Why would you want to calculate visual range? To determine how much smog has affected visibility, of course!
First, calculate the light extinction coefficient:
bext = bsg + bag + bsp + bap
Where,
bsg = light scattering of atmospheric gases (Rayleigh scattering)
bag = light absorption by gases (NO2 in urban atmosphere)
bsp = light scattering by particles
bap = light absorption by particles
Then�
Using Koshmieder�s equation, estimate the visual range, (Lv):
Lv = (-ln 0.02)/bext.
*Note: 0.02 represents the minimum contrast threshold for detection by a typical human observer that an object is just barely visible for the case of a large black object silhouetted against the horizon sky.
(Visual range does not convey info about coloration and light intensity of sky and nearby objects).
Each component of the light extinction coefficient can by calculated theoretically when there is information on the concentration of nitrogen dioxide, airborne particle size, and the chemical composition distribution.
Atmospheric gas light scattering is a function of the wavelength of light and atmospheric pressure.
Particle light scattering is found through a Mie scattering calculation where the refractive index and diameter of the particles must be known. This is the simplest type of scattering, since one must assume that the particles are spherical. To find out more about a Mie scattering calculation, click here.
Particle light absorption is a function of the amount of elemental carbon and mineral matter present in the particles.