In which atmospheric layer is the protective ozone primarily located?

• A. Troposphere
• B. Stratosphere
• C. Mesosphere
• D. Thermosphere

Answer: (B)

Ozone in the atmosphere tends to accumulate where UV-driven chemistry can both create and sustain it, and that place is the stratosphere. In this layer, high-energy ultraviolet light breaks apart oxygen molecules (O2) so single oxygen atoms are available to combine with O2 to form ozone (O3). The same UV radiation also controls ozone destruction, but the overall balance in the stratosphere leads to a relatively stable, elevated concentration—a region we call the ozone layer. This layer sits above the troposphere, where air is more turbulent and ozone tends to be produced as a pollutant and is not kept in high, protective concentrations. The other layers—mesosphere and thermosphere—have far less ozone, so they don’t serve as the protective shield. Thus, the protective ozone is primarily located in the stratosphere, roughly 10 to 50 kilometers above the surface, with the strongest concentration around 20 to 30 kilometers.