What is a common component of photochemical smog and is a reddish-brown gas with a pungent odor?

• A. Nitrogen monoxide NO
• B. Ozone O3
• C. Nitrogen dioxide NO2
• D. Sulfur dioxide SO2

Answer: (C)

The main idea here is recognizing nitrogen dioxide as the brown, pungent gas that commonly marks photochemical smog. Nitrogen dioxide is produced from nitrogen oxide in vehicle exhaust and reacts in the atmosphere under sunlight to help drive smog chemistry. Its reddish-brown color is a hallmark of urban smog, and it has a sharp, irritating odor. In sunlight, NO2 can break apart to form an oxygen atom, which quickly combines with O2 to form ozone—a central secondary pollutant in photochemical smog. So NO2 fits both the color and the odor description and its role as a precursor to ozone explains why it’s a typical component of this smog type. The other gases listed are either colorless or have different odors and roles, so they don’t match the described characteristics.