"Molecules and small particles scatter the same way as long as the particle is sufficiently small," Bohren says. If the particle is small compared with the wavelengths of visible light, it will scatter short wavelengths, such as blues and violets, more than long wavelengths, such as red. Many man-made aerosols are small enough to meet this criterion, so they contribute to the deep crimson sunsets of Los Angeles and other polluted cities across the globe.
However, "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.
"Particles of any kind, even much smaller than the wavelength of visible light, will, as a rule, make the sky brighter but at the expense of its purity of color," Bohren says, noting that the effect is more pronounced when there is a high concentration of large aerosols. So, although aerosols may make a sunset red, excess pollution will also dampen the overall sunset experience. In fact, the transition from day to night might be a whole lot peachier—and healthier—without all that atmospheric flotsam.