Which gas is commonly used as an indicator of NOx in urban air and is a component of photochemical smog?

Which gas is commonly used as an indicator of NOx in urban air and is a component of photochemical smog?

• A. Carbon monoxide CO
• B. Ozone O3
• C. Nitrogen dioxide NO2
• D. Ammonia NH3

Answer: (C)

Nitrogen dioxide is the best indicator of NOx in urban air because NOx, which includes NO and NO2 from combustion, is a primary emission source in cities. NO converts quickly to NO2 in the atmosphere, and NO2 is a stable, reactive gas that is readily measured, making it a practical proxy for overall NOx levels. Since NOx drives many photochemical smog processes, including the formation of ozone under sunlight, NO2 serves as a useful gauge of smog potential in urban environments. Carbon monoxide signals combustion activity in general but doesn’t specifically track NOx levels. Ozone is a key component of photochemical smog itself, but it results from NOx and VOC reactions rather than indicating NOx presence. Ammonia is not the typical urban NOx indicator and is more associated with agricultural sources.

Nitrogen dioxide is the best indicator of NOx in urban air because NOx, which includes NO and NO2 from combustion, is a primary emission source in cities. NO converts quickly to NO2 in the atmosphere, and NO2 is a stable, reactive gas that is readily measured, making it a practical proxy for overall NOx levels. Since NOx drives many photochemical smog processes, including the formation of ozone under sunlight, NO2 serves as a useful gauge of smog potential in urban environments.

Carbon monoxide signals combustion activity in general but doesn’t specifically track NOx levels. Ozone is a key component of photochemical smog itself, but it results from NOx and VOC reactions rather than indicating NOx presence. Ammonia is not the typical urban NOx indicator and is more associated with agricultural sources.