When the sun shines on our planet, it first hits the molecular oxygen and nitrogen molecules in our atmosphere. These molecules, which are so lovely to breathe, are small compared to the wavelengths of light that we see emitted by the sun. So when sunlight reaches our atmosphere, only the smaller light waves are scattered. This phenomenon is what we see as blue and purple; it is why we perceive the sky as blue and the sun as yellow.
In reality, the sun is white and emits all wavelengths in the visible spectrum. But when the molecules in our atmosphere scatter the blue out of the sunlight, the remaining wavelengths of light are all but blue, appearing to us as yellow. This exact phenomenon occurs when the sun sets, and is why the colors appear in such striking opposition.
When the sun is near the horizon, as it is when it is setting, its light is more scattered by the atmosphere because it passes through the atmosphere sideways. So for us in Colorado Springs, when the sun is setting, the atmosphere from California and even further into the Pacific Ocean is in between us and the sun’s rays. This means that an extremely large amount of blue is leeched from the sun’s white light, leaving a deep red result; red is the longest wavelength in the visible spectrum, and therefore, it is the least likely to be scattered by the atmosphere.
So, how do these phenomena interact with pollution in our atmosphere? The answer depends on the chemical makeup of the pollution. In general, however, pollution consists in larger particulate matter, like smoke. Smoke in the atmosphere is generally gray or white because it reflects all light in the visible spectrum at relatively the same amount. Therefore, pollution generally mutes the colors of a sunset because it does not add to the color separation that occurs with sunlight in the atmosphere. This is why sunsets seen from a window of a plane and after a storm are more vivid — there is less pollution above the clouds and storms wash out larger particles of pollution from the air.
Beautiful sunsets should thus be added to the list of things we need to preserve in our continuous battle against climate change.