Causes of Poor Visibility
When we visit a national park or look at the skyline of a city, often we do not
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
manmade 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 it reaches the observer. As the number of fine particles increases,
more light is absorbed and scattered, resulting in less clarity, color, and visual
For a detailed treatise on visibility, visit http://www.epa.gov/air/visibility/.
Contribution of Various Particulates to Haze
Winston-Salem, North Carolina Speciated PM2.5 at site 37-067-0022
Fine particle concentrations are highest in Forsyth County during spring and summer,
when warm temperatures aid photochemistry and westerly trajectories bring in polluted
air masses. Speciated PM2.5 fractions measured in Winston-Salem between September
2001 and December 2002 are detailed above.
Following information courtesy of
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
are very efficient at scattering light, thereby exacerbating the problem in the
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 (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 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. 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.
Health Effects of Particulate
Some of the pollutants that form haze have been linked to serious health effects
and environmental damage. Exposure to fine particles in the air have been linked
with 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. New studies are being published that update our knowledge about
particulate related health problems. For the latest information, check with the
Health Effects Institute
or conduct a web search on keywords like PM2.5 and health.
What You Can 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 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 the Southeast
U.S., visibility issues are being handled by VISTAS.
We are challenged to do our part to help reduce air pollution. To learn more about
what you can do to reduce air pollution, click on http://www.epa.gov/air/actions.