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Acid Rain

The term acid rain describes any from of precipitation containing high levels of sulfuric and nitric acids. All precipitation, including snow, fog, hail, and sleet, can be acidic. The terms acid deposition or wet disposition are frequently used as synonyms. This form of pollution has caused severe environmental problems in many parts of North America, Europe, and Asia. It affects mainly heavily industrialized regions and densely populated urban areas, but substantive amounts of gases causing acid precipitation can travel hundreds of miles before falling.

Causes and Formation

The formation of acid rain generally starts with the emission of sulfur dioxide and nitrogen oxide gases into the atmosphere. These substances react with the water molecules in clouds and become sulfuric and nitric acid. By the time they reach the ground again in the form of precipitation, they are highly acidic. The degree of acidity is measured on a pH scale, ranging from 0 to 14.

The number indicates the hydrogen ion concentration in kilograms per cubic meter, thus lower numbers represent greater acidity. In severe cases, the pH value of rain can be as low as 2 or 3. Some parts of western Europe and the eastern part of the United States suffer from this highly acidic rain. In certain areas, such as Los Angeles, California, or Whiteface Mountain, New York, acid fog causes much more damage, showing about 10 times higher degrees of acidity than rain.

Acid rain is mainly caused by human activities. The largest share of these pollutant gases come from the combustion of fossil fuels, such as oil and coal, used by electric power plants as well as by cars and other motor vehicles. Livestock production is another important factor. A much smaller amount comes from wildfires. Natural phenomena can also cause acid precipitations. Emissions from active volcanoes can lead to fog and rain with high degrees of acidity, making human settlement and vegetation impossible in surrounding areas. A small amount of those gases causing acid precipitation occurs naturally in wetlands and oceans.

Effects and Damage to Nature

Acid precipitation contaminates streams and lakes and damages fish and other aquatic life. It is especially harmful in areas where soil is thin or mainly composed of granite rock, since there are few natural filters to buffer the acidic components of water. Acid precipitation further has the effect of dissolving aluminium from the soil, which aquatic organisms then absorb. Not all species can tolerate the same amount of acidity, and some are more vulnerable to increasing aluminium and lower pH values than others. Fish eggs are affected, as they will not hatch when the water is too polluted. Higher acidity can also kill adult aquatic organisms. As a result, there is reduced biodiversity in contaminated lakes and rivers, and some species have already been eliminated.

Acid precipitation causes great damage to many kinds of vegetation. It inhibits nitrogen fixation and leaches out nutrients from foliage. Trees can then become less tolerant to other outside influences, such as cold weather. High altitude forests are most affected, as they are more often immerged in contaminated clouds and fog, which are much more acidic than rain. Even though agricultural crops can be damaged, the harmful effects are counterbalanced by additional applications of lime and fertilizers that replace the nutrients. Some microorganisms are sensitive to changing degrees of acidity. Their enzymes are denatured, and they cannot survive. While toxic substances such as aluminium dissolve from the soil and enter the watershed (causing damage to marine life), useful and important nutrients, such as magnesium and calcium, are also leached away. This loss of nutrients especially affects sensitive plants. The addition of limestone helps stabilize the pH of soil—a method mainly used for agricultural soil.

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