Which statement best describes the relationship between NOx and secondary pollutant formation in urban air?

• A. NOx reacts with VOCs under sunlight to form ozone and secondary organic aerosols.
• B. NOx has no role in secondary pollutant formation.
• C. NOx only forms primary pollutants.
• D. NOx reduces ozone formation.

Answer: (A)

NOx acts as a precursor to secondary pollutants in urban air through photochemical reactions driven by sunlight. In sunlight, NO2 is broken down to NO and an oxygen atom, and that oxygen atom quickly joins O2 to form ozone. This process relies on NOx being present, because it provides the NO2 that starts the cycle. VOCs supply radicals (like OH and RO2) that help convert NO to NO2 and keep the cycle going, allowing ozone to accumulate rather than simply being destroyed. The same oxidation chemistry of VOCs produces products that are low in volatility and can condense intoparticles, forming secondary organic aerosols. So NOx is central to creating both ozone and SOA, making it a driver of secondary pollutant formation in urban environments. The idea that NOx has no role, forms only primary pollutants, or reduces ozone in general does not fit how urban photochemical smog develops.