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.

How is Visibility Impairment Measured?

Total light extinction consists of the sum of light scattering and light absorption components:

These components are added together to determine the total light extinction (bext) due to particles/gases.

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 and absorbed 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 from extinction data as follows:

Deciview (dv) - The deciview is a visual index designed to be linear with respect to perceived visual air quality changes over its entire range in 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 person. This is approximately a 10% change in the light extinction (Mm^-1) reading. Deciviews can be calculated from extinction data as follows: