Previous Year Question

Pollutants are broadly classified according to their formation into Primary Pollutants and Secondary Pollutants.

Primary Pollutants are pollutants that are emitted directly into the atmosphere from a source. For example, Carbon Monoxide, Carbon Dioxide, Sulphur Dioxide and particulate matter can be released directly from various combustion processes.

In contrast, Secondary Pollutants are not emitted directly in the same form from a source. They are produced in the atmosphere through reactions among primary pollutants or through chemical reactions occurring in the presence of sunlight. NIOS also defines Secondary Pollutants as products formed by reactions between primary pollutants and normal atmospheric constituents.

The term Smog originally combines Smoke + Fog, but in the modern air-pollution context, Photochemical Smog is especially important. In it, Nitrogen Oxides and Volatile Organic Compounds undergo reactions in the presence of sunlight and can produce Ozone, PAN and other photochemical oxidants.

Therefore, in examination classification, Smog is identified as a Secondary Pollution phenomenon.

Carbon Monoxide (CO) can be emitted directly through incomplete combustion of fuels and is therefore a Primary Pollutant.

Carbon Dioxide (CO₂) can be emitted directly through combustion of fossil fuels and other carbon-containing substances and is therefore treated as a Primary Pollutant in this classification. NIOS also gives CO₂ and CO from fossil-fuel burning as examples of primary pollutants.

Fly Ash is a fine particulate residue produced during coal combustion and is directly released from sources such as thermal power plants; therefore, it is not formed as a secondary atmospheric pollutant.

The key distinction is:

Primary Pollutant → emitted directly from a source
Secondary Pollutant → formed through reactions in the atmosphere