The traditional explanation of the red sky at sunset is that as the sunlight travels a longer path from the horizon to the pier, "most of the blue has been scattered out of that beam" explains Stephen Corfidi, a meteorologist at the National Oceanic & Atmospheric Administration (NOAA). What remains are the warmer hues of yellow, orange and red, which blend into a yellowish-orange sunset.
But how red? "In an atmosphere with no junk at anytime, you'll never get a sunset that would make someone with normal color vision say, 'Wow that's red!'" says Craig Bohren, professor emeritus of meteorology at Pennsylvania State University. "It is certainly true that the 'pollution' results in redder sunsets."
In cities, "you can ignore natural aerosol products for the most part" because the number of aerosols produced by human activity far exceeds natural sources, says Sergey Nizkorodov, a chemist at the University of California, Irvine. Human-generated aerosols can enter the atmosphere directly, as is the case with soot emitted by internal combustion engines in cars and trucks, he explains. Aerosols are also produced when molecules in the gaseous state enter the atmosphere and react with other chemicals, he adds. A classic case: burning fossil fuels releases sulfur dioxide gas into the air, which then turns into sulfuric acid aerosols.
However like everything else, you an have too much of a good thing, and "at some point, the air pollution is so bad, and the sky is so saturated, you don't even see the sun clearly anymore," Nizkorodov says. For example, the sunset can appear bright but washed out when large numbers of big particles accumulate in the troposphere, the layer of the atmosphere closest to the ground. Aerosols that are close in size or larger than the wavelengths of visible light tend to scatter all colors indiscriminately, increasing the overall brightness of the sky but dampening color contrast.
Conclusion: although aerosols may make a sunset red, excess pollution will also dampen the overall sunset experience. ::Scientific American