How do volatile organic compounds (VOCs) and nitrogen oxides (NOx) contribute to tropospheric ozone formation?

• A. VOCs and NOx react in sunlight to form tropospheric ozone
• B. Tropospheric ozone forms when VOCs and NOx are emitted directly as ozone gas
• C. Ozone is formed primarily in the stratosphere, not by VOC/NOx
• D. NOx alone forms ozone without VOCs

Answer: (A)

This question hinges on how sunlight drives the chemical reactions that create ozone in the lower atmosphere, using both VOCs and NOx. In sunlight, NO2 from NOx splits to NO and an oxygen atom, and that oxygen atom quickly binds with O2 to form ozone. But if nothing else happens, NO would also destroy ozone, limiting its buildup. VOCs help by reacting to produce radicals that convert NO to NO2 without removing ozone, so the cycle can keep producing more ozone as long as sunlight and the right mix of reactants are present. Because ozone is formed inside the atmosphere rather than being released directly, and because the presence of VOCs is essential to sustain the photochemical production, the combination of VOCs, NOx, and sunlight leads to tropospheric ozone formation. The other statements aren’t correct because ozone isn’t emitted directly as a gas, it isn’t formed mainly in the stratosphere, and NOx alone cannot produce ozone without VOCs to drive the necessary radical chemistry.