US Forest Service Causes of Poor Visibility

forest scene

When we visit a wilderness or national park, or look at the skyline of a city, we do not often enjoy a clear vista; a white or brown haze hangs in the air and affects the view. This haze is not natural. It is caused by man-made air pollution, often carried by the wind hundreds of miles from where it originated.

Typical visual range in the eastern U.S. is 15 to 30 miles, or about one-third of what it would be without human-caused air pollution. In the West, the typical visual range is 60 to 90 miles, or about one-half of the visual range under natural conditions. Haze diminishes the natural visual range.

Haze is caused by fine particles that scatter and absorb light before reaching the observer. As 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. The typical importance of each particle type in the eastern and western U.S. is shown in the figure to the right. Details on each particle type are provided below.

Contribution of Various Particulates to Haze

Eastern U.S. Western U.S.

Sulfate Particles form in the air from sulfur dioxide (SO2) 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 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 fires, but these sources have only a small effect on overall visibility.

Nitrate Particles form in the air from nitrogen oxide (NO) gas. This gas is released from virtually all combustion activities, especially those involving cars, trucks, off-road engines (e.g., 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 waste.

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. Whereas other types of particles form from the condensation and growth of microscopic particles and gasses, 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 conditions, where the air is stable or constrained.

Some of the pollutants that form haze have been linked to serious health effects and environmental damage. Exposure to fine particles in the air has been linked to increased respiratory illness, decreased lung function and premature death. In addition, sulfate and nitrate particles contribute to acid rain, which can damage forests, reduce fish populations, and erode buildings, historical monuments and even car paint.

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.

We are challenged to do our part to help reduce air pollution. Click Here to learn more about what you can do to reduce air pollution.