Animation of the formation and effects of acid rain. Acid rain occurs when pollutant gases emitted from industrial processes react with water in the atmosphere, forming acids. These dissolve in rain water and fall to the ground, where they have numerous detrimental effects. The power plants and vehicles seen here emit gases including sulfur dioxide, sulfur trioxide, nitrogen dioxide and carbon dioxide. In the moist atmosphere of the atmosphere, these react with water to form, respectively, sulfurous acid, sulfuric acid, nitric acid and carbonic acid. When these acids fall in rainwater, they harm forests by acidifying the soil and attacking foliage. The rain also acidifies groundwater, killing fish and other aquatic life. The rain can also seriously damage masonry, particularly buildings and monuments made of limestone. Since serious research into acid rain's effects began in the mid-to-late 20th century, many power plants remove many of these gases at source, and international treaties have been put in place to establish emissions standards from power plants and vehicles.