Traveling pollution has far-reaching impacts.

KEY CONCEPT 20.6

Acid deposition is a secondary pollutant that results from fossil fuel burning. This pollution can travel long distances and can harm plants and animals that are exposed.

One of the most problematic characteristics of air pollution is that it moves. Air pollution produced in one city can end up harming humans and other species halfway around the globe. For example, as much as half of the air pollution that falls on the Great Smoky Mountains of Tennessee and North Carolina originates in the Ohio Valley, where it is released by tall smokestacks of coal-burning electrical power plants. Prevailing winds bring the pollution southeast, and the tall mountains in the southern Appalachians eventually stop it. There, it not only pollutes the air but also produces acid deposition— sulfur and nitrogen emissions that react with oxygen and water to form acids that can fall back to ground as acid rain or snow. Acidification of soil due to acid deposition can change the soil chemistry and mobilize toxic metals such as aluminum, hindering plants’ ability to take up water.

acid deposition

Precipitation that contains sulfuric or nitric acid; dry particles may also fall and become acidified once they mix with water.

Acids leach nutrients from the soil, too, reducing the amount of calcium, magnesium, and potassium available to plants in topsoil. Taken together, these impacts can decrease plant growth, weaken plants so they are more vulnerable to disease or pests, and even kill them. Many aquatic organisms are also vulnerable to the acidification of their water habitat, especially the eggs and young of many fish and amphibians. This acid deposition is also a problem throughout the northeastern United States. (See LaunchPad Chapter 29 for more on the pH scale and ocean acidification.) INFOGRAPHIC 20.4

ACID DEPOSITION

Why do you think that acid deposition has been a bigger problem in the eastern United States than the western part of the country? Why might acid rain be increasing in some western areas?

The population density is much greater in the eastern U.S., especially the northeast. More people mean more energy production and use (more coal burning and more vehicles). The western states seeing an increase in acid deposition may be experiencing population increases, greater per capita energy use, or may have switched from a low- or no-sulfur electricity production method to one that uses coal.

Evidence that air pollution can travel long distances can be found in the far north. Prevailing air currents pick up pollutants from the western United States, conveying them all the way to Lake Laberge in Canada’s Yukon Territory, where the moisture condenses, forms clouds, and falls on the lake as rain or snow. Thus, even the most isolated regions on Earth are vulnerable to the effects of air pollution because atmospheric and hydrologic circulation moves chemical and particulate pollutants around the globe.

Appalachian acid rain, pollution in Lake Laberge, and stratospheric ozone depletion are transboundary pollution problems because regions that suffer from the pollution are not necessarily the ones that released the pollutants. This means that even if an area does not produce pollution itself, its air may still be toxic. With the EPA’s Air Quality Index available online, people can search for up-to-date air quality reports about any U.S. region. The index also alerts local communities about air quality problems from ground-level ozone and particulate pollution.

transboundary pollution

Pollution that is produced in one area but falls in a different state or nation.

These trees in an experimental test plot show damage from the ozone pumped out of the tall vertical pipes (white) that ring the area.
David Hay Jones/Science Source
Exposure to acid rain has resulted in yellowing and loss of needles, decreasing overall photosynthesis and stunting the growth of these conifers at high elevations in the Austrian Alps.
Tony Craddock/Science Photo Library/Science Source

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