As I've written about before (in particular here and here), ozone absorption has a big influence on the color of the sky during twilight. In particular, ozone absorption gives the sky its increasingly saturated blue color as the sun sinks below the horizon. You can easily see this effect in real life, especially if you are looking out the window from inside and your eyes are adjusted to the unchanging hue of indoor light. I recently rendered a new pair of images (below) to clearly illustrate the effects of ozone. I hadn't rendered before-and-after ozone pictures since adding aerosols to the atmosphere, so I wanted to create these updated images. The solar elevation angle in these renders is −4°. These are display-linear images—I haven't done anything to increase the saturation or contrast.