Note: 3 converters are given below, dependent on input (VR, bext, and DV).

Caution: The converters cannot be used for average values.  E.g. you cannot calculate average DV from average b_ext; instead, you must calculate DV for each daily measurement, and then calculate an average DV from the individual DV values.

Visual Range (V.R.):The greatest distance at which an observer can just see a black object viewed against the horizon sky.

Units:  Distance, e.g. kilometers (km)

Benefit:  This metric is most useful in situations where only the distance that one can see is important, such as for aviation.

Drawback:  Does not account for changes in scene, such as loss of textures and discoloration as haze increases.

Relationships:

V.R. = K/b_ext

Where K is the Koschmieder Coefficient – the natural log of the contrast threshold of the human eye,  K = 3 – 3.9.

Note that a unit conversion is required in converting between V.R. and b_ext.

Light Extinction (b_ext): The attenuation of light due to scattering and absorption as it passes through a medium.

Units: inverse distance, e.g. inverse mega meters (Mm^-1)

Benefit:  Light extinction can be directly related to gaseous and aerosol concentrations.

Drawback:  Light extinction is non-linearly related to a person’s perception of changes in haze.  For example, a 10 Mm^-1 increase in b_ext will have a larger perceived impact on a scene at b_ext = 20 Mm^-1 than at b_ext = 100 Mm^-1

Relationships:

b_ext = K/V.R.

b_ext = 10*exp(DV/10); where b_ext is calculated in Mm^-1.

Where K is the Koschmieder Coefficient – the natural log of the contrast threshold of the human eye,  K = 3 – 3.9.

Note that a unit conversion is required in converting between V.R. and b_ext.

Deciview (DV): A metric of haze proportional to the logarithm of the atmospheric extinction (b_ext).

Units: Unitless

Benefit: Under many circumstances a change in one deciview will be perceived to be the same on clear and hazy days.

Drawback:  Deciview is not easily related to gaseous and aerosol concentrations

Relationships:

DV = 10*ln(b_ext/10);   where b_ext is expressed in Mm^-1.

Perceived Visual Air Quality (PVAQ): An index that relates directly to how human observers perceive changes in visual air quality.

Units:  Unitless

Benefit: Quantitatively, measures haze or visibility impairment to how human observers perceive changes in visual air quality.