Most have heard about the effects of acid rain on plants and trees. But how does it affect buildings?
This article explains one of the pressing problems facing architecture and construction today and the different ways to minimize its harmful impact.
What Is Acid Rain?
Acid rain is the highly diluted pollution that flows through the atmosphere from a smokestack or tailpipe and falls to earth as precipitation. Acid rain is not just one chemical type but refers to any polluted precipitation with acidity as one of its properties.
By definition, therefore, acid rain is a relative term. Rain isn’t acidic in comparison with other types of precipitation until it’s understood what type(s) constitute the norm.
For example, if nitrogen dioxide, sulfur dioxide, and other pollutants are present in the rain at concentrations 1/1,000th of those found naturally in clean air, this water would be considered mildly acidic. However, in comparison with unpolluted rain, the same water is considered highly acidic. Whether it’s found in a polluted or unpolluted area, however, acid rain is anything but natural.
The Acid Cycle
Acid rain occurs when pollutants emitted from smokestacks and tailpipes are attacked by oxygen molecules present in the atmosphere. The natural role of these “free radicals,” known to chemists, is to react with other chemical compounds. They play an important part in many biological processes (the destruction of bacteria near skin wounds being one example) and can even be thought of as nature’s way of neutralizing poisons that might otherwise build up in living things.
In the upper atmosphere, just above the Earth’s surface, about 10 percent of all oxygen molecules are free radicals. In polluted areas near the ground, however, this percentage can rise as high as 50 percent. Once in the atmosphere, these oxygen molecules react with pollutants to form acids and other new compounds. Because of this reaction, chemists call air pollution a “chemical cycle.”
When acidic compounds (which include acids and oxidants) reach the earth’s surface, they commonly fall to earth as rain or snow. Both normally are very slightly acidic. Pollutants that don’t react with atmospheric oxygen tend to remain airborne long enough to directly settle into soil or water. Still, those in the chemical cycle become aerosols (fine particles suspended in a gas).
When such aerosols travel through clouds, they become solid droplets of acidic liquid. In cases where pollutants are carried over very long distances, the relative proportions of acid rain’s constituents may be quite different from those existing at ground level.
Generally speaking, though, polluted areas have a higher concentration of oxidants than do unpolluted areas. Even when chemically pure water is released into an atmosphere that contains no other matter, it almost always acquires some organic impurities. It becomes mildly acidic due to contact with free radicals in the air.
How Acid Rain Affects Homes and Buildings
Acid rain corrodes building materials such as asphalt and metals. It also damages plant life, including plants in gardens around houses. Many older buildings have been built using lime mortar, which has a high calcium content and tends to resist attack by sulfuric acid found in acid rain.
But modern concretes used widely since the Second World War contain much less lime, so they do not last as well against this type of pollution. Roofing made of asphalt or concrete tiles is especially vulnerable.
Many popular buildings are already suffering from the effects of acid rain. These include Ely Cathedral, the Houses of Parliament, and even Windsor Castle. One can see these changes not only on the surface of buildings but also internally through damage to wallpapers and paints and bricks and mortar.
The good news is some steps can help protect a house or a building from acid rain:
- Retractable rolling awnings help provide shade and protect the flooring and even the garden from the ill effects of acid rain by using powder-coated thermo-fused polyester.
- To make roofing tiles less vulnerable, it is possible to coat them with a layer of cement containing limestone (known as a “whitewash”). This will not completely stop them from corroding but cuts down the time they will take to do so by at least 30 percent.
- If you have just had a new roof fitted using asphalt or concrete tiles, it is a good idea to whitewash them to ensure they do not deteriorate too quickly. If you choose instead to paint the tiles, use an acrylic-based paint and not one based on alkyd resins, as this will crack badly when attacked by sulfuric acid.
- It is best to apply new coats at least once every ten years, even if there is no sign of corrosion. Never use paint on external walls more than once because it will crack and peel away from the surface prematurely.
- If you want to keep your old building looking its best, regular washing down (every two years, for example) should help delay the onset of major damage.
To prevent acid rain from getting worse, it needs a long-term solution, including decreasing carbon emissions. But the ideas above can already mitigate the effects, at least in the short term.