An homogeneous mixture is composed of a solvent and one or more solutes. Measurements of concentrations of very dilute solutions, such as ozone in the atmosphere, are often stated in parts per million or parts per billion. You can create a solution of one part per million, and an operational definition of ppm.
Stratospheric ozone absorbs harmful ultraviolet light.
Reactions that include reactive molecules have faster reaction rates. CFC's are stable molecules in the troposphere and do not readily dissociate until they reach the stratosphere. A chlorine atom is highly reactive, allowing it to destroy several thousand ozone molecules. An increase in temperature increases reaction rates. The Spring sunrise over Antarctica in October increases temperature which increases the rate of ozone depletion.