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, please click on:
http://www.epa.gov/air/visibility/introvis.pdf (requires Adobe Reader)
Contribution of Various
Particulates to Haze
Fine particle concentrations are
highest in Berks County during spring and summer, when warm
temperatures aid photochemistry and westerly trajectories bring in
polluted air masses.
Following information courtesy
of MidWest Hazecam:
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.
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
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
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