Educational Material
What is Visibility Impairment?
“
Visibility” is a measure of how the air looks. It is usually
described as the maximum distance that a dark object can be perceived
against the
background sky. Visibility can also refer to the clarity of objects in
the distance, middle, or foreground. Visibility is unique among air pollution
effects because it involves human perception and judgment. The typical
visual range in the western U.S. is 60 to 90 miles.
Reduced visibility
is an effect of air pollution, but cannot be directly measured as an
air quality
health standard like carbon monoxide or ozone. “Visibility
impairment” is
defined by the Grand
Canyon Visibility Transport Commission
as: “The loss of clarity in the air that results when gases or
aerosols scatter and absorb light.” Aerosols include liquid droplets
and very fine solid particles dispersed in a gas. These aerosols either
scatter or absorb light coming from an object before it reaches an observer’s
eyes. As the amount and type of aerosols increases, more light is absorbed
and/or scattered, resulting is less clarity, color, and visual
range.
Loss of clarity in the air can be described through a parameter known
as “light
extinction.”
Sulfates, nitrates, and elemental
and organic carbon are most effective at scattering or absorbing light. Human-caused sources
of these particles
include wood burning, emissions from automobiles, boats, airplanes, and
locomotives, soot from burning fields, and electric power generation.
Tiny gas and/or liquid droplets that are formed by chemical reactions
between
sulfate or nitrate and ammonia also degrade visibility. Nitrogen
dioxide and sulfur dioxide gases from burning of fossil fuels also contribute
to the brown cloud. Nitrogen dioxide gas is brown, giving that color
to the
haze. Chemical reactions in the atmosphere convert these gases to fine
particles.