Fine Particulates and Visibility

With the Clean Air Act of 1977, Congress established a national goal of remedying existing and preventing future human-caused visibility impairment in most of the large Wilderness Areas, National Parks, and National Wildlife Refuges in the US. The Boundary Waters Canoe Area Wilderness (BWCAW) is an area where visibiilty is protected. 

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, 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.

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.

For the BWCAW the following chart shows how the contribution of the five types of particles vary through the year.  It can be seen that sulfates are the major cause of visibility impairment while nitrates and organic carbon are minor contributors. 




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.

Nitrate particles form in the air from emissions gases of nitrogen oxides. These gases are released from virtually all combustion activities including, power plants, industrial boilers turbines and vehicles. Like sulfates, nitrates scatter more light in humid environments.

Organic carbon particles are emitted into the air and also form there as a reaction of various gaseous hydrocarbons. Major human sources of organic carbon particles include vehicle exhaust, vehicle refueling and solvent evaporation. Natural hydrocarbon emissions from forests and wildland fire smoke are additional sources. The majority of organic carbon fine particulates at the BWCAW have been found to be natural in origin.

For more general information about visibility and causes of impairment, visit

What can you do?

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, society is demanding more power and driving their vehicles more miles which tends to offset this progress. Projections are showing that existing air quality regulations by themselves are probably insufficient to make steady progress toward natural conditions in the BWCAW.

The following graph is a prediction of how much the visibility on the worst days (blue line) is predicted to improve in the BWCAW over the next ten years compared to a line that shows a constant rate of improvement toward the goal of natural visibility.


The Superior National Forest is working closely with Minnesota and each of the neighboring states as they develop plans to reduce regional haze. For more information on Minnesota’s plan click here.

Current and historical visibility information for the BWCAW or any other Class I area can be found here.

The current visibility conditions at the BWCAW and a number of other Class I areas are documented with digital cameras.  They can be seen here.

Plume Blight

Another type of visibilty impairment is called plume blight.  Plume blight occurs when an emission source such as a smoke stack emits particulate matter or nitrogen dioxide into a stable atmosphere. These pollutants can form a thin, dark, coherent plume obscuring the view. This picture captures a classic example of plume blight. Blight happens before the plume has been dispersed so widely that it is indistinct from the background. Both contrast and coloration may vary depending upon the plume constituents, the viewing background, the viewer angle, and the angle of the sun. It is the goal of the Clean Air Act to prevent this type of impact to the BWCAW.

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. For more information on the health effects of particulates and current air quality conditions visit.