Sources and Effects of Air Pollution

• Introduction
• Visibility and Fine Particulates in the Air
• Contribution of Various Particulates to Haze
• Human Health Effects of Fine Particulates
• Atmospheric Deposition and Acidic Rain
• Toxic Pollutants in Atmospheric Deposition
• Ozone
• Natural Resource Effects of Ozone
• Human Health Effects of Ozone

Introduction

Air pollutants affecting our National Forests can be divided into two groups: primary and secondary pollutants. Primary pollutants come directly from sources such as industrial facilities, automobiles and forest fires. These include sulfur and nitrogen compounds, particulate matter, volatile organic compounds (VOC’s) like paints and toxic metals such as mercury. Secondary pollutants, such as ozone, are formed when primary pollutants undergo chemical reactions in the atmosphere. Most pollutants can be transported great distances from their source to impact lands far away.

Visibility and Fine Particulates in the Air

With the Clean Air Act of1977, Congress established a national goal of remedying existing and preventing future human-caused visibility impairment in most of our large Wilderness Areas, National Parks, and National Wildlife Refuges. Air pollution likely impairs visibility to some degree on all federal lands. The visual range within the eastern U.S. is often just 15 to 30 miles, estimated at one-third of what it would be without human caused air pollution. In the West, the visual range averages between 60 and 90 miles, or about one-half of the visual range under natural conditions. Visibility information can be found at http://vista.cira.colostate.edu/views.

Haze is caused by fine particles in the air that scatter and absorb light. When the number of fine particles increases, more light is absorbed and scattered, resulting in a shorter visual range, less clarity and altered color.

Five types of fine particles contribute to haze: sulfates, nitrates, organic carbon, elemental carbon, and crustal (soil) material. The importance of each type of particle varies across the U.S. and from season to season (see graphic below).

Contribution of Various Particulates to Haze

Sulfate particles form in the air from sulfur dioxide gas. Most of this gas is released from industrial sources such as coal-burning power plants, smelters, and oil refineries. Sulfates are the largest contributor to haze in the eastern U.S. In humid environments, sulfate particles grow rapidly to a size that is very efficient at scattering light.

Organic carbon particles are emitted into the air and also form there as a reaction of various gaseous hydrocarbons. Major sources of organic carbon particles include vehicle exhaust, vehicle refueling and solvent evaporation.

Nitrate particles form in the air from nitrogen oxide gas. This gas is released from virtually all combustion activities, especially those involving cars, trucks, and motors like those in lawn mowers, and boats, power plants, and other industrial sources. Like sulfates, nitrates scatter more light in humid environments.

Elemental carbon particles like soot are smaller than most other particles and absorb rather than scatter light. The brown clouds often seen in winter over urban areas and some mountain valley towns can be largely attributed to elemental carbon. These particles are emitted directly into the air from virtually all combustion activities. They are especially prevalent in diesel exhaust and smoke from the burning of wood and wastes.

Crustal material, like dust, enters the air from dirt roads, fields, and other open spaces as a result of wind, traffic, and other surface disturbing activities.

To reduce haze we must reduce emissions of haze-forming pollutants across broad areas of the country. Cars, trucks, and industries are much cleaner than they were in the past, and programs are in place to maintain this progress over the next several years. However, these programs are probably insufficient to restore visibility to its natural conditions in many protected areas.

Human Health Effects of Fine Particulates

Many of the same pollutants that impact visibility can also have serious human health effects. Exposure to fine particles in the air increases the chances of respiratory and cardiovascular illness. Even relatively brief exposures to particulate matter may aggravate asthma and bronchitis and cause heartbeat irregularities and heart attacks. Some particulate pollution also has carcinogenic effects to humans.

Atmospheric Deposition and Acidic Rain

Deposition is the amount of pollution that reaches the surface of the earth. It is affected both by the concentration of pollution in the air and the amount of moisture that carries it to the earth. Deposition also occurs without precipitation.

Sulfate is the primary component of acidic rain in the eastern U.S. with the highest levels of emissions coming from the heavily industrialized Ohio River Valley. In spite of recent reductions across the eastern U.S., sulfate deposition is still higher than the ecosystems of the Appalachian states can tolerate. Nitrogen deposition is more of a factor in acidic rain in the mid and western U.S. In addition to contributing to acidic rain, nitrogen can cause other ecosystem impacts by unnaturally fertilizing land and water disrupting the natural flora and fauna.

For more on acid rain, visit www.epa.gov/acidrain.

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