Atmospheric gas composed of 3 oxygen atoms (O3). There are two « ozones » in Earth’s atmosphere. In the upper atmosphere (stratospheric), naturally occurring O3 serves as a buffer from some of the sun’s ultraviolet (UV) rays. At ground-level (tropospheric), this man-made « ozone » is caused by chemical reactions caused by polluting emissions from industrial activity such as cars, powerplants, refineries, etc.