Visibility has traditionally been calculated using the Koschmieder formula that relates the distance at which a black object can just be distinguished against the horizon to the extinction coefficient of the assumed homogeneous air path between the object and observer. The light extinction coefficient is the sum of the medium's absorption and scattering coefficients. The components of light extinction include light scattering by air molecules (Rayleigh scattering) and by particles (Mie scattering), and light absorption by gases and by particles. Scattering by particles is the dominant component of light extinction. The authors demonstrate the use of accurate and approximate methods for calculating light extinction to estimate regional visibility. Concentrations of sulfates, which are known to be the major contributors to visibility degradation in the eastern United States, are simulated using the Regional Particulate Model (RPM).