It is often written that natural and manmade dust and pollution cause colorful sunrises and sunsets. Indeed, the brilliant twilight `afterglows` that follow major volcanic eruptions owe their existence to the ejection of small particles high into the atmosphere more will be said on this a bit later. If, however, it were strictly true that low-level dust and haze were responsible for brilliant sunsets, cities such as New York, Los Angeles, London, and Mexico City would be celebrated for their twilight hues. The truth is is that tropospheric aerosols --- when present in abundance in the lower atmosphere as they often are over urban and continental areas --- do not enhance sky colors --- they subdue them. Clean air is, in fact, the main ingredient common to brightly colored sunrises and sunsets.
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