Acid Rain What Is Acid Rain? Rain and snow are naturally slightly acidic due to chemical reactions with carbon dioxide in the atmosphere. The term "acid rain" is used to describe rain or snow that has a pH lower than what is natural for a given area. pH is a measurement of how acidic or basic a material is and ranges from 0 to 14. Precipitation with a pH value less than 5 is considered acid rain. How Does Acid Rain Form? Enormous quantities of manmade and natural material are added to the air every day. Most of the materials added to the atmosphere return to the ground through a process known as deposition. Deposition occurs when it rains and snows, but it can also occur when dust settles out of the atmosphere during dry periods. The earth's gravity works to continually pull dry particles back to earth. Uneven heating of the earth results in global winds in the atmosphere. Global winds provide the energy for long-range travel of the gases, liquids, and dust in the atmosphere, which can travel great distances before falling back to the ground. The burning of fossil fuels like coal and oil products by automobiles and power plants releases large amounts of sulfur dioxide and nitrogen oxides into the air. While being transported by winds, some of these particles get caught up in clouds. When sulfur dioxide and nitrogen oxides gases and particles come in contact with water droplets in clouds, chemical reactions can occur, resulting in acid rain. Additional processes called rainout and washout mix these acidic gases, liquids, and particles into rain drops and snowflakes and carry them to the ground. Because of global winds and mixing in the atmosphere, every country's air pollution contributes to some degree to the earth's problem of acid rain. In 1993, the United States released approximately 90 billion pounds of sulfur dioxide and nitrogen oxides into the air. Rain and snow that falls in the Eastern U.S. typically has a much lower pH than precipitation in other parts of the country. The pH of rain and snow in the Eastern U.S. is much lower than in less populated areas. What Are The Effects Of Acid Rain? More than thirty years ago, scientists noticed that in certain lakes in remote wilderness areas, fish populations were mysteriously declining. Some lakes that once teemed with fish were found to contain none at all. In their search for what caused the fish to die, scientists concluded that acid rain was the problem. Researchers continue to document that acid rain is harmful or fatal to fish (Walk and Godfrey, 1990). Acid rain also has been found to speed up the natural decay of stone monuments and historical buildings. Other materials such as iron, steel, zinc and paint also can be damaged by acid rain. The human health effects of acid rain are also of concern. Although people aren't directly in danger from exposure to acid rain, the particles in air that lead to acid rain may be a risk to human health. Source: U.S. Geological Survey