Acid Rain

           Any form of precipitation (rain, snow, sleet, or hail) containing sulfuric or nitric acid, generally with a pH value less than 5.6 (normal rain has a pH of 5.6 to 6.0 from the weak carbonic acid — H₂CO₃ — in the atmosphere). Sulfur dioxide (SO₂) and nitrogen oxides (NO) are the primary constituents of acid rain. In the US, About ²/₃ of all SO₂ and ¼ of all NO originate from electric power generating plants that rely on burning fossil fuels, especially coal. Other acid rain sources include various industrial processes, especially oil refining and ore smelting and emissions from gasoline engines in motor vehicles. Acid rain results when those emitted gases react in the atmosphere with water vapor, oxygen, and other chemicals to form various complex acidic compounds. Energy from the Sun increases the rate of most of those reactions. The result is a mild solution of sulfuric and nitric acids in the atmosphere that affects the Earth in the form of precipitation. Many of those pollutants can remain in the atmosphere for five or six days and thus can be transported by the winds far from their source regions.

Real World Problem: All industrialized and many developing nations have been affected by acid rain. Among the best-known effects of those kinds of airborne pollution is the acidification of lakes. The higher acid levels destroy fish eggs and also leach aluminum from surrounding areas into the lakes, which proves toxic to adult fish. Consequently, many lakes in the northeastern part of North America and northern Europe are devoid of fish and other aquatic organisms. If that’s not bad enough, acid rain reduces forest and agricultural productivity in North America and Europe (as an example, about 80 percent of all the trees in Germany’s Black Forest have been adversely affected). In addition, since the 19^th Century in European and North American cities statutes, monuments, and other ornamental stonework have been corroded by the acid rain. Marble, limestone, or dolostone structures are especially vulnerable to that attack, literally dissolving stone details until entire sections are unrecognizable. This situation is complicated by the reality of atmospheric transport, which carries pollution from its source downwind, sometimes for thousands of miles. Therefore, Japan complains about pollution from China, Sweden complains about British pollution, and Canada complains about pollution generated in the Midwest. Although long-term human health effects are not known with great specificity, short-term effects include asthma and other lung-related problems, headaches, and eye, nose and throat irritations.

Author’s Note: Since the pH scale ranges from 0 to 14, a pH of 7 is neutral. Normal rain has a pH of ranging from 7 to 5.5 and acid rain can be as much as or even more than 100 times more acidic. It’s important to remember that the scale is logarithmic, meaning that a difference of a single whole integer represents a ten-fold change in absolute value; in other words, a drop of a single pH unit equals a 10-fold increase in the concentration of hydrogen ions, making the substance more acidic. As an aside, the pH scale was originally introduced in 1909 by the Danish biochemist Søren Peter Lauritz Sørensen when he used the symbol pH in the measurement of the acidity or alkalinity of solutions, with strongly alkaline registering at the upper end of the scale and highly acidic at the lower end. The letter p was derived from the German word, potenz, meaning power or exponent or potential of a number, in this case 10, and the H was, of course, hydrogen.

Historical Background: The chemist Robert Angus Smith was England’s first alkali inspector. In the late 1860s and early 1870s he discovered a connection between air polluted by heavy industries over the city of Manchester and the high level of acidity in precipitation in the region. Smith published what is now regarded as a classic study, Air and Rain: The Beginnings of a Chemical Climatology, in which he coined the term, acid rain, and found that building taller smokestacks relieved the situation locally but spread the problem over much larger areas and completely failed to dilute the toxic fumes to a point where they were no longer harmful. In 1959 the Swedish soil chemist Svante Oden and meteorologist Erik Eriksson traced Sweden’s soil and lake problems to emissions from industrial plants in Germany and England and ushered in the first modern awareness of the serious problem of long-distance transport of toxic chemicals. Since the 1980s, tall stacks used in electrical power stations and industrial smelters have grown even taller, with many reaching heights over 1,000 feet. Tests of several industrial smelter stacks demonstrated that the stack gases were as hot as 700° F and exited the stacks at speeds of up to 55 miles per hour. The result of the transformation from short to tall stacks was that local air quality improved but regional and even international pollution increased dramatically, which provides yet another example of the revenge of unintended consequences. See Edward Tenner, Why Things Bite Back: Technology and the Revenge of Unintended Consequences, New York: Alfred A. Knopf, Inc, 1996.

Additional Author’s Note: Very high levels of acidity in rain have been recorded in the U.S. in numerous states, but among the lowest pH readings were in Kane, Pennsylvania (pH 2.7 or as acid as lemon juice), and Wheeling, West Virginia (pH 1.5 or about the level of stomach acid).