Hello Matthieu,
quoting from @bhutzell (who is the expert here) in this thread, I believe the calculation equivalent to earlier versions of CMAQ would be
visibility = 10.0 log( (aerosol_extinction_550nm+Rayleigh_Extinction_550nm)/Rayleigh_Extinction_550nm)
where aerosol_extinction_550nm is EXT_AERO_W550 from CCTM_PHOTDIAG3 and Rayleigh_Extinction_550nm is 0.014 1/km
@bhutzell , please chime in if I’m misrepresenting your previous answer.