Can smog enhance the beauty of sunsets?

Air pollution offers few benefits. It harms health, mars landscapes, and damages the planet we rely on. But could there be a silver lining? Could the sky-obscuring effect of smog actually make sunsets more vibrant?

Sunset Image Gallery

Many believe that smog-filled cities like Los Angeles and Beijing, with their perpetually hazy skies, enjoy more vivid sunsets as a result. However, this belief is largely incorrect. The phenomenon begins with the science behind sunsets — how the colors spread across the sky as the sun sets.

When the sun is positioned high above, its light traverses a relatively short distance through the atmosphere before reaching your eyes, the point where the light is observed. This sunlight contains all wavelengths of visible light, each perceived as a distinct color. The molecules in Earth's atmosphere, primarily nitrogen and oxygen, scatter certain wavelengths while leaving others unaffected. Since these molecules are tiny compared to the wavelengths of visible light, they only scatter the shorter wavelengths, dispersing them in various directions away from the direct path to your eye. This selective scattering of wavelengths, or colors, is known as Rayleigh scattering. During the day, the sky appears blue because violet and blue wavelengths, the shortest in the spectrum, are scattered more than other colors. As these wavelengths spread across the sky, it takes on a blue hue. (The sky would actually appear violet, but the human eye is more sensitive to blue [source: NOAA].)

As the sun progresses across the sky, this phenomenon shifts.

Since the Earth is spherical, the distance between the sun and the ground varies as the sun moves. At noon, this distance is minimal; at sunset, when the sun is on the horizon, it becomes the longest. With a greater distance, sunlight must pass through more atmosphere before reaching your eyes. Rayleigh scattering still occurs, but it yields a completely different outcome.

Sunset and Smog

Recall how atmospheric molecules scatter blue and violet light more than the longer wavelengths like yellow, orange, and red? This Rayleigh scattering effect transforms the sky from blue to hues of red, orange, and yellow as the sun moves farther away, increasing the distance its light must travel through the atmosphere.

As sunlight passes through the atmosphere and interacts with these molecules, more of the shorter wavelengths are scattered away. By the time the light reaches your eyes, the blue and violet wavelengths have been entirely scattered, leaving only the longer wavelengths visible in the sky. This is why a setting sun paints the sky in shades of red, orange, yellow, and everything in between. Meanwhile, the scattered blue and violet light creates a blue sky in another part of the world, in a different time zone.

This phenomenon occurs in a clear sky, where nitrogen and oxygen molecules are the primary scatterers. However, introducing other molecules, such as those found in smog, alters the entire scenario.

Technically, smog isn't as sinister as it's often portrayed. The term 'smog' is a blend of 'smoke' and 'fog,' and it originates from various sources, some entirely natural. It primarily consists of aerosols, which are solid or liquid particles suspended in the air. For instance, volcanic eruptions generate dense smog by releasing massive amounts of ash into the atmosphere. Similarly, forest fires produce thick smoke that rises, turning the sky almost opaque.

However, in urban areas, natural causes of smog are insignificant compared to human activities. Emissions from factories, coal-powered plants, vehicles, and gas leaks, along with the byproducts of burning plastics and aerosol sprays, release far more particles into the air than natural events. This type of smog is a toxic blend of chlorofluorocarbons (greenhouse gases), hydrocarbons, sulfuric acid, and numerous other harmful pollutants [source: Health and Energy]. In large cities, smog is predominantly a human-made cocktail of chemicals.

This is where the misunderstanding arises. While smog from volcanic eruptions and forest fires consists of relatively uniform aerosols, human-made smog contains a wide variety of molecules and materials, with aerosol particles differing significantly in size. The size of these particles determines whether they amplify or diminish the colors of a sunset.

When events like volcanic eruptions release particles into the atmosphere, the resulting smog blocks specific wavelengths of light. As sunlight travels through this polluted air, it interacts with nitrogen and oxygen, which scatter the blue end of the spectrum, and volcanic ash, which is larger and also scatters a significant portion of the longer yellow wavelengths. The outcome is a strikingly red sunset that persists until the ash dissipates.

However, smog from sources like smokestacks or vehicles contains particles of varying sizes, leading to nearly indiscriminate scattering. Rayleigh scattering no longer applies, as the atmosphere scatters all wavelengths of light rather than selectively. The aerosols in this type of smog are diverse and relatively large, scattering every wavelength of light. Consequently, the sunset loses its vibrant colors, appearing instead as a dull, grayish-white haze, with the yellow, orange, and red hues scattered away before they can reach the observer.

If you happen to witness a particularly vivid sunset in Los Angeles, it's likely due to low smog levels that day, not because the smog is dense.