In smog formation, what is the role of VOCs and nitrogen oxides?

• A. VOCs dissolve in water to form harmless compounds.
• B. NOx directly emits ozone without VOC involvement.
• C. VOCs react with nitrogen oxide to form photochemical oxidants, which together with ozone contribute to smog.
• D. NOx neutralizes VOCs to prevent smog.

Answer: (C)

In photochemical smog, sunlight drives reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx). VOCs provide reactive radicals that enable NO to be converted to NO2, and the NO2 then photolyzes under sunlight to release an oxygen atom that quickly bonds with O2 to form ozone. The radical chemistry from VOCs also sustains this cycle, producing various photochemical oxidants beyond ozone, such as other reactive oxygen species, which together create the smog mix. So, VOCs and NOx must interact under light to produce the array of oxidants that make up smog.

The other ideas don’t fit because smog formation isn’t driven by VOCs dissolving in water, nor is ozone formed simply by NOx emissions without the sunlight-driven chemistry that VOCs enable. And NOx doesn’t neutralize VOCs to prevent smog; rather, NOx participates in the chemical cycles that generate ozone and other oxidants.