When the sun is low on the horizon, as it is at sunrise or sunset, its light must pass through many miles of atmosphere before reaching your eyes. As I explained in my discussion of the blue sky, tiny particles in the atmosphere deflect blue light much more efficiently than they deflect red light. Therefore, long-wavelength light such as reds and oranges makes it through the atmosphere pretty well and you see this light still heading toward you from the solar disk. But in its long passage to your eyes, the short-wavelength blue light is just too likely to bounce off air particles and little of it makes its way to your eyes without being redirected elsewhere. When you look at the sun as it sits just above the horizon, all you see is light that hasn't bounced on its way to your eyes and that light is overwhelmingly reddish in color. The missing blue light is making the sky look blue somewhere far to your east at sunrise or to your west at sunset. And after a volcanic eruption, when there are many tiny dust particles in the atmosphere, this filtering effect is particularly strong and you see the most beautifully colored sunrises and sunsets.
Answered by Louis A. Bloomfield of the University of Virginia