Direct answer. Two important examples of secondary air pollutants are: (i) Ozone (O₃) (tropospheric or ground-level ozone) and (ii) Peroxyacetyl nitrate (PAN). Both are not emitted directly from any source, but are formed in the atmosphere by chemical reactions between primary pollutants in the presence of sunlight.
Primary and secondary air pollutants (basic idea)
In Environmental Studies, air pollutants are often divided into:
- Primary pollutants: Substances emitted directly into the air from identifiable sources such as factories, vehicles, thermal power plants, burning of fuels, etc. Examples: sulphur dioxide (SO₂), nitrogen oxides (NO and NO₂), carbon monoxide (CO), particulate matter, hydrocarbons.
- Secondary pollutants: Pollutants which are not emitted directly, but are formed in the atmosphere by chemical reactions (often photochemical) between primary pollutants and normal atmospheric constituents. Examples: ozone (O₃), PAN, some components of photochemical smog, acids in acid rain (H₂SO₄, HNO₃), etc.
Secondary pollutants are therefore a result of atmospheric chemistry and may sometimes be more harmful than the original primary pollutants from which they are formed.
1. Ozone (O₃) as a secondary pollutant
Ozone is a gas consisting of three oxygen atoms. It is important to distinguish between:
- Stratospheric ozone: Present high up in the stratosphere, forming the “ozone layer”. It is beneficial because it absorbs harmful ultraviolet (UV) radiation from the sun.
- Tropospheric or ground-level ozone: Present in the lower atmosphere near the earth’s surface; it is a secondary air pollutant.
Ground-level ozone is formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs) (coming from vehicle exhausts, industrial emissions, petrol vapours, solvents, etc.) react in the presence of sunlight. The reaction is a part of the process that produces photochemical smog in big cities.
Ozone is a powerful oxidising agent and can:
- Cause irritation of eyes, nose and throat, chest discomfort and breathing problems.
- Aggravate asthma and other respiratory diseases.
- Damage plant leaves, reduce crop yield and affect forest health.
- Corrode materials such as rubber and some metals.
2. Peroxyacetyl nitrate (PAN) as a secondary pollutant
Peroxyacetyl nitrate (PAN) is another typical secondary pollutant found in photochemical smog. It is formed when:
- Nitrogen oxides (NOx) emitted from vehicle exhausts and industries, and
- Unburnt hydrocarbons or volatile organic compounds (VOCs)
react under strong sunlight to form a group of compounds called peroxyacyl nitrates, of which PAN is the most common.
PAN has the following harmful effects:
- It is a strong eye irritant, causing burning sensation and watering of eyes.
- It affects the respiratory system and may cause headaches and fatigue.
- It is toxic to plants and can damage leaves, thereby affecting agricultural productivity.
- Due to its relative stability, PAN can be transported over long distances and spread pollution to areas far from the original source.
Significance of secondary pollutants in Environmental Studies
Understanding secondary pollutants like ozone and PAN is very important because:
- They are formed by the interaction of different primary pollutants, so effective control requires an integrated approach to emissions.
- They often form under specific conditions (e.g. strong sunlight, stagnant air), making urban areas with heavy traffic highly vulnerable.
- They are key components of photochemical smog, which is now a serious problem in many large cities.
Exam-oriented recap
- Secondary pollutants are formed in the atmosphere by chemical reactions between primary pollutants.
- Two examples: Ozone (O₃) and Peroxyacetyl nitrate (PAN).
- Both are important constituents of photochemical smog and have harmful effects on human health, vegetation and materials.
Conclusion: Thus, ozone (O₃) and peroxyacetyl nitrate (PAN) are classic examples of secondary air pollutants. They are not emitted directly, but are formed in the atmosphere through complex reactions of nitrogen oxides and hydrocarbons under sunlight, and they play a major role in modern urban air pollution problems.