Which of the following correctly distinguishes primary pollutants from secondary pollutants, with examples?

• A. Primary pollutants are emitted directly (CO, NOx); secondary pollutants form in the atmosphere (O3, sulfate aerosols).
• B. Primary pollutants form in the atmosphere (O3, sulfate aerosols); secondary pollutants are emitted directly (CO, NOx).
• C. Primary pollutants include noise and heat; secondary pollutants include ozone and smog.
• D. Primary pollutants are always visible; secondary pollutants are always invisible.

Answer: (B)

The main idea is that some pollutants go into the air directly from a source, while others are created in the air through chemical reactions after those pollutants are released.

Primary pollutants are emitted directly into the atmosphere from sources like motor vehicles, power plants, and industrial processes. They include carbon monoxide, nitrogen oxides, sulfur dioxide, and particulate matter. These pollutants arrive at the air exactly as they leave the source, so reducing their emissions tackles them at the source.

Secondary pollutants are not released directly. They form in the atmosphere when primary pollutants react with sunlight, water, oxygen, and other chemicals. Ground-level ozone is a well-known secondary pollutant that forms from nitrogen oxides and volatile organic compounds in sunlight. Sulfate and nitrate aerosols also form from the oxidation of sulfur dioxide and nitrogen oxides, respectively.

So, the correct way to distinguish them is: primary pollutants are emitted directly; secondary pollutants form in the atmosphere after chemical reactions.