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
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
clarity, color, and visual range.
For a detailed treatise on visibility, please click on: http://www.epa.gov/air/visibility/
Contribution of Various Particulates to Haze
Winston-Salem, North Carolina Speciated PM2.5 at
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 MidWest Hazecam:
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 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 (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
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
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
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.