animals and even humans.

In contrast, the rain in New Zealand is only very slightly acidic and then only due to carbonic acid, a weak acid which is produced when carbon dioxide in the air dissolves in water. (This is the same as occurs in gassy mineral water or in fizzy drinks). It is naturally occurring and so weak that plants and animals are not affected by it.