Albedo attenuation factor
The albedo is only visible from the collectors if no close obstacle is present till the level of the ground. This is why we have to integrate the shading factor at zero height, on the virtual portion of the sphere under the horizon, included between the horizontal plane and the plane of the collectors. As for diffuse, this factor is independent of the sun's position, and therefore constant over the year.
It is however to be remembered that for non-vertical planes, the energetic contribution of the albedo is weak in the global incident energy, and that errors in its estimation will therefore only have secondary repercussions.