Air pollution

Structure of the atmosphere

From the ground upward:

• Troposphere (up to about 8–18 km; thicker at the equator): contains most of the air mass and nearly all weather. Temperature falls with height.
• Stratosphere (up to about 50 km): contains the ozone layer; temperature rises with height because ozone absorbs UV.
• Mesosphere (up to about 80–85 km): temperature falls again; the coldest layer.
• Thermosphere: very thin air; temperature rises sharply.

Primary and secondary pollutants

• Primary pollutants are released directly: carbon monoxide (CO), sulphur dioxide (SO₂), oxides of nitrogen (NOₓ), particulate matter, hydrocarbons.
• Secondary pollutants form in the air by reactions among primary ones: ground-level ozone (O₃), peroxyacetyl nitrate (PAN), sulphuric and nitric acids.

Acid rain

Normal rain is slightly acidic (pH about 5.6) because CO₂ dissolves to form carbonic acid. Acid rain has pH below 5.6. SO₂ and NOₓ from burning coal and fuel form H₂SO₄ and HNO₃. Effects: acidified lakes kill fish, leaching of soil nutrients, forest damage, and corrosion of marble and limestone, e.g. concern over yellowing of the Taj Mahal from industrial emissions near Agra and Mathura.

Smog

• Classical (London) smog: smoke + fog + SO₂, in cool, damp weather; chemically reducing. The London smog of December 1952 killed thousands.
• Photochemical (Los Angeles) smog: NOₓ + hydrocarbons + strong sunlight → O₃, PAN and aldehydes; forms in warm, sunny weather; chemically oxidising; causes eye irritation and breathing problems.

Tropospheric (ground-level) ozone is “bad ozone”: it harms lungs and damages crops. Stratospheric ozone is “good ozone” because it shields us from UV.

A temperature inversion (warm air above cool air) stops vertical mixing and traps pollutants near the ground, as often happens in winter in north Indian cities.

Key points

• Order: troposphere → stratosphere → mesosphere → thermosphere.
• Ozone layer is in the stratosphere.
• Acid rain pH < 5.6; caused by SO₂ and NOₓ.
• Classical smog = SO₂ + smoke + fog (reducing); photochemical smog = NOₓ + hydrocarbons + sunlight (oxidising).

Exam tips

Common questions: which pollutant is secondary (O₃, PAN); which layer has the ozone layer; which smog needs sunlight. Note the weather that goes with each smog type.

Common mistakes

• Calling ozone a primary pollutant; at ground level it is secondary.
• Thinking all ozone is harmful; stratospheric ozone protects life.
• Saying rain with pH 6 is acid rain; the cut-off is 5.6.