The vaguely defined region of the upper atmosphere in which photochemical reactions take place. It is generally considered to include the stratosphere (or the top thereof) and the mesosphere, and sometimes the lower part of the thermosphere. This entire region is the seat of a number of important photochemical reactions involving atomic oxygen O, molecular oxygen O2, ozone O3, hydroxyl OH, nitrogen N2, sodium Na, and other constituents to a lesser degree. See atmospheric shell.