Transmissometer Transmitter Station
The transmissometer is used to assess
visibility impairment by measuring the amount of light lost over
a known distance. The instrument consists of a light source (transmitter)
and light detector (receiver) which are generally located on an
elevated surface, such as a tall building, and are separated by
a distance of 1-3 miles.
The transmitter emits a uniform light
beam of constant intensity at regular intervals for a programmed
duration. The light beam is
carefully aimed at the detector. The amount of light transmitted
and received is precisely measured. The loss of light (or light
extinction) is measured as the sum of light scattering and light
the transmissometer path length.
Transmissometer Receiver Station
The receiver includes a telescope that gathers
the transmitter light, which includes both background illumination
and the transmitter
signal, and converts it to an electrical signal. The receiver computer “locks-on” to
the transmitter light’s frequency and separates the
transmitter light from ambient (background) lighting. The computer
compares the measured transmitter light with the known transmitter
the transmission of the intervening atmosphere. The measured transmission
can be related to the light lost along the path due to scattering
The Optec NGN-2 integrating nephelometer is used to assess visibility
impairment by estimating the particle scattering coefficient (bsp)
at a point location. The nephelometer provides a direct measurement
of the light scattered by aerosols and gases in a sampled air volume.
Scattered radiation from an illumination source is integrated over
a large range of scattering angles, in a defined band of visible
wavelengths. Because the total light scattered out of a path is the
same as the reduction of light along a path due to scattering, the
integrating nephelometer gives a direct estimate of bsp.
High Resolution Digital Camera
Photographic documentation is an important aspect of evaluating
visibility. Photography is an effective way to document events
and trends on a media that is easily interpreted. Digital images
are readily available for viewing on a computer, can be conveniently
distributed via the Internet, and can be easily stored, managed,
and duplicated without degradation.
The digital cameras used in this website are capable of capturing,
storing, and transmitting high-resolution digital images (up to
1792 x 1200, 24-bit true color) from monitoring sites in the Phoenix
area. Each system consists of a high-resolution digital camera
housed in a weatherproof, temperature controlled environmental
enclosure, and a supporting image capture computer, powered by
a low voltage
volt) power supply.
Digital images are captured every 15 minutes, stored on the system’s
internal hard drive, and uploaded by telephone to the Web site.
GLOSSARY / TERMS
Absorption (of Light): A
process by which light is taken up by another material.
Area A: In
accordance with Arizona Revised Statutes (ARS)
§49-541 the part of the greater Phoenix metropolitan
area where specific pollution control programs are in place for ozone,
carbon monoxide, and particulate matter. Area A includes Maricopa County
and parts of Yavapai and Pinal Counties.
dispersion of microscopic solid or liquid particles in a gas, such as
smoke or fog.
or organic): A
naturally occurring abundant nonmetallic element that occurs in many
inorganic and all organic and compounds.
(dv): A measurement scale representing perceptible
changes in visibility calculated from light extinction measurements.
Extinction (of light): The
loss of light due to scattering and absorption as it passes through
Inverse megameter (Mm-1): The
amount of light lost as it travels over one million meters. This unit
is most useful for relating
to particle concentrations in the air.
Inversion: An anomaly in the normal change of temperature with increasing
altitude. This usually refers to a thermal inversion, in which temperature
of the atmosphere increases rather than decreases with height.
group of aerosols that originate as nitrogen dioxide gas and are converted
in the atmosphere.
Nitrogen dioxide: Gas
consisting of one nitrogen atom and two oxygen atoms. Nitrogen dioxide
absorbs blue light
to the human eye. Nitrogen dioxide gives the Phoenix Brown Cloud its
color. Most emissions come from combustion of fuels in mobile and stationary
Particulate matter: Solid
or liquid material in the atmosphere that includes wind blown dust
and soot from combustion sources. Very
those that are under 2.5 microns in size (PM2.5), are the most
effective for causing visibility impairment.
Rayleigh scattering: The
scattering of particles much smaller than the wavelength of light,
such as air
molecules. Because of the small
of such particles, light is generally scattered in equal amounts
in front of and behind the particle. This type of scattering is
scattering because it is natural scattering not related to air
(of light): An interaction of a light wave
with an object that causes the light to be redirected.
groups of aerosols that originate as sulfur dioxide gas and
are converted to aerosols in the atmosphere.
dioxide: Gas consisting of one sulfur atom
and two oxygen atoms. Sulfur dioxide is important because
is particularly efficient at scattering light.
measure of how far and how well an observer can see through
Range: The distance at which a large
black object just disappears from view.