Why Winter Mornings in North Indian Cities Turn Grey With Smog

Burning coal, petrol and crop stubble releases gases that react in sunlight, dissolve in rain, trap heat and even thin the ozone layer high above. Environmental chemistry studies these reactions so that their causes can be understood and controlled.

This lesson covers tropospheric pollution — smog, acid rain and the greenhouse effect — and stratospheric pollution, including ozone depletion.

Tropospheric pollution comes mainly from burning fuels, which releases oxides of sulphur and nitrogen, hydrocarbons, carbon monoxide and particulates that form smog, acid rain and an enhanced greenhouse effect.

Main pollutants:

- Sulphur dioxide from burning coal and oil — irritates the lungs and forms acid rain
- Nitrogen oxides from vehicle engines and power plants — form photochemical smog and acid rain
- Carbon monoxide from incomplete combustion — binds to haemoglobin far more strongly than oxygen, cutting the blood's oxygen supply
- Particulates such as soot, dust and smoke — lodge deep in the lungs

Classical smog. Forms in cool, humid air from smoke, fog and sulphur dioxide, and is chemically reducing.

Photochemical smog:

- Forms in warm, sunny, dry conditions from vehicle exhaust, and is chemically oxidising
- Sunlight splits nitrogen dioxide, $\mathrm{N}{\mathrm{O}}_2\stackrel{\mathrm{h}\nu }{\to }\mathrm{NO}+\mathrm{O}$, and the oxygen atom forms ozone, $\mathrm{O}+{\mathrm{O}}_2\to {\mathrm{O}}_3$
- Ozone and nitrogen oxides react with unburnt hydrocarbons to form irritants such as formaldehyde, acrolein and peroxyacetyl nitrate
- It irritates the eyes and lungs, cracks rubber and damages plants

Acid rain:

- Rain with a pH below 5.6, formed when oxides of sulphur and nitrogen dissolve in water:

$$2S{\mathrm{O}}_2+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_{4}4N{\mathrm{O}}_2+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 4HN{\mathrm{O}}_3$$

- It harms crops, forests and aquatic life, and corrodes marble and metal: $\mathrm{CaC}{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4\to \mathrm{CaS}{\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2$

Greenhouse effect and global warming:

- Carbon dioxide, methane, water vapour, nitrous oxide and CFCs absorb infrared radiation given out by the Earth and re-emit it, warming the lower atmosphere
- The natural greenhouse effect keeps the Earth habitable; extra gases from burning fuels and deforestation strengthen it, causing global warming, melting glaciers and rising sea levels

An everyday example. Winter smog over North Indian cities builds up when cool, still air traps vehicle exhaust, smoke from burning crop stubble and dust close to the ground.

The substance. Ozone is harmful in the troposphere but protective in the stratosphere — the same molecule is a pollutant at street level and a shield many kilometres above.

Stratospheric ozone absorbs harmful ultraviolet radiation, and chlorofluorocarbons destroy it because ultraviolet light releases chlorine radicals from them, each of which can break down many ozone molecules in a chain reaction.

How ozone forms and protects:

- Ultraviolet radiation splits oxygen molecules, ${\mathrm{O}}_2\stackrel{\mathrm{UV}}{\to }\mathrm{O}+\mathrm{O}$, and the atoms combine with oxygen, $\mathrm{O}+{\mathrm{O}}_2\rightleftharpoons {\mathrm{O}}_3$
- Ozone absorbs ultraviolet radiation and breaks down again, keeping a natural balance that shields the surface

Ozone depletion by CFCs:

- CFCs, used as refrigerants and aerosol propellants, are so unreactive that they drift unchanged up to the stratosphere
- Ultraviolet light breaks them apart: $\mathrm{C}{\mathrm{F}}_2\mathrm{C}{\mathrm{l}}_2\stackrel{\mathrm{UV}}{\to }\mathrm{C}{\mathrm{F}}_2\mathrm{C}{\mathrm{l}}^{\bullet }+\mathrm{C}{\mathrm{l}}^{\bullet }$
- The chlorine radical destroys ozone and is regenerated:

$$\mathrm{C}{\mathrm{l}}^{\bullet }+{\mathrm{O}}_3\to \mathrm{Cl}{\mathrm{O}}^{\bullet }+{\mathrm{O}}_2\mathrm{Cl}{\mathrm{O}}^{\bullet }+\mathrm{O}\to \mathrm{C}{\mathrm{l}}^{\bullet }+{\mathrm{O}}_2$$

- Because chlorine comes back after each cycle, a single chlorine atom can destroy many ozone molecules

The ozone hole over Antarctica:

- In the polar winter, polar stratospheric clouds form, and on their surfaces chlorine nitrate and hydrogen chloride react to release chlorine: $\mathrm{ClON}{\mathrm{O}}_2+\mathrm{HCl}\to \mathrm{C}{\mathrm{l}}_2+\mathrm{HN}{\mathrm{O}}_3$
- When sunlight returns in spring, it splits the chlorine into radicals that destroy ozone rapidly, thinning the layer sharply over the continent

Effects of ozone depletion:

- More ultraviolet radiation reaches the surface, raising the risk of skin cancer, cataracts and sunburn
- Plankton, crops and other plants are harmed, and plastics and paints weaken faster

An everyday example. Newer refrigerators and air conditioners sold in India increasingly use chlorine-free refrigerants such as isobutane, replacing the CFCs that damaged the ozone layer.

The substance. Ozone depletion and global warming are different problems — one lets in more ultraviolet light and the other traps more infrared, though CFCs contribute to both.