Why most of the ultraviolet waves emitted by the Sun do not strike Earth's surface?

Ozone (O3) is a gas that occurs in two layers of the atmosphere, the stratosphere and the troposphere. The stratospheric or "good" ozone layer, which extends upward from about 10 to 30 miles above the earth's surface, protects life on earth from the sun's harmful ultraviolet rays (UV-b). However, ozone found in the troposphere, the layer of the atmosphere that extends from the earth's surface to about 10 miles up, is deemed ground level or "bad" ozone. At ground level, ozone is an air pollutant that damages human health, vegetation, many common materials, and is a key ingredient of smog.