ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY
The effects of acid rain on stone. Rainfall is rendered acid by sulphur dioxide and nitrogen dioxide which mix with atmospheric moisture. Nitrogen oxide comes from car exhausts and sulphur dioxide is emitted by fossil-fueled power stations and heavy industry, often from tall chimneys built to reduce local pollution. Normal rain has a pH of 5.6; in parts of Europe and North America the average pH is now 4-4.5. This acidity attacks carbonate-bearing stone such as limestone and sandstone, converting the calcium carbonate to calcium sulphate. The gypsum formed is dissolved by rainwater causing pitting and corrosion, as seen here on a stone lion outside Leeds Town Hall.
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