Acid Rain

The critical issue of acid rain is a good example of a transnational environmental problem. An increase in acidic gases and particles in the atmosphere produced in one country can impact other countries further downwind of these acidifying emissions.

The acidic nature of rain was first observed near industrial cities in England and Scotland by an English chemist, Robert Angus Smith, in 1832. However, it was not until the late 1960s and early 1970s that acid rain was recognized as a widespread environmental problem in Europe and eastern North America. According to Gene Likens, F. Herbert Bormann, and Noye Johnson (1972), acid rain is a general or popular term, now known to include not only wet deposition from rain, snow, sleet, hail, cloud, and fog water but also dry deposition from acidifying particles and gases. Acid deposition is the more scientific term for this phenomenon.

The sources of most acid deposition are anthropogenic, mainly the burning of fossil fuels for power generation, industrial activity and transportation, and the smelting of metal ores. These activities produce SO2 and/or NOx (NO + NO2) gases, which are oxidized and converted to sulfuric and nitric acid, respectively. In their simplest forms, the reactions are:

pH is a measure of hydrogen ion (H+) concentration, and the scale is logarithmic (pH = −log H+ concentration), so precipitation with a pH of 4.0 has 10 times more H+ ions (100 μeq/l) than precipitation with a pH of 5.0 (10 μeq/l). The pH of rain in industrialized regions is mainly the result of sulfuric and nitric acid. Precipitation is naturally somewhat acidic because of the carbonic acid from CO2 and organic acids from biological activity. Volcanic activity can increase atmospheric acidity by producing hydrochloric acid (HCl), as well as sulfuric acid. Lightning and forest fires can convert molecular nitrogen (N2), the main component of the atmosphere, to nitrogen oxides, which then can form nitric acid. These natural sources of acidity are generally small and reduce the pH of precipitation from 5.6 or higher to about 5.2. However, in many industrialized areas of the world, the average pH of precipitation is 4.0 to 4.5, with individual events often much lower. Organic acids can contribute to acidity (Likens, Keene, Miller, & Galloway, 1987), but their contribution is usually less than 20% of the total acidity in industrial areas.

The largest amounts of acid deposition occur in eastern North America, central and northwestern Europe, and more recently in Southeast Asia (see Figure 1). [Page 18]These are all areas downwind of large emissions of SO2 and NOx resulting mainly from fossil fuel–based power production and industrial activities. China is the leading producer of SO2, having produced 33.2 million metric tons in 2006 (Lu et al., 2010), and the United States produces the most NOx, 16.6 million metric tons in 2005 (U.S. Environmental Protection Agency, 2010).

Figure 1 Areas sensitive to acid deposition (shaded) and areas receiving acid deposition (hatched areas show pH of rain < 4.5).;

Source: Based on Kuylenstierna, Rhode, Cinderby, & Hicks (2001); Rhode, Dentener, & Schulz (2002).

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