Definition
editOpacity is the amount of light obscured by particulate pollution in the air (atmosphere). Opacity in this definition will be related to visibility degradation that is caused by the scattering and absorption of light with particles and gases in the atmosphere. The reduction visibility through the atmosphere is mostly caused by air pollution. Visibility in the atmosphere can be described as visual range. Visual range is the distance where a large black object disappears from view. Opacity can be used as an indicator of changes in performance of particulate control systems.
Formula
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T = fractional transmittance of light across the duct
The equation above is derived from Beer-Lambert Law
Those equations show relation between transmission (T) and the product of absorption coefficient of the substance (α), and the distance of light travel through the dimension (path length, ℓ). The product of absorption coefficient can also be written as product of molar absorptivity of the absorber (ε) and concentration (c) of absorbing species in material, or absorption cross section (σ) and the density (N).
Fact
editMinnesota Pollution Control Agency has stated emission standard for visible air contaminant. Any emission facility cannot discharge any gases into atmosphere more than 20%. There are two ways to measure opacity:1.) by instruments and 2.)by human observers. EPA requires some companies to install instruments in their smokestacks to measure opacity. Or, usually opacity can be determined through visual observation by people who have been trained and certified as Visible Emission Observers. They have been trained on reading opacity.