When sulfur dioxide, nitrogen oxides or other chemicals combine with water in the atmosphere it becomes a weak solution of nitric or sulfuric acid. This mixture becomes rain, dew or fog. The chemicals are in the air because of natural processes, such as a volcano. Car exhaust and smoke produce these components, too. Materials exposed to this acid may corrode or deteriorate when in contact with the rain, dew or fog. Gravestones are made of many things. Marble, granite, limestone and sandstone are the most common.
The amount of minerals dissolvable in acid is the key as to how fast the stone deteriorates. If there are many chemicals which react with acids in the stone, it will be faster. The rate also depends on stone density. The denser the material, the less acid rain it will absorb. A porous rock will allow acid precipitation deeper into it and will deteriorate quicker.
Limestone is a common stone for gravestones. It is a porous rock with calcium carbonate in its composition. Calcium carbonate is a base and neutralizes acids. To do this, part of the calcium carbonate changes and this causes deterioration of the tombstone or anything made of limestone. Because the stone is porous, the water can go deeper into the material and cause deterioration.
Granite and marble are common stones and used for gravestones and buildings. They can contain calcites that will neutralize acids. The more of these they contain, the acid will eat more of the rock away. By themselves, granite and marble are resistant to acid rain, but they will show effects of acid rain. It loses its shine and luster and becomes rougher to the touch. Acid rain will soften the edge sharpness and gradually wear away the stone. Marble also contains calcites. When acid rain contacts calcites, the calcites dissolve.