INSTRUMENTATION
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 absorption over 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 light to calculate the transmission of the intervening atmosphere. The measured transmission can be related to the light lost along the path due to scattering and absorption.
Nephelometer
	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 (24 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.

Aerosol: A dispersion of microscopic solid or liquid particles in a gas, such as smoke or fog.

Carbon (elemental or organic): A naturally occurring abundant nonmetallic element that occurs in many inorganic and all organic and compounds.

Deciview (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 the atmosphere.

Inverse megameter (Mm-1): The amount of light lost as it travels over one million meters. This unit is most useful for relating visibility directly 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.

Nitrates: The group of aerosols that originate as nitrogen dioxide gas and are converted to aerosols in the atmosphere.

Nitrogen dioxide: Gas consisting of one nitrogen atom and two oxygen atoms. Nitrogen dioxide absorbs blue light and therefore appears reddish or brown to the human eye. Nitrogen dioxide gives the Phoenix Brown Cloud its color. Most emissions come from combustion of fuels in mobile and stationary sources.

Particulate matter: Solid or liquid material in the atmosphere that includes wind blown dust and soot from combustion sources. Very fine particulates, 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 size of such particles, light is generally scattered in equal amounts in front of and behind the particle. This type of scattering is also called atmospheric scattering because it is natural scattering not related to air pollution.

Scattering (of light): An interaction of a light wave with an object that causes the light to be redirected.

Sulfates: The groups of aerosols that originate as sulfur dioxide gas and are converted to aerosols in the atmosphere.

Sulfur dioxide: Gas consisting of one sulfur atom and two oxygen atoms. Sulfur dioxide is important because it converts to an aerosol that is particularly efficient at scattering light.

Visibility: A measure of how far and how well an observer can see through the atmosphere.

Visual Range: The distance at which a large black object just disappears from view.