Visibility


				Air ~ David C. Richie Visibility



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Visitors rank “breathing fresh, clean air” a principle reason for visiting the national forests. Poor air quality and impaired visibility are an economic impediment to local communities who rely upon tourism.

In 1977, Congress established a national goal of remedying any existing and preventing any future human-caused visibility impairment in our largest wilderness areas, national parks, and national wildlife refuges. Air pollution impairs visibility to some degree on all federal lands. The visual range within the eastern U.S. is often just 15 to 30 miles, or about 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.

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

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.

Contribution of Various Particulates to Haze

click graphic to enlarge

Eastern US

Western US

Sulfate Particles form in the air from sulfur dioxide gas. Most of this gas is released from coal-burning power plants and other industrial sources, such as smelters, industrial boilers, and oil refineries. Sulfates are the largest contributor to haze in the eastern U.S., due to the region's large number of coal-fired power plants. In humid environments, sulfate particles grow rapidly to a size that is very efficient at scattering light, thereby exacerbating the problem in the East.

Organic Carbon Particles are emitted directly into the air and also form there as a reaction of various gaseous hydrocarbons. Sources of direct and indirect organic carbon particles include vehicle exhaust, vehicle refueling, solvent evaporation (e.g., paints), food cooking, and various commercial and industrial sources. Gaseous hydrocarbons are also emitted naturally from trees and from fires, but these sources have only a small effect on overall visibility.

Nitrate Particles form in the air from nitrogen oxide gas. This gas is released from virtually all combustion activities, especially those involving cars, trucks, off-road engines (like construction equipment, lawn mowers, and boats), power plants, and other industrial sources. Like sulfates, nitrates scatter more light in humid environments.

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

Crustal Material is very similar to dust. It enters the air from dirt roads, fields, and other open spaces as a result of wind, traffic, and other surface activities, like agricultural work. Whereas other types of particles form from the condensation and growth of microscopic particles and gases, crustal material results from the crushing and grinding of larger, earth-born material. Because it is difficult to reduce this material to microscopic sizes, crustal material tends to be larger than other particles and tends to fall from the air sooner, contributing less to the overall effect of haze.

Haze generally appears either as uniform haze, layered haze, or plumes.

A uniform haze degrades visibility evenly across the horizon and from the ground to a height well above the highest features of the landscape. Uniform haze often travels long distances and covers large geographic areas, in which case it is called a regional haze.

In a layered haze, you can see the top edge of the pollution layer. This is often the case when pollution is trapped near the ground beneath a temperature inversion.

Plumes result from local sources. Plumes and plume-like layers of elevated pollution take their shape under certain meteorological condition where the air is stable or constrained.

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 several programs are in place to maintain this progress over the next several years. Nonetheless, these programs by themselves are unlikely to restore visibility to its natural conditions in many protected areas.

In April 1999 the U.S. Environmental Protection Agency (EPA) issued regulations to further reduce haze and protect visibility across the country. The EPA and federal land managers from other agencies are working with state, local and tribal authorities to promote steady improvements in visibility for decades to come. In light of current population and economic growth trends, restoring and maintaining good air quality will continue to be a challenge.
Poor visibility from air pollution is an indicator of other effects that can injure trees and plants, acidify streams and lakes, leach nutrients from soils, and damage cultural resources. And some of the same pollutants that form haze may cause serious human health effects. Exposure to fine particles in the air increases the chances of respiratory illness, lung dysfunction, and premature death.

WELCOME TO THE USDA FOREST SERVICE
AIR QUALITY IMAGE WEB SITE

This web site provides public access to real-time air quality information collected by the Forest Service. These data are used to monitor air quality conditions in and near Forest Service managed lands with the goal of protecting, and where necessary, working to improve visibility and other resources affected by air pollution.

Human Health Effects of Fine Particulates

Particulate matter is associated with respiratory, cardiovascular, and carcinogenic pathologies. Even relatively brief exposures to particulate matter may aggravate asthma and bronchitis and can cause heartbeat irregularities and heart attacks. Air quality in some Forest Service areas may exceed the standard for fine particulate matter (PM2.5)—those particles less than or equal to 2.5 micrometers (µm). Elevated levels of PM2.5 may also emanate from forest fire smoke.

In July 1997, the EPA established a new federal public health standard for PM2.5. The new standard is based on a 24-hour and annual concentration. The 24-hour standard is used as a basis for health advisories in the parks. The 24-hour standard is set at 40 micrograms per cubic meter parts (µg/m3). The EPA has also proposed rules for air quality index reporting (Federal Register Notice Volume 63, Number 236, 12/9/98).

JAMA March 6, 2002 - Lung Cancer, Cardiopulmonary Mortality, and Long Term Exposure to Fine Particulate Air Pollution (PDF 157 kb)

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