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
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.
is an effect of air pollution, but cannot be directly measured as an
health standard like carbon monoxide or ozone. “Visibility
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
Loss of clarity in the air can be described through a parameter known
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
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
haze. Chemical reactions in the atmosphere convert these gases to fine
How is Visibility Impairment Measured?
Total light extinction
consists of the sum of light scattering and light absorption components:
Light scattering by particles (bsp) - Numerous
field studies of urban visibility, including the studies in the
Phoenix and Tucson areas, have shown that particles less than or
2.5 microns in aerodynamic diameter (PM2.5) cause the vast majority
of light scattering.
Atmospheric, or Rayleigh scattering (bRayleigh) is natural scattering
not related to air pollution.
Light absorption (babs) is due to particles and gases, and is determined
by analysis of elemental carbon.
These components are added together to
determine the total light extinction (bext) due to particles/gases.
bext = bsp +
bRayleigh + babs
Light extinction can be measured with a transmissometer.
A nephelometer measures
only the light scattering component of light extinction.
Light extinction data
can be reported three ways:
Inverse megameter (Mm-1) -
Inverse megameter is the direct measurement unit for visibility impairment
data. It is
the amount of light scattered
as it travels over a distance of one million meters.
Visual range (VR) - Visual range is an expression of visibility impairment defined as
the distance in miles or kilometers at which a large, black
object just disappears
from view. Visual range values are calculated from direct measurement
data, or are estimated directly by observers. Visual range can be calculated
data as follows:
Visual Range (km) = 3912 / bext(Mm-1)
Deciview (dv) - The deciview is
a visual index designed to be linear with respect to perceived visual
air quality changes over its entire
a way that
is analogous to the decibel index for sound. The
deciview scale is zero for pristine
conditions and increases as visibility degrades. Each deciview
change represents a perceptible change in visual air quality to the average
is approximately a 10% change in the light extinction (Mm-1) reading.
Deciviews can be calculated
from extinction data as follows:
Deciview (dv) = 10 x ln (bext(Mm-1)/10)