Pollution is defined as the accumulation of substances or forms of energy in an environment in quantities or at flow rates, which exceed an ecosystem’s capacity to either neutralise or disperse them to harmless levels. (RF Dasmann – 1975).

What do the terms used in this definition mean?

1. Accumulation of Substances: For example, the Environment has a natural capacity to absorb CO₂ through Photosynthesis, as we saw in the Carbon Cycle. If we release more of it in the atmosphere than the environment’s natural absorption capacity, it will create global warming and ocean acidification.
2. Forms of Energy: For example, if we release clean water but at a temperature of 80^oC, it will harm the ecosystem and living biomass; It will exceed the capacity of the ecosystem to handle that temperature.

Pollution can happen through various mediums, such as air, water, or soil. An Average human being requires nearly 12-15 times more air than food; therefore, Atmospheric pollution is the most vital dimension in the study of pollution.

Atmospheric pollution:

The troposphere is the lowest region of the atmosphere where all humans live. It extends up to 8-18km from sea level. It is a turbulent dusty zone containing air, most of the water vapour, and clouds, and it has strong air movement.

The stratosphere extends up to 50km above sea level beyond the troposphere. It contains N₂, O₂, Ozone and little water vapour. In fact, the ozone layer in the stratosphere prevents about 99.5% of the sun’s harmful UV radiations.

Atmospheric pollution is generally studied as tropospheric and stratospheric pollution.

Tropospheric pollution:

Due to the presence of undesirable solid or gaseous particles in the air. Pollutants:

1. Gaseous air pollutants
2. Particulate matter

Gases Air Pollutants

Through human activities, such as Burning fossil fuels, various pollutants such as oxides of Sulphur, Nitrogen and Carbon. Further, unburned fuel releases hydrogen sulphides and hydrocarbons during the year. Ozone and other oxidants are mainly released during various industrial processes.

Let’s consider each pollutant one by one:

A. Oxides of Sulphur (SO₂)

• Sulphur dioxide (SO₂) gas is poisonous to both plants and animals. It causes respiratory ailments even in low concentrations. It readily creates Sulphurous Acid upon coming in contact with water in cells.
  • Creation of Sulphuric Acid: In the Atmosphere, SO₂ slowly oxidises to form Sulphur trioxide (SO₃) gas. It eventually creates sulphuric acid upon reacting with water.

2SO₂ (g) + O₂ (g) → 2SO₃(g)

SO₃ (g) + H₂O (l) → H₂SO₄ (aq)

• Impact on Human, Plant and Animal Life:
  • Respiratory Disorders – It harms the alveoli sacs in the lungs, causing asthma, bronchitis, and emphysema in humans;
  • Impact on Eyes: Irritation to eyes, resulting in tears & redness.
  • Impact on Plants: High concentration leads to stiffness of flower buds, which eventually fall.
• Impact on Material Objects: Sulphuric acid (H₂SO₄) often precipitates with rain, causing Acid Rain. It corrodes the surface of material objects, including rocks.

B. Oxides of Nitrogen:

Oxides of Nitrogen are another class of corrosive gases present in the atmosphere. It occurs in two ways: 

1. Naturally: At high altitudes during lightning strikes, O₂ & N₂ combine, and NO₂ forms. Oxidises to form NO₃^– which is washed into soil that serves as fertilizers.
2. Anthropogenic causes:
   1. Automobile engine (high temp) when fossil fuel is burnt N & O combine: 

              N₂ (g) + O₂ (g) → 2NO(g) +O₂ (g) → 2NO₂ (g)

            2. Agriculture: Emissions from Chemical fertilizers and livestock manure.

                 Burning of fossil fuels: NO_x emissions in India grew by 52% from 1991 to 2001 & 69% from 2001 to 2011.

Effects of Oxides of Nitrogen:

1. Green House effect: NO₂ is 300 times more potent greenhouse gas than CO₂ but isn’t as prevalent.
2. Pollution 
   • Pollution of rivers & streams: also known as Nutrient Pollution – Eutrophication (caused by compounds of Nitrogen & phosphorous); It causes algae to grow faster than ecosystems can handle (they produce powerful toxins apart from Oxygen).
   • Irritant red haze in traffic & congested places,
   • Higher concentration Of NO₂ damages the leaves of plants & is toxic to living tissues.
   • Retard the rate of photosynthesis,
   • It’s a lung irritant that can lead to acute respiratory disease in children.
   • Harmful to various textile fibres & metals.
3. Biodiversity Loss
4.  

                 NO (g) + O₃ (g) → NO₂ (g) + O₂ (g)

        South Asian Nitrogen Hub (SANH) (of 50 institutes, of which 18 are from India) is studying the impact of Nitrogen                emissions in South Asia.

C. Hydrocarbons:

Hydrocarbons are compounds that only consist of carbon and hydrogen. These gases are generally energy-dense and thus act as fuel. It is released into the atmosphere only in two ways:

1. Artificially: These are only formed by incomplete combustion of fuel in automobiles.
2. Naturally, Methane is the most important gaseous hydrocarbon. It is sometimes released into the atmosphere naturally through sedimentary formations.

Effects: –

• Hydrocarbonsare Carcinogenic.
  • These generally harmplantsbycausing ageing, breakdown of tissues, and shedding of leaves, flowers and twigs.
  • Global warming potential: Methane is 25 times more potent greenhouse gas than Carbon dioxide.

