Ground-level or "bad" ozone is not emitted directly into the air, but is created by chemical reactions between

• A. NOx and VOCs in the presence of sunlight
• B. CO2 and CH4 in the presence of heat
• C. SO2 and NO2 in rain
• D. O3 and NO2 in darkness

Answer: (A)

Ground-level ozone is a secondary pollutant formed by photochemical reactions in sunlight. The key players are nitrogen oxides (NOx) and volatile organic compounds (VOCs) released from vehicle exhaust, industry, and solvents. In sunlight, NO2 can be broken apart to produce an oxygen atom, which quickly combines with O2 to form ozone. VOCs drive additional radical chemistry (like peroxy radicals) that oxidizes more NO to NO2, sustaining continuous ozone production. Without sunlight, these photochemical steps don’t proceed, so ozone isn’t formed in large amounts.

Other options don’t fit because CO2 and CH4 don’t drive this photochemical ozone formation, SO2 and NO2 in rain relate to acid rain chemistry rather than ozone creation, and ozone formed in darkness cannot be sustained since light is required to drive the NO2 photolysis that starts the process.