Yay for pollution! According to this Scientific American article, airborne particle pollutants are responsible for wonderful sunsets:
To get a red sky, you need aerosols, explains A. R. Ravishankara, director of chemical sciences at the NOAA Earth System Research Laboratory in Boulder, Colo. Aerosols are solid or liquid particles suspended in the air that originate from both natural processes and human activity.
Natural aerosols come from forest fires, mineral dust kicked up by sandstorms, sea spray and volcanic eruptions, among other things. Volcanoes, which have produced some of the most spectacular sunsets in history, can inject sulfuric acid droplets into the stratosphere, the layer of the atmosphere between 10 to 35 miles in altitude. These droplets can be swept across the globe, painting brilliant crimson twilights wherever they go. Following the 1883 eruption of Indonesia's Krakatoa, brilliant sunsets appeared around the world, one of which is said to have inspired Norwegian artist Edvard Munch's painting, The Scream.
Link (Image Credit: Eduardo Amorim [Flickr]) - Thanks Scott!