Earlier versions of CMAQ output visibility estimated near 550 nm using the below formula.
visibility = 10.0 log( (aerosol_extinction_550nm+Rayleigh_Extinction_550nm)/Rayleigh_Extinction_550nm)
The aerosol extinction was estimated two ways. One used a volume mixing model and an approximation of Mie Scattering Theory. The other used the IMPROVE reconstruction method. In current version of CMAQ, the In-line calculation of photolysis gives diagnostic outputs for the first estimate of aerosol extinction. In the PHOTDIAG3 file, the variable to used is EXT_AERO_W550 which has units of 1/km. For Rayleigh_Extinction_550nm, the visibility calculation should use 0.014 1/km. The number corresponds to what the In-line calculation computes for Rayleigh Scattering Extinction at 550nm.
Note that (aerosol_extinction_550nm+Rayleigh_Extinction_550nm) roughly corresponds to the total extinction. The PHOTDIAG3 file also contains the total extinction at several wavelengths but not at 550 nm. The value includes extinction from aerosols, ozone, nitrogen dioxide, Rayleigh Scattering and clouds. If the user wishes to use the PHOTDIAG3 values for total extinction, they have to interpolate EXT_W607 and EXT_W381 values for the total extinction at 550nm by using the Angstrom Exponents.