Which of the following is a secondary pollutant?

Option 2 is correct, i.e. Smog.

• Smog is an air pollutant that is composed mainly of particulate matter such as pollen and dust along with other particulate matter such as Sulphur oxides, Volatile organic compounds, nitrogen oxides, and ammonia gas.
• Smog happens due to smoke, dust, and practices of carbon entering into the atmosphere. It is also due to fog on cool nights.
• Photochemical smog happens when nitrogen oxides and volatile organic compounds react together in the presence of sunlight as a catalyst and form ozone at lower levels.
• The effects of smog on human health and the environment are serious and harmful.
• The main components of photochemical smog are nitrogen oxides, Volatile Organic Compounds (VOCs), tropospheric ozone, and PAN (peroxyacetyl nitrate).

Note:

• Primary Pollutants: Those pollutants which are directly emitted from a source are called primary pollutants. Particulate matter, oxides of nitrogen, and toxic metals are all examples of primary pollutants as they can be directly associated with the burning of fossil fuels, volcanic eruptions or industrial effluents.
• Secondary Pollutants: Secondary pollutants are those pollutants that arise when primary pollutants react with one another or with other substances in the surroundings. Smog, which is the combination of smoke and fog is an example of a secondary pollutant.