Primary Sources for Acid Depositions

Nitrogen oxides (also known as oxides of nitrogen, and abbreviated as NOx) is a collective term used to refer to two species of oxides of nitrogen: nitric oxide (NO) and nitrogen dioxide (NO₂). Nitric oxide is a colorless, flammable gas with a slight odour. Although somewhat toxic, its odour is insufficient to provide warning.

Nitrogen dioxide is a reddish brown, nonflammable, gas with a detectable smell. In significant concentrations it is highly toxic, causing serious lung damage with a delayed effect. Nitrogen dioxide is a strong oxidizing agent that reacts in the air to form corrosive nitric acid, as well as toxic organic nitrates. It also plays a major role in the atmospheric reactions that produce ground-level ozone or smog.

Globally, quantities of nitrogen oxides produced naturally by bacterial and volcanic action, and lightning, outweigh man-made emissions. Man-made emissions are mainly due to fossil fuel combustion from both stationary sources, such as power generation (24%), and mobile sources, such as transport (49%). Other atmospheric contributions come from non-combustion processes, for example nitric acid manufacture, welding processes and the use of explosives.

In the atmosphere, nitrogen oxides mix with water vapour producing nitric acid. This acidic pollution can be transported by win over many hundreds of miles, and deposited as acid rain.

Sulfur dioxide (SO₂) is a colourless gas, belonging to the family of gases called sulpfur oxides (SOx). It reacts on the surface of a variety of airborne solid particles, is soluble in water and can be oxidised within airborne water droplets.

Natural sources of sulfur dioxide include releases from volcanoes, oceans, biological decay and forest fires. The most important man-made sources of sulfur dioxide are fossil fuel combustion, smelting, manufacture of sulfuric acid, conversion of wood pulp to paper, incineration of refuse and production of elemental sulfur. Coal burning is the single largest man-made source of sulfur dioxide accounting for about 50% of annual global emissions, with oil burning accounting for a further 25 to 30%.

The major health concerns associated with exposure to high concentrations of sulfur dioxide include effects on breathing, respiratory illness, alterations in pulmonary defenses, and aggravation of existing cardiovascular disease. In the atmosphere, sulfur dioxide mixes with water vapour producing sulfuric acid. This acidic pollution can be transported by wind over many hundreds of miles, and deposited as acid rain.

Transboundary Pollution

Stationary emissions sources, such as coal-fired and oil-fired power stations, and mobile sources, such as cars, ships and aircraft emit a complex mixture of pollutants, including sulfur dioxide and nitrogen oxides (the precursors to acid rain). It is now well established that this air pollution is transported over hundreds or even thousands of kilometres. Consequently, when acidic pollution is finally deposited, its environmental impacts are felt in areas far removed from their sources. Since this air pollution has no regard for national boundaries, it has been termed transboundary pollution.

Because acidic pollution is transboundary, there is no clear relationship between how much pollution a country emits and how much is deposited there.

Throughout Europe, the prevailing wind direction is generally westerly or southwesterly. Consequently, much of the pollution emitted in the UK travels across the North Sea and is deposited in Scandinavia. Whilst the UK emits much more pollution than it receives through acid deposition, Norway and Sweden experience proportionally a much greater amount of acid rain compared to their lower emissions.

To control the spread of transboundary pollution the United Nations Economic Commission for Europe (UNECE) implemented the Convention on Long-Range Transboundary Pollution (1979). Since that time, emissions of sulfur dioxide across Europe have been lowered dramatically, but increases in the volume of traffic have meant that emissions of nitrogen oxides have not fallen as quickly. Consequently, whilst other environmental issues such as global warming and ozone depletion have received more attention in recent years, transboundary acid rain remains a problem today.