dOES SMOG CREATE BEAUTIFUL SUNSETS?
To get a red sky, you need aerosols. Aerosols are solid or liquid particles suspended in the air that originate from both natural processes and human activity. In a large city, you can ignore natural aerosol products for the most part, because the number of aerosols produced by human activity far exceeds natural sources. Human-generated aerosols can enter the atmosphere directly, as is the case with soot emitted by internal combustion engines in cars and trucks. Aerosols are also produced when molecules in the gaseous state enter the atmosphere and react with other chemicals. A classic case: burning fossil fuels releases sulfur dioxide gas into the air, which then turns into sulfuric acid aerosols.
Most particles suspended above cities scatter radiation, preferentially removing the cooler violets and blues in the spectral palette and enhancing the red. In this sense, these particles scatter light much the same as do oxygen and nitrogen molecules. Molecules and small particles scatter the same way as long as the particle is sufficiently small. 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. 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. 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.
Pictures 1.David McNew/Getty Images 2. ISTOCKPHOTO.COM/JEREMY WEE
Most particles suspended above cities scatter radiation, preferentially removing the cooler violets and blues in the spectral palette and enhancing the red. In this sense, these particles scatter light much the same as do oxygen and nitrogen molecules. Molecules and small particles scatter the same way as long as the particle is sufficiently small. 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. 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. 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.
Pictures 1.David McNew/Getty Images 2. ISTOCKPHOTO.COM/JEREMY WEE