Which statement best describes tropospheric ozone?

• A. It blocks UV-B radiation and protects life
• B. It is a man-made secondary air pollutant formed in the troposphere and part of photochemical smog
• C. It is formed only by natural sources
• D. It is a stable greenhouse gas in the upper atmosphere

Answer: (B)

The main idea is that tropospheric ozone is a secondary pollutant formed in the lower atmosphere by sunlight-driven chemical reactions, not a directly emitted gas. When nitrogen oxides and volatile organic compounds from vehicles, industry, and solvents react in sunlight, they produce ozone in the troposphere. This ozone is a key part of photochemical smog and harms human health and vegetation.

This differs from stratospheric ozone, which sits higher up and protects life by absorbing UV-B radiation, so choosing the statement about shielding UV-B would describe stratospheric, not tropospheric, ozone. It’s not formed only from natural sources—the human-caused emissions of precursors drive much of its formation. While ozone acts as a greenhouse gas, it’s not best described as a stable, long-lived greenhouse gas in the upper atmosphere, and its main concern is its role as a pollutant in the lower atmosphere.