As the sun gets lower in the sky it must pass through more and more of the earths atmosphere with its particulates, thus the lower the sun is, the more it is colored by particulates, giving us the red light of some of sunsets. Adjusting the color of light is fairly easy if it is unidirectional, but reflected sources of different colors can be difficult to match. Reflected Light of various colors, as the Impressionists have shown us, is an important part of the coloration of the objects we see. Rough surfaces with many planes have more varieties of light. When you are matching similar surfaces, both need a similar mottling of colors, and to achieve this you can use the clone tool at an appropriate setting to borrow from one for the other.
Fog is perhaps the best know atmospheric, but all air contains some amount of particulate matter.
In clear air, distant scenery will take on a bluish tint. Smog will turn scenery more and more yellowish the farther away it is. A figure photographed on a clear day cannot be composited into a scene photographed on a hazy day without making adjustments to the figure by desaturating the color and blurring the image appropriate to how near the figure is to the viewer in the scene. It is rarely as successful to intensify the colors and sharpen a picture taken in haze so it will match an image taken in clear light. Atmospherics will be discussed again under Aerial Perspective.