Tropospheric Pollution

Two Broad Classes of Tropospheric Pollutants

Tropospheric pollution arises from undesirable solid or gaseous particles building up in the air we breathe. These fall into two categories:

1. Gaseous air pollutants — oxides of sulphur, nitrogen and carbon, hydrogen sulphide, hydrocarbons, ozone and other oxidants.
2. Particulate pollutants — dust, mist, fumes, smoke, smog, etc.

1. Gaseous Air Pollutants

1. Oxides of sulphur. Burning sulphur-containing fossil fuels releases sulphur dioxide (SO₂), a gas poisonous to plants and animals. Even low concentrations trigger respiratory diseases such as asthma, bronchitis and emphysema, and irritate the eyes. High SO₂ levels stiffen flower buds, causing them to drop from plants. Uncatalysed oxidation of SO₂ is slow, but particulate matter in polluted air catalyses its conversion to sulphur trioxide:

   $2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)$

   Ozone and hydrogen peroxide can also drive this oxidation:

   $SO_2(g) + O_3(g) \rightarrow SO_3(g) + O_2(g)$

   $SO_2(g) + H_2O_2(l) \rightarrow H_2SO_4(aq)$

2. Oxides of nitrogen. Dinitrogen and dioxygen do not react at ordinary temperatures, but at the high temperatures created by lightning or inside an automobile engine they combine to form nitric oxide:

   $N_2(g) + O_2(g) \xrightarrow{1483\,K} 2NO(g)$

   NO reacts rapidly with oxygen to give nitrogen dioxide:

   $2NO(g) + O_2(g) \rightarrow 2NO_2(g)$

   and reacts even faster with stratospheric ozone:

   $NO(g) + O_3(g) \rightarrow NO_2(g) + O_2(g)$

   The reddish haze seen over traffic and congested areas is due to NO₂. High NO₂ levels damage plant leaves, slow photosynthesis, irritate the lungs (especially in children), and corrode textiles and metals. NO₂ that is oxidised further to nitrate ion (NO₃⁻) is washed into soil, where it can act as a natural fertilizer.
3. Hydrocarbons. Formed by incomplete combustion of automobile fuel, hydrocarbons are carcinogenic (cancer-causing) and harm plants by accelerating ageing, breaking down tissue, and causing leaves, flowers and twigs to shed prematurely.
4. Oxides of carbon

• Carbon monoxide (CO) is colourless, odourless, and one of the most dangerous air pollutants because it binds to haemoglobin roughly 300 times more strongly than oxygen does, forming carboxyhaemoglobin. Once carboxyhaemoglobin reaches about 3–4% of blood haemoglobin, the blood's oxygen-carrying capacity drops sharply, causing headache, weak eyesight, nervousness and cardiovascular problems. In pregnant smokers, elevated CO can cause premature birth, spontaneous abortion or birth defects. CO mainly comes from vehicle exhaust and incomplete combustion of coal, firewood and petrol.
• Carbon dioxide (CO₂) enters the atmosphere through respiration, fossil-fuel combustion, cement manufacture (decomposition of limestone) and volcanic eruptions. It normally makes up about 0.03% of the atmosphere by volume and is confined to the troposphere. Green plants absorb CO₂ for photosynthesis, releasing oxygen and helping keep the natural balance — but deforestation combined with rising fossil-fuel use is pushing CO₂ levels up, which is the chief driver of global warming.

Global Warming and the Greenhouse Effect

About 75% of incoming solar energy is absorbed by the earth's surface, warming it; the rest radiates back toward space. Gases such as CO₂, methane, ozone, chlorofluorocarbons (CFCs) and water vapour trap part of this outgoing heat, warming the atmosphere — this is the greenhouse effect, named after the way glass in a greenhouse lets sunlight in but partly reflects and absorbs the infrared heat radiated back by the warmed soil and plants inside, trapping warmth. A natural greenhouse effect keeps Earth's temperature stable and habitable; the concern is the enhanced effect caused by rising concentrations of these gases.

• CO₂ is the single largest contributor; if its share rises much above 0.03% the natural balance is disturbed.
• Methane forms when vegetation burns, rots or is digested without oxygen — released from paddy fields, coal mines, rotting garbage and fossil-fuel use.
• CFCs are synthetic industrial chemicals (used in air conditioning, refrigeration) that also damage stratospheric ozone.
• Nitrous oxide occurs naturally but its levels have risen due to chemical fertilizers and fossil-fuel burning.

Continued warming risks melting polar ice, flooding low-lying land, and increasing infectious diseases such as dengue, malaria, yellow fever and sleeping sickness. Everyday steps that help — using bicycles, public transport or carpools instead of individual vehicles, planting more trees, avoiding the burning of dry leaves and wood, and not smoking in public places — can slow the rate of warming.

Acid Rain

Ordinary rainwater is already mildly acidic (pH ≈ 5.6) because atmospheric CO₂ dissolves in it:

Rain is classified as acid rain once its pH falls below 5.6. This happens when sulphur and nitrogen oxides from burning fossil fuels (coal, oil, petrol, diesel) are oxidised and react with atmospheric water:

These acids reach the ground either as wet deposition (dissolved in rain, fog or snow) or dry deposition (particulates and gases settling directly on surfaces).

Effects of acid rain:

• Dissolves and washes away nutrients needed by trees, crops and plants.
• Causes respiratory ailments in humans and animals.
• Harms aquatic ecosystems when it reaches rivers and lakes as run-off.
• Corrodes water pipes, leaching heavy metals such as iron, lead and copper into drinking water.
• Damages buildings and stone/metal structures — the Taj Mahal's marble (CaCO₃) reacts directly with the sulphuric acid in polluted Agra air:

This has slowly discoloured and disfigured the monument. India's 'Taj Trapezium' action plan (from 1995) pushed over 2,000 polluting industries around Agra, Firozabad, Mathura and Bharatpur to switch from coal/oil to natural gas or LPG, supplied low-sulphur diesel to vehicles near the Taj, and required catalytic converters (a ceramic honeycomb coated with Pd, Pt and Rh) to convert exhaust CO and NOₓ into CO₂ and N₂ at 573 K. Reducing soil acidity by adding powdered limestone is another mitigation measure.

2. Particulate Pollutants

Particulates are minute solid particles or liquid droplets suspended in air, released by vehicles, fires, dust and industrial ash.

• Viable particulates — living organisms such as bacteria, fungi, moulds and algae; these can trigger allergies and plant disease.
• Non-viable particulates are classified by how they form:
  • Smoke — solid/liquid particle mixtures from combustion (cigarette smoke, burning fuel or garbage, oil smoke).
  • Dust — fine solid particles (>1 µm) from crushing, grinding or attrition (sand, sawdust, pulverised coal, cement, fly ash).
  • Mists — formed by spray liquids or condensing vapours (sulphuric acid mist, drifting pesticide/herbicide sprays).
  • Fumes — condensation products of vapours from sublimation, distillation, boiling or chemical reactions (organic solvents, metals, metal oxides).

Particle size determines health impact: particles larger than 5 microns tend to lodge in the nasal passage, while ~10-micron particles can reach deep into the lungs. Lead, once a major vehicle-emitted air pollutant from leaded petrol, interferes with the development and maturation of red blood cells; switching to unleaded petrol has greatly reduced this problem in India.

Smog

Smog = smoke + fog. There are two distinct types:

• Classical (reducing) smog — occurs in cool, humid climates; a mixture of smoke, fog and SO₂ that is chemically reducing. …