Compare the roles of the ozone layer and tropospheric ozone in environmental chemistry.

• A. Ozone in the stratosphere protects life by absorbing UV radiation, while tropospheric ozone at ground level is a pollutant formed from NOx and VOCs.
• B. Both layers block UV radiation identically.
• C. Ozone layer is formed by NOx reactions; tropospheric ozone originates from sunlight.
• D. Stratospheric ozone increases UV exposure.

Answer: (A)

Ozone behaves very differently depending on where it sits in the atmosphere. Up in the stratosphere, ozone forms a protective layer that absorbs most of the sun’s harmful ultraviolet (UV) radiation, especially UV-B, helping to shield living organisms from DNA damage and other UV-related harm. Near the surface, in the troposphere, ozone is not a shield but a pollutant. It is created by chemical reactions that involve NOx and volatile organic compounds (VOCs) in the presence of sunlight, and this ground-level ozone can irritate lungs, worsen asthma, damage crops, and contribute to smog.

The best description says exactly this: stratospheric ozone protects life by absorbing UV radiation, while tropospheric ozone at ground level is a pollutant formed from NOx and VOCs. The other statements mix up where ozone comes from or what it does—ground-level ozone doesn’t block UV like the stratospheric layer does, stratospheric ozone isn’t formed primarily by NOx, and stratospheric ozone doesn’t increase UV exposure.