Monday, June 18, 2012

Acid Rain Effects on Architecture

Acid rain has profound effects on architecture and statues. As you can see in the photo above, continuous exposure to acid rain causes statues the disintigrate. What causes the unfortunate weathering of these statues? The answer lies in the chemcal reaction that happens between acid rain and limestone.

Acid rain undergoes a series of chemical and physical changes when it falls to the earth. These changes can lessen the acidity of the rain; this is called neutralization. Alkaline and basic soils, particulalry soils that are rich in limestone or calcium carbonate, can help to neutralize the acididy directly. The reason why acid reacts so violently with limestone, is due to the chemical reaction.

Limestone: CaCO3 + H2SO4---> CaSO4 + H2CO3

H2CO3---> CO2 gas +H2O

The sulfuric acid reacts with limestone in a neutralization reaction. The calcium sulfate that is produced in the reaction is soluable in water. This causes the limestone to dissolve and crumble. In this reaction, the original acid converts to water. This reaction is similar to the reaction that happens when acid rain reacts with limestone in natural surroundings. When there is less calcium carbonate in a region, acid rain tends to affect this area more heavily than in an area where limestone or marble is part of the environment.

Acid precipitation affects stone in two different ways: dissolution and alteration. When sulfurous, sulfuric, and nitric acids in air react with calcite in marble and limestone, the calcite disolves. We see this on surfaces that are rough and loss of carved details. Limestone and marble buildings sometimes show blackened crusts that have peeled off to reveal crumbling stone beneath it. This black crust is called "gypsum". Gypsum is a mineral that forms from the reaction between calcite, water and sulfuric acid. Gypsum is soluable in water, and it can form anywhere on carbonate stone surfaces that have come into contact with sulfur dioxide gas. It is usually washed away with time. It remains in places that are not directly exposed to rain. Gypsum is white, but crystals form networks that trap particles of dirt and other pollutants, so the crust has a black appearance. 

a picutre of gypsum with dirt and pollution particles trapped by the network

Many buildings have also been destroyed because of this destructive element, but more important than the buildings acid rain destroys, environmentalists are becoming more concerned for human health. This rain has the acidity to wear away the surfaces of buildings, but this has become an increasing problem. Researchers have recorded showers in Japan that have the acidity of vinegar. Indicating that the already problematic nature of acid rain is getting worse. 

A building that has been exposed to acid rain. The detailing on the sides of this structure have gradually been worn away by continual exposure to acid rain.

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