The spraying is not being done by commercial aircraft. The Military uses small planes that fly at high altitudes and release the chemical spray, then the commercial jets with their jet engines fly through the invisible spray, and when the engines mix the air and the chemicals up, like a blender they leave massive "chemtrails" that are visible. It is pretty ingenious because you only need a few small planes and then you rely on the normal commercial traffic patters to spread the chemicals for you unaware. See this film:

My video of this taking place during the Boston Marathon weekend, breathe up runners of the world: