Which gas is a key precursor to ground-level ozone formation in urban areas?

Which gas is a key precursor to ground-level ozone formation in urban areas?

• A. Nitrogen oxides
• B. Carbon monoxide
• C. Sulfur dioxide
• D. Lead

Answer: (A)

Ground-level ozone forms in the presence of sunlight through photochemical reactions that involve nitrogen oxides and volatile organic compounds released into urban air. The gas that acts as the key precursor is nitrogen oxides. When sunlight breaks down nitrogen dioxide, it produces reactive oxygen atoms that quickly combine with molecular oxygen to form ozone. VOCs provide the radicals needed to keep this reaction going and to prevent ozone from being scavenged back, so NOx is the essential driver of the process in urban smog. Other listed gases aren’t the main drivers of ozone formation. Carbon monoxide can influence atmospheric chemistry but is not the primary precursor for ozone production. Sulfur dioxide and lead are important pollutants with other health and environmental effects, but they don’t play the central role in creating ground-level ozone.

Ground-level ozone forms in the presence of sunlight through photochemical reactions that involve nitrogen oxides and volatile organic compounds released into urban air. The gas that acts as the key precursor is nitrogen oxides. When sunlight breaks down nitrogen dioxide, it produces reactive oxygen atoms that quickly combine with molecular oxygen to form ozone. VOCs provide the radicals needed to keep this reaction going and to prevent ozone from being scavenged back, so NOx is the essential driver of the process in urban smog.

Other listed gases aren’t the main drivers of ozone formation. Carbon monoxide can influence atmospheric chemistry but is not the primary precursor for ozone production. Sulfur dioxide and lead are important pollutants with other health and environmental effects, but they don’t play the central role in creating ground-level ozone.