A particularly important consequence of the release of gaseous pollutants into the atmosphere is the local acidification of the environment. Rainwater reaching Earth's surface has a pH of about 5.6 (it is lower than 7 because of the dissolved carbon dioxide). In some parts of North America, Europe, and even Asia, however, acidic rainwater of pH < 5.6 is quite widespread. This is believed to be due to the presence of acidic gases, particularly oxides of sulfur (SOX) and oxides of nitrogen (NOX) in the atmosphere. The contribution of HCl, if any, is relatively minor. A biogenic contribution to acidification caused by emissions from volcanos or forest fires also exists, but is relatively small.
The nitric oxides, from vehicle exhaust, are insoluble in water but can be oxidized (either by peroxides or ozone) into NO2 that can dissolve in water to yield dilute nitric acid. The NOX emissions are relatively small amounting to only about 28 million tons, compared to the 71 million tons of sulfur emitted globally in 1994. This includes biogenic emissions, due to microbiological activity. The SOX emissions are responsible for about 60% of the acid rain obtained worldwide . A particular concern is the increased threat of acidification in the developing countries that use low grades of fuel and rely on biomass burning. For instance, the brown coal used in China has a sulfur content of 5 percent. In the United States, most of the SOX emissions (an estimated 75%) occur from east of the Mississippi River where most industry is located. While emission of hazardous gases is generally considered a local or regional problem, the SOX/NOX emission can also result in impacts across national boundaries. For instance, the emissions from northern states in the United States are responsible for about half the acid deposition in southeastern Canada,40 and those from industries in northeastern China and South Korea affect Japan.
The impacts of acidification reach well beyond the direct toxic effects of high hydrogen ion concentration on plants and animals. Decline in the quality and growth rates of forests due to acid rain is well documented in several regions of the world41 . For instance, in the Sichuan province (particularly in Omei Mountain) of China, 87% of the cedar trees are seriously affected by acidification. Scandinavian scientists in 1960s first observed reduction of fish in lakes affected by acidification. Even a moderate acidification (pH ~ 6.0) of surface waters can kill crustacean species. At pH < 5.5 loss of economically important fish species such as salmon and trout is expected. Hardy species such as the
40 Under the 1991 Air Quality Accord between Canada and the United States, emissions that cause acid deposition were permanently capped in both countries (13.3 million tonnes of sulfur for the United States and 3.2 million tonnes for Canada).
41 In Germany over 50% of the forests, particularly the fir forests in the southern part of the country, show some signs of decline. In the U.S. red Spruce is the most-affected species.
eel and the brook trout succumb at a pH ~ 4.5 . Fish, particularly the eggs and the fry are very sensitive sudden changes in the pH of their medium. A particularly damaging phenomenon (sometimes referred to as "acid shock") is seen in winter when acidic deposits build up in the snowpack. In spring with the melting of snowpack the large amounts of acid released in a short period of time contaminate bodies of water. An important indirect effect of acid rain is the solubilization of metal compounds that can be toxic to fish and other aquatic organisms . Aluminum solubilizes and leaches out of the soil at pH < 5.5 yielding Al3+ and Al(OH)3 that can be toxic to plants. A similar increase in concentration in the soil water is expected for other metals such as cadmium, mercury, and lead. Acidity-induced decreases in the solubility of minerals such as calcium can also be detrimental to the ecosystem. With low levels of dissolved calcium in water, invertebrates with shells (molluscs and crustaceans) can suffer poor growth. The deleterious effects of acidity on historical buildings and monuments (e.g., the Roman architecture in Italy, the Greek monuments such as the marble sculptures in Athens, and the Taj Mahal in Agra, India) are well known. International efforts are already under way to address the loss of this world cultural heritage.
The emissions of SOx and NOx are declining in the United States in recent years, reducing the threat of acid rain in the region. This is mainly a result of Title IV of the Clean Air Act that set a goal of reducing annual SO2 emissions by 10 million tons below the 1980 emissions levels. The phase I effort (regulating 110 large sources, concluded in 1995) was successful, having reduced SOx levels 40% below the required levels. It is important to continue the remedial and pollution prevention efforts globally to keep this trend going in future years.
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