Secondary pollutant

Figure 1. Photochemical smog, consisting of various secondary pollutants, over Shanghai.

Secondary pollutants are any type of pollutant that is formed in the atmosphere. These pollutants are not emitted directly from a source, such as vehicles or power plants, instead they form as a result of the pollutants emitted from these. Pollutants that are directly input into the environment are called primary pollutants.

Secondary pollutants are concerning as they take many forms and can be formed from many different compounds. The phenomena of photochemical smog seen over densely populated cities is a result of the interactions of primary pollutants with other molecules in the air such as molecular oxygen, water and hydrocarbons, which combine to form yellow clouds that are harmful to humans. Photochemical smog is made up of various secondary pollutants like ozone, peroxyacyl nitrates (PANs) and nitric acid (seen in Figure 2).

Different types of secondary pollutants include:

• Ozone (O₃)
• Sulfuric acid and nitric acid (component of acid rain)
• Particulate matter
• Nitrogen dioxide (NO₂)
• Peroxyacyl nitrates (PANs)
• and more

These substances essentially "cook up" in the atmosphere, and are typically found downwind of primary emissions due to the time it takes to produce them. When primary pollutants cannot be dispersed due to inversion layers in the atmosphere, smog is formed over the area where they were produced, which is why smog is so prominent in warm, dense cities. Secondary pollutants are very sensitive to weather patterns.

Figure 2. Photochemical smog formation; sunlight reacts with NO₂ which then interacts with other molecules in the air to form smog.