The most efficient absorber of ultraviolet radiation in the atmosphere is

Analyze the question: It asks which gas in the atmosphere absorbs ultraviolet (UV) radiation most efficiently. This is about the ozone layer.

Explain the concept:

• The stratosphere contains the ozone layer, a region rich in ozone (O₃).
• Ozone absorbs harmful UV-B and UV-C radiation from the sun. This protects living things from skin cancer, cataracts, and damage to crops.
• Ozone absorbs UV and breaks apart: O₃ + UV → O₂ + O. The fragments later recombine, releasing heat.
• Ordinary oxygen (O₂) absorbs only the very short-wavelength UV (below about 240 nm), so it is far less efficient across the harmful UV range.

Evaluate the options:

• A. CO₂: absorbs infrared radiation, so it is a greenhouse gas. It is not a UV absorber.
• B. H₂: negligible in the atmosphere and not a UV absorber.
• C. H₂O: absorbs mainly infrared, which also makes it a greenhouse gas.
• D. O₂: absorbs only some of the far UV, so it is less efficient than ozone.
• E. O₃: strongly absorbs UV in the 200-310 nm range. This is the most efficient absorber.

Answer: The correct option is E. O₃ (ozone).

Tip: Remember that O₃ = "shield", because the ozone layer shields us from UV, while CO₂ and H₂O = "heat trappers", because they absorb infrared. In an exam, if "UV" appears in the question, think "ozone".