Air

Acid deposits

The atmosphere contains acid-reaction substances that deposit on the Earth’s surface and contaminate it: they are the so-called “acid deposits”. The substances that make these deposits acid are generally nitric acid and sulphuric acid, that form by the reaction of water and nitrogen oxides and sulphur oxides (SOx) contained in polluted air. Nitrogen oxides are produced by the combustion of fossil energy sources rich in sulphur – especially coal and lignite – and by volcanic eruptions. Instead sulphur oxides can have a natural origin (lightning, fires, bacterial decomposition of organic materials, biological processes of the oceans), or an anthropogenic origin, deriving from the combustion of fossil energy sources. Acid deposits alter the acidity of lake and river waters (making it impossible for fish and other water organisms to live there) and that of soils (by altering the availability of nutritional elements, resulting in the reduction of fertility and productivity). Acid deposits can also directly damage the vegetation (for instance by melting the wax that protects the leaves and making them more vulnerable to parasitic attacks), buildings, monuments and, if particularly intense, man and animals as well.