Sulphur oxides (SO2), SO2
The most common sulphur oxide is sulphur dioxide (SO2). Sulphur trioxide (SO3) is an intermediate product during the manufacture of sulphuric acid (contact process). Sulphur dioxide is a colourless gas with a penetrating, choking odour. It dissolves readily in water to form an acidic solution (sulphurous acid).
Sulphur dioxide is a reducing agent and is used for bleaching and as a fumigant and food preservative. Large quantities of sulphur dioxide are also used in the contact process for the manufacture of sulphuric acid. It is also used in bleaching wool or straw, and as a disinfectant. Liquid sulphur dioxide has been used in purifying petroleum products. Sulphur dioxide was used as a blanketing gas in magnesium production to prevent its oxidation in air, this use was replaced some time ago by sulphur hexafluoride, which is essentially non-toxic (but an extremely potent greenhouse gas).
The main emission source of sulphur dioxide is the burning of fossil fuels. Power stations, oil refineries and other large industrial plants contribute the majority of the total mass released. Motor vehicles and domestic boilers, as well as natural sources such as active volcanoes and forest fires, release sulphur dioxide. Oxidation of other sulphur compounds (such as hydrogen sulphide) released into the atmosphere by natural and man-made processes provide another emission source. From 1970 to 1998 the amount of sulphur dioxide being released into the atmosphere annually has been reduced by 75%. This reduction was largely a result of the decreasing use of coal for power generation and its replacement by natural gas.
Excessive exposure to sulphur dioxide may cause health effects on the eye, lung and throat. Sulphur dioxide is toxic to a variety of plants and may produce visible signs of injury and/or reduce yields of certain crops. Paradoxically, beneficial effects may also be seen on some plants where sulphur dioxide can reduce the incidence of some fungal diseases-for example- the incidence of 'black spot' on roses is low in areas of high sulphur dioxide. Sulphur dioxide emitted in sufficient quantities at low or ground level can combine with air moisture to cause gradual damage to some building materials (such as limestone) by forming an acid solution that gradually dissolves the stonework if it is constantly exposed.
Sulphur dioxide gas dissolves in the water droplets in clouds causing the rain to be more acidic than usual. Pollutants can be transported thousands of kilometres due to the introduction of tall chimneys, dispersing pollutants high in the atmosphere. Acid rain affects the natural balance of rivers, lakes and soils, resulting in damage to wildlife and vegetation.