Scottish chemist Robert Angus Smith earned the title "Father of Acid Rain" in 1852 when he noted the connection between damaged forests and the acidic waste products of London’s factories. Twenty years later, Smith published the book “Air and Rain: the Beginnings of a Chemical Climatology,” in which he coined the term “acid rain” to describe rainwater polluted by high concentrations of sulfuric and nitric acids.
Acid Rain and pH
To understand acid rain, you have to understand the pH scale. The pH scale ranges from 0 to 14. Pure water has a neutral pH of 7. A solution with a pH higher than 7 is referred to as an alkaline, or base. Solutions with a pH lower than 7 are acidic. The pH scale is logarithmic, so each whole pH value below 7 is 10 times more acidic than the next higher value.
Normal, unpolluted rain is naturally acidic, with a pH between 5.0 and 5.5. Polluted rainwater with a pH lower than 5.6 is referred to as acid rain.
Chemistry of Acid Rain
Water collects in the Earth's atmosphere as water vapor. When the vapor condenses, it forms particles that fall toward the ground as rain, snow, fog or hail. Both natural and man-made activities release sulfur dioxide and various nitrogen oxide gases into the atmosphere. When these gases come in contact with water particles, they dissolve into the water.
Sulfur dioxide in solution forms sulfuric acid, while nitrogen oxides in solution primarily form nitric acid. These two acids combined with water form a weak acidic solution. In turn, a higher concentration of dissolved acids lowers the pH of rainwater. In the heavily industrialized Northeast, rain pH commonly measures between 4.0 and 4.5, about 10 times more acidic than normal rain.
In nature, bacterial action in soil as well as forest fires form nitrogen oxides. Volcanic activity or lightning can also oxidize atmospheric nitrogen. And erupting volcanoes, sea spray, marine plankton and rotting vegetation all discharge sulfur dioxide.
Natural sources of sulfur and nitrogen compounds account for the slightly acidic pH of normal rainwater. In its unpolluted state, rainwater acidity is generally counteracted by the alkalinity of the soil where it falls, resulting in a more neutral environmental pH.
Agricultural nitrate fertilizers, aerosol spray propellants and even cigarettes contribute to the load of nitrogen oxides in the atmosphere. Several industrial processes produce sulfur dioxide byproducts. But most of the man-made acidic pollutants in the Earth's atmosphere come from the combustion of fuels containing sulfur and nitrogen compounds: coal, oil and gasoline. According to the U.S. Environmental Protection Agency, electric power generation alone accounted for 69 percent of total sulfur dioxide emissions in the United States in 2007 and 20 percent of nitrogen oxides.
Burning fossil fuels pumps higher than normal concentrations of sulfur dioxide and nitrogen oxides into Earth's atmosphere, resulting in more frequent and more concentrated acid rains.
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