part of has increased levels of ozone stratosphere ozone layer "The ozone layer contains less than 10 parts per million of ozone, while the average ozone concentration in Earth's atmosphere as a whole is only about 0.3 parts per million. The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 20 to 30 kilometres (12 to 19 mi) above Earth, though the thickness varies seasonally and geographically"- https://en.wikipedia.org/wiki/Ozone_layer The ozone layer is a layer of gases in the stratosphere which contains high concentrations of ozone gas relative to other parts of the atmosphere. ozone shield ozone layer atmospheric layer