Visibility (aka background visual range) varies widely geographically. You might look at the USEPA "Workbook for Estimating Visibility Impairment," EPA-450/4-80-031. It's aging at this point, but the physics haven't changed. Much of it is oriented to estimating plume perceptibility impacts, but there's a pretty extensive treatment of light scattering and absorption, including the appendix on regional haze.