Scientific Research
You might have heard that dirt causes a good sunset but that’s not entirely true.  Pollution does play a role in sunsets though; pollution softens the sky’s colors.  This is because the particles of dust and dirt vary from different sizes so the wavelengths of sunlight pass through and the particles are mixed.  Although pollution helps make a good sunset, clean air is the best.  Another aspect that makes a good sunset is when there are at least a few high clouds.  The higher the clouds are, the more of the sun’s rays will pass through before the lower atmosphere.  A sunset ranges from violet and blue at one end and red and orange at the other.

Sources
Google.com
http://www.rps.psu.edu/probing/sun.html
http://phoenix.about.com/od/weather/a/sunset.htm