Tropospheric Pollution
A deep dive into Gaseous Pollutants, Acid Rain, Particulate Matter, and the chemistry of Smog.
The troposphere is the lowest region of the atmosphere, extending from the Earth's surface up to about 10 km. It contains roughly 80% of the total mass of the atmosphere and practically all the water vapor. Tropospheric pollution occurs due to the presence of undesirable solid or gaseous particles in the air.
Tropospheric pollutants are broadly classified into two categories:
- Gaseous air pollutants: Oxides of sulphur, nitrogen, and carbon, hydrogen sulphide, hydrocarbons, ozone, and other oxidants.
- Particulate pollutants: Dust, mist, fumes, smoke, smog, etc.
1. Gaseous Air Pollutants
A. Oxides of Sulphur ($SO_x$)
Primarily produced by the burning of fossil fuels (coal, oil) containing sulphur. Sulphur dioxide ($SO_2$) is the most common species and is highly toxic to both animals and plants.
- Effects: Causes respiratory diseases (asthma, bronchitis) in humans. In plants, it leads to chlorosis (yellowing of leaves) and stiffness of flower buds.
The oxidation of $SO_2$ to $SO_3$ in the atmosphere is slow but is catalyzed by particulate matter.
B. Oxides of Nitrogen ($NO_x$)
At high altitudes, lightning strikes cause $N_2$ and $O_2$ to combine. However, the major anthropogenic source is automobile engines burning fossil fuels at high temperatures.
$2NO (g) + O_2 (g) \rightarrow 2NO_2 (g)$ (Brown gas)
- Effects: $NO_2$ is a lung irritant, causes acute respiratory disease in children, and is toxic to living tissues. It also damages plant leaves and retards photosynthesis.
Formed by the incomplete combustion of fuel in automobiles. They are composed of carbon and hydrogen.
Effects: Many are carcinogenic (cause cancer). They also harm plants by causing aging, breakdown of tissues, and shedding of leaves.
- Carbon Monoxide ($CO$): Highly poisonous, colorless, and odorless. Produced by incomplete combustion. It binds to hemoglobin to form carboxyhemoglobin, which is ~300 times more stable than oxyhemoglobin, leading to oxygen starvation.
- Carbon Dioxide ($CO_2$): Mostly released by respiration, burning of fossil fuels, and decomposition of limestone. Excessive levels contribute heavily to the Greenhouse Effect and global warming.
2. Acid Rain
Normally, rain water has a pH of about 5.6 due to the presence of $H^+$ ions formed by the reaction of rain water with atmospheric carbon dioxide.
$H_2CO_3 (aq) \rightleftharpoons H^+ (aq) + HCO_3^- (aq)$
When the pH of the rain water drops below 5.6, it is called acid rain. This happens when oxides of sulphur and nitrogen in the atmosphere react with water to form sulphuric acid and nitric acid.
$4NO_2 (g) + O_2 (g) + 2H_2O (l) \rightarrow 4HNO_3 (aq)$
Harmful Effects of Acid Rain:
- Harmful for agriculture, trees, and plants as it dissolves and washes away nutrients.
- Causes respiratory ailments in human beings and animals.
- Toxic to aquatic life when it falls into rivers and lakes.
- Corrodes water pipes resulting in the leaching of heavy metals (Fe, Pb, Cu) into drinking water.
- Damages buildings and historical monuments made of stone or metal (e.g., The Taj Mahal phenomenon: $CaCO_3 + H_2SO_4 \rightarrow CaSO_4 + H_2O + CO_2$).
3. Particulate Pollutants
Particulates are minute solid particles or liquid droplets suspended in the air. They can be viable (living microorganisms like bacteria, fungi, molds) or non-viable.
| Type | Description | Examples |
|---|---|---|
| Smoke | Solid or mixture of solid and liquid particles formed during combustion of organic matter. | Cigarette smoke, smoke from burning fossil fuels, garbage. |
| Dust | Fine solid particles (over 1 $\mu m$ in diameter) produced during crushing, grinding, or attribution of solid materials. | Sand from sandblasting, saw dust, fly ash from factories. |
| Mists | Produced by particles of spray liquids and by condensation of vapors in air. | Sulphuric acid mist, herbicides, insecticides. |
| Fumes | Generally obtained by the condensation of vapors during sublimation, distillation, boiling. | Metallurgical operations (metal oxides fumes). |
Note on Health: Particles larger than 5 microns usually lodge in the nasal passage. Particles smaller than 1.0 micron penetrate deeply into the lungs.
4. Smog
The word smog is a portmanteau of smoke and fog. It is the most common example of air pollution occurring in many cities worldwide. There are two major types of smog:
- Conditions: Occurs in cool, humid climates.
- Composition: Mixture of smoke, fog, and sulphur dioxide ($SO_2$).
- Nature: Chemically, it acts as a reducing mixture. Hence, it is also called reducing smog.
- Conditions: Occurs in warm, dry, and sunny climates.
- Composition: The main components result from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides produced by automobiles. Key components include ozone, nitric oxide, acrolein, formaldehyde, and PAN.
- Nature: It has a high concentration of oxidizing agents. Hence, it is called an oxidizing smog.
Formation of Photochemical Smog
When fossil fuels are burnt, unburnt hydrocarbons and nitric oxide ($NO$) are released. Sunlight causes these to react in a chain sequence:
2. $[O] + O_2 (g) \rightleftharpoons O_3 (g)$ (Formation of Ozone)
3. $NO (g) + O_3 (g) \rightarrow NO_2 (g) + O_2 (g)$
Both $NO_2$ and $O_3$ are strong oxidizing agents. They react with unburnt hydrocarbons to produce chemicals like formaldehyde, acrolein, and PAN (Peroxyacetyl nitrate).
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