Tropospheric ozone is a secondary pollutant formed under the influence of sunlight from reactions involving which primary pollutants?

• A. CO2 and water vapor
• B. NOx and methane
• C. Sulfates and nitrates
• D. NOx and VOCs in the presence of sunlight

Answer: (D)

Tropospheric ozone forms through sunlight-driven photochemistry that requires two main kinds of pollutants: nitrogen oxides and volatile organic compounds. When sunlight is present, NO2 can be split to produce an O atom, which then combines with O2 to make ozone. VOCs supply organic radicals that keep the cycle going by regenerating NO2 from NO without consuming the ozone, allowing more ozone to accumulate near the ground. This is why ozone becomes a secondary pollutant tied to the presence of both NOx and VOCs in sunlight.

The other options don’t fit because CO2 and water vapor aren’t the reactive partners in this ozone-forming chemistry, sulfates and nitrates are secondary aerosols formed from sulfur and nitrogen oxides rather than ozone, and while methane is a VOC, the broader and more accurate description is NOx together with VOCs in the presence of sunlight.