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Acid Rain
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In 1856, Robert Angus Smith - the scientist who first used the term acid rain - wrote:

"it has often been observed that the stones and bricks of buildings crumble more readily in large towns where much coal is burnt.... I was led to attribute this effect to the slow but constant action of acid rain."

Buildings have always been subject to attack by weathering; the effects of rain, wind, sun, and frost. Acid rain can accelerate the rate of this damage. Throughout the world, emissions of sulphur dioxide and nitrogen oxides contribute to the international problem of acidification. Acid deposition affects most materials to some degree. Limestone, marble and sandstone are particularly vulnerable, whilst granitic-based rocks are more resistant to acidity. Other vulnerable materials include carbon-steel, nickel, zinc, copper, paint, some plastics, paper, leather and textiles. Stainless steel and aluminium are more resistant metals. Structural damage to underground pipes, cables and foundations submerged in acid waters can also occur, in addition to damage to buildings, bridges and vehicles above ground.

Whilst dry deposition contributes to the corrosion of materials, in most areas with substantial rainfall it is the effect of wet deposition on building surfaces which is more damaging. Building stone can be damaged when calcium carbonate in stone dissolves in acid rain to form a crust of calcium sulphate or gypsum. The sulphated layers are more readily washed away by rainfall or removed by the action of frost and other weather conditions, resulting in more stone being exposed. This permanent alteration of stone surfaces by the action of acid deposition is known as sulphation.

Sulphur dioxide is the main pollutant in respect to corrosion but others also take their toll, including nitrogen oxides, carbon dioxide, ozone (on organic materials) and sea salt from sea spray. Research has revealed that when nitrogen dioxide is present with sulphur dioxide, increased corrosion rates occur. This is because the nitrogen dioxide oxidises the sulphur dioxide to sulphite thereby promoting further sulphur dioxide absorption.

The interactions between building materials and pollutants are very complex and many variables are involved. Deposition of pollutants onto surfaces depends on atmospheric concentrations of the pollutants and the climate and microclimate around the surface. Once the pollutants are on the surface, interactions will vary depending on the amount of exposure, the reactivity of different materials and the amount of moisture present. The last factor is particularly important because the sulphur dioxide that falls as dry deposition is oxidised to sulphuric acid in the presence of moisture on the surface.

The effects of acid deposition on modern buildings are considerably less damaging than the effects on ancient monuments. Limestone and carbonate stones which are used in most heritage buildings in the UK are the most vulnerable to corrosion and need continued renovation. Cathedrals such as York Minster and Westminster Abbey have been severely eroded in recent years. A five-year research program in the UK has suggested that if sulphur dioxide emissions were reduced by 30%, savings over 30 years could be as high as 9.5 billion. Many other countries have noticed an acceleration of damage to their cultural heritage. The Taj Mahal in India, the Colosseum in Rome and monuments in Krakow, Poland are continuing to deteriorate. In Sweden, medieval stained glass windows are thought to have been affected by acid rain.

Taj Mahal