Which of the following is a secondary pollutant?

O3 is a secondary pollutant. Secondary pollutants are pollutants which form in the atmosphere. These pollutants are not emitted directly from a source (like vehicles or power plants). Ground-level ozone is a colorless and highly irritating gas that forms just above the earth's surface. It is called a "secondary" pollutant because it is produced when two primary pollutants (nitrogen oxides (NOx) and volatile organic compounds) react in sunlight and stagnant air.