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Air Pollution
To learn more about the major threats to aquatic biodiversity please select from the following:
Air pollution can originate from many sources, especially electrical utilities (power plants) from the burning of fossil fuels like coal. Power plants and factories burn coal and oil. Power plants use coal and oil to produce the electricity we need to heat and light our homes and to run our electric appliances. We also burn natural gas, coal, and oil to heat our homes. Cars, trucks, and airplanes use gasoline, another fossil fuel.
What is Acid Rain?
Air pollution can take many forms, but one of the most detrimental to freshwater biodiversity is acidic deposition, sometimes referred to as acid rain. Acidic deposition, in both dry (e.g., particulates) or wet (e.g., rain, snow) forms can harm aquatic ecosystems by causing the water to become more acidic, and thus affecting the aquatic life.
The smoke and fumes from burning fossil fuels rise into the atmosphere and combine with the moisture in the air to form acid rain. The main chemicals in air pollution that create acid rain are sulfur dioxide and nitrogen oxides. Acid rain usually forms high in the clouds where sulfur dioxide and nitrogen oxides react with water, oxygen, and oxidants. This forms a mild solution of sulfuric acid and nitric acid. Sunlight increases the rate of most of these reactions. Rainwater, snow, fog, and other forms of precipitation containing those mild solutions of sulfuric and nitric acids fall to the earth as acid rain.
Water moves through every living plant and animal, streams, lakes, and oceans in the hydrologic cycle. In that cycle, water evaporates from the land and sea into the atmosphere. Water in the atmosphere then condenses to form clouds. Clouds release the water back to the earth as rain, snow, or fog. When water droplets form and fall to the earth they pick up particles and chemicals that float in the air. Even clean, unpolluted air has some particles such as dust or pollen. Clean air also contains naturally occurring gases such as carbon dioxide. The interaction between the water droplets and the carbon dioxide in the atmosphere gives rain a pH of 5.6, making even clean rain slightly acidic. Other natural sources of acids and bases in the atmosphere may lower or raise the pH of unpolluted rain. However, when rain contains pollutants, especially sulfur dioxide and nitrogen oxides, the rain water can become very acidic.
Acid rain does not account for all of the acidity that falls back to earth from pollutants. About half the acidity in the atmosphere falls back to the earth through dry deposition as gases and dry particles. The wind blows these acidic particles and gases onto buildings, cars, homes and trees. In some instances, these gases and particles can eat away the things on which they settle. Dry deposited gases and particles are sometimes washed from trees and other surfaces by rainstorms. When that happens, the runoff water adds those acids to the acid rain, making the combination more acidic than the falling rain alone. The combination of acid rain plus dry deposited acid is called acid deposition.
The chemical reactions that change air pollution to acid rain can take from several hours to several days. Years ago, when smokestacks were only a few stories high, pollution from smokestacks usually stayed near the ground and settled on land nearby. This caused unhealthy conditions for plants and animals near the smokestacks. To reduce this pollution, the government passed a law permitting the construction of very tall smokestacks. At that time, people thought that if the pollution were sent high into the air it would no longer be a problem. Scientists now know that this is incorrect.
Sending pollution high into the sky increases the time that the pollution stays in the air. The longer the pollution is in the air, the greater are the chances that the pollutants will form acid rain. In addition, the wind can carry these pollutants for hundreds of miles before they become joined with water droplets to form acid rain. For that reason, acid rain can also be a problem in areas far from the polluting smokestacks. Dry deposition is usually more abundant near the cities and industrial areas where the pollutants are released.
There are also natural sources of acids such as volcanoes, natural geysers and hot springs. Nature has developed ways of recycling these acids by absorbing and breaking them down. These natural acids contribute to only a small portion of the acidic rainfall in the world today. In small amounts, these acids actually help dissolve nutrients and minerals from the soil so that trees and other plants can use them for food. The large amounts of acids produced by human activities overload this natural acidity.
Effects of Acid Rain
Acid rain can affect streams through many ways including rainfall, soils washed into streams, and urban area runoff. Acid rain alters lake/stream chemistry by lowering pH. Most healthy lakes and streams have a pH between 6 and 8, while acid rain has a pH less than 5. As the rain falls or particulates are deposited, many lake ecosystems become less able to buffer this acid. Many lakes and rivers become more acidic as time goes on, as their neutralization abilities are compromised. This in turn affects the ecosystem as a whole.
Healthy freshwater ecosystems have a diverse number of species, such as zooplankton, fish, and aquatic birds like loons that depend on the freshwater environment for life. As the pH falls below 6, these ecosystems will begin to see a decline in the number of aquatic species, and their populations. Some species are more tolerant of acidic conditions than others, as shown in the chart below. Clams and snails will be the first species to disappear. This in turn will affect other species, including those that are acid tolerant, due to collapses in the food chain. As lakes become more acidic (pH levels drop), the following changes can be seen:
(Source: U.S. Environmental Protection
Agency Acid Rain Program: Effects
of Acid Rain on Water)
| As water pH approaches | Effects |
|---|---|
| 6.0 |
|
| 5.0 |
|
| Less than 5.0 |
|
(Source: Environment Canada: Acid Rain...and Water) 
Those species that do not tolerate acidic environments will first begin to lose the ability to reproduce. Even if a female is able to spawn, the young of the year might not be able to survive the harsh acidic environment, and may be more susceptible to disease or deformity.Some species do thrive in acidic environments. Blackfly larvae, mosses and lake-bottom plants find acidic conditions beneficial to growth. (Source: Environment Canada: Acid Rain...and Water)
Because the weather systems in the US tend to travel in a southwest to
northeast pattern, many acidification problems tend to occur in the northeast.
Approximately 14 percent of lakes greater than 10 acres in size in the
Adirondack Mountains are chronically acidic, as are about 12 percent of
streams in the mid-Atlantic Highlands and the mid-Appalachians. Florida
and the Upper Peninsula of Michigan are also plagued with this problem.
Episodes of acidification is another problem that occurs in multiple lakes
and streams, including those in the high elevation West. Of those lakes
that are acidic, acid rain has been shown to be the problem in 75% of
them, while 50% of acidic streams have acid rain as the cause.
(Sources: http://www.epa.gov/rgytgrnj/programs/artd/air/acidrain/acidrn2.htm#Question3 and http://www.epa.gov/airmarkets/progsregs/arp/basic.html)
To give a better picture of acid deposition problems within the United
States, The National Atmospheric Deposition
Program/National Trends Network developed a map
of hydrogen ion concentrations as pH ,
from measurements made at the field laboratories during 2006.
Additional Resources