Present concern over acid rain is concentrated mainly on its effect on the natural environment, but acid rain also contributes to deterioration in the built environment. Naturally acid rain has always been involved in the weathering of rocks at the earth's surface. It destroys the integrity of the rock by breaking down the mineral constituents and carrying some of them off in solution. All rocks are affected to some extent, but chalk, limestone and marble are particularly susceptible to this type of chemical weathering. Inevitably, when these rocks are used as building stone, the weathering will continue. In recent years, however, it has accelerated, in line with the increasing acidity of the atmosphere.
Limestone is a common building stone, because of its abundance, natural beauty and ease of working. Its main constituents are calcium and magnesium carbonates, which react with the sulphuric acid in acid rain to form the appropriate sulphate. These sulphates are soluble, and are washed out of the stone, gradually destroying the fabric of the building in the process. Further damage occurs when the solutions evaporate. Crystals of calcium and magnesium sulphate begin to form on or beneath the surface of the stone. As they grow, they create sufficient pressure to cause cracking, flaking and crumbling of the surface, which exposes fresh material to attack by acid rain (Anon. 1984). Limestone and marble suffer most from such processes. Sandstone and granite may become discoloured, but are generally quite resistant to acidity, as is brick. Acid damage to the lime-rich mortar binding the bricks may weaken brick-built structures, however (Park 1987). Structural steel and other metals used in modern buildings may also deteriorate under attack from acid rain (Ontario: Ministry of the Environment 1980).
By attacking the fabric of buildings, acid rain causes physical and economic damage, but it does more than that; it also threatens the world's cultural heritage. Buildings which have survived thousands of years of political and economic change, or the predation of warfare and civil strife, are now crumbling under the attack of acid rain. The treasures of ancient Greece and Rome have probably suffered more damage in the last 50 years than they did in the previous 2,0003,000 years (Park 1987). On the great cathedrals of Europe—such as those in Cologne, Canterbury and Chartres—the craftsmanship of medieval stone masons and carvers may now be damaged beyond repair (Pearce 1982a; Park 1987). Few buildings in the industrialized regions of the world are immune, and damage to the Taj Mahal in India from sulphur pollution may be only the first indication that the problem is spreading to the developing world also (Park 1987). The faceless statues and crumbling cornices of the world's famous buildings receive most publicity, but less spectacular structures may provide important information on the rate at which damage is occurring and its relationship to acid emissions. In the United States, for example, researchers investigating the effects of acid rain on building materials have used the standardized marble gravestones in national military cemeteries, as controls in their field studies (Anon. 1984).
The chemical processes involved in acid rain attacks on the built environment are essentially the same as those involved in the natural environment, but there are some differences in the nature and provenance of the acidity. Damage in urban areas is more often associated with dry deposition than with wet, for example (Park 1987). Acidic particles falling out of the atmosphere close to their source land on buildings, and, once moisture is added, corrosion begins. Damage is usually attributed to deposition from local sources, such as the smelters or power stations commonly found in urban areas, with little of the long range transportation associated with acidification in the natural environment. However, in cities with little industrial activity, such as Ottawa, Canada, most of the damage will be caused by wet deposition originating some distance upwind (LaBastille 1981), and the same probably applies to acid corrosion of isolated rural structures (Park 1987).
Since the various Clean Air Acts introduced in Europe and North America in the 1960s and 1970s had their greatest impact on urban pollution levels, it might be expected that the effects of acid rain on the built environment would be decreasing. There is some indication that this is so, but there appears to be a time lag involved, and it may be some time before the reduction in emissions is reflected in a reduction of acid damage to buildings and other structures (Park 1987).
Was this article helpful?