Which statement best defines secondary air pollutants?

• A. Air pollutants formed when primary air pollutants react with one another or with other chemicals in the air
• B. Pollutants emitted directly from a source, such as exhaust from a car
• C. Gases that deplete the ozone layer
• D. Pollutants produced by natural processes only

Answer: (A)

Secondary air pollutants are pollutants formed in the atmosphere when primary pollutants react with each other or with other chemicals in the air. They aren’t released directly from a source; instead, chemical reactions driven by sunlight and atmospheric conditions create them. A classic example is ground-level ozone, which forms from reactions between nitrogen oxides and volatile organic compounds in sunlight. Other examples include sulfates and nitrates formed from SO2 and NOx.

That definition fits the statement that describes pollutants formed when primary pollutants react with each other or with other chemicals in the air. In contrast, pollutants emitted directly from a source, like car exhaust, are primary pollutants. Gases that deplete the ozone layer refer to ozone-depleting substances, related to stratospheric ozone chemistry rather than secondary pollutant formation. And pollutants produced by natural processes only is incorrect because secondary pollutants can arise from human activities as well as natural reactions.