D. Oxides of Carbon:

1. Carbon Monoxide (CO): It is a colourless and odourless gas, generally released through Incomplete combustion of Carbon.
   • Source: Automobile exhaust, incomplete combustion of coal, firewood, petrol, etc.
   • Effects:
     • CO is highly poisonous to living beings because of its ability to block the delivery of oxygen to organs & tissues. It binds with Haemoglobin 300 times more than Oxygen, forming Carboxyhaemoglobin. When its concentration reaches 3-4%, the blood’s oxygen-carrying capacity is greatly reduced, causing headaches, weak eyesight, nervousness and cardiovascular disorders in the long run.
     • Pregnant women: premature birth, spontaneous abortions & deformed babies.
   • Prevention:
     • Using efficient Engines.
     • Prevention of wildfires in forests and replacement of fuelwood for cooking with fuels like LPG.
     • Prevention of Smoking
2. Carbon Dioxide (CO₂): It is naturally released through respiration. Artificially, it is released through the burning of fossil fuels for energy, the decomposition of limestone during cement manufacturing, and volcanic eruptions.
   • Excess CO₂ is removed by green plants, which creates a delicate balance. However, the burning of fossil fuels disturbs this balance.
   • During the pre-industrial level, the Normal concentration of CO₂ was 0.028% (or 280 ppm); However, currently, it is around 412 ppm.

We shall study the Greenhouse Effect in detail in the coming chapters.

What is Smoke?

Smoke is a suspension of airborne particles and gases emitted when a material undergoes combustion. Smoke always leaves a shadow, which means that it has a large number of solid and liquid particles. What creates smoke? Generally, unburnt fuel particles are released into the air during burning. These particles can be fluid as well as solid, creating a smoky texture.

Particulate Matter

Minute solid particles or liquid droplets in the air such as dust, mist, fumes, smoke and smog.

Acid rains:

Acid rain refers to the ways in which acid from the atmosphere is deposited on Earth’s surface. Oxides of Nitrogen and Sulphur are acidic. Normally, the pH of rain is 5.6 due to H⁺ ions formed by the reaction of rainwater with CO₂ in the atmosphere.

When the pH of rain is less than 5.6, it is known as Acid rain.

Acid in the rain can be blown by wind along with Solid particles in the atmosphere:

• Finally, settle down either on the ground as Dry deposits or
• In water, fog & snow as Wet deposition.

Production of Acid in Atmosphere:

• Burning of fossil fuels: coal, oil in power stations & furnaces or petrol & diesel in motor engines,

2SO₂ (g) + O₂ (g) + 2H₂O (l) → 2H₂SO₄ (aq)

4NO₂ (g) + O₂ (g)+ 2H₂O (l) → 4HNO₃ (aq)

• Ammonium salts also formed: Atmospheric haze (aerosol);
  • Aerosol particles of oxides or ammonium salts in rain drops: wet-deposition. 
  • SO₂ is absorbed directly on both solid & liquid ground surfaces: Dry deposition.

Effects of Acid Rain

• It dissolves and washes away nutrients needed for growth and is thus harmful to Agriculture, trees, and plants.
• Causes respiratory ailments in human beings & animals.
• Affects plants & animal life in aquatic ecosystem = Oceanic Acidification.
• It corrodes metal and rock surfaces, resulting in its leaching (a process in which metal becomes soluble in water).
• Iron, lead and copper are thus in drinking water. It affects groundwater, rivers, lakes, and more.
• It damages buildings & other structures.

Leaching of Taj Mahal

The air around Agra used to have high concentrations of oxides of Sulphur and Nitrogen due to a large number of industries and power plants, the release of air pollutants at the Mathura oil refinery, and the use of poor-quality coal, kerosene and firewood as fuel. Its marble started depleting due to the leaching by Acid rain: CaCO3 +H2SO4 → CaSO2 + H2O+ CO2; Monument getting discoloured & lustreless. Thus, the Government of India announced an action plan in early 1995 to prevent historical monuments from creating a ‘Taj Trapezium Zone.’ Mathura Oil Refinery has taken suitable measures to check the emission of toxic gas. The Zone includes the cities of Agra, Firozabad, Mathura, and Bharatpur (Rajasthan), having around 2000 industries that have been switched to use CNG or LPG from coal/oil. A New Natural gas pipeline currently brings more than 0.5Mn m3 of Natural gas/per day.Vehicles near Taj would ply on low-sulphur diesel. Taj Mahal Declaration: To beat plastic in 500m around Agra, the Taj Mahal Declaration was made in 2018 by the M/o Culture.

Other Compounds:

Lead:

• Lead used to be a major air pollutant due to Leaded petrol. Earlier, Tetra ethyl led (TEL) was used in petrol as an octane rating booster, which improved the engine’s performance. It was the primary source of airborne lead emission. Now Unleaded petrol is used in India.
• Lead interferes with the development and maturation of blood cells.

Smog:

The term ‘Smog’ is derived from the association of two terms, Smoke and Fog. Fog is a situation when water vapour condenses in the air near the ground, reducing visibility. When Fog mixes with smoke, it creates smog.

Smog is created through two mechanisms:

• Classical smog: Cool, humid climate: smoke, fog & SO₂. It is chemically a reducing mixture, i.e. it removes oxygen from the substances with which it reacts. Therefore, it is known as Reducing smog.
  •  

Formation of Photochemical smog:

• Burning of fossil fuel: hydrocarbons (unburnt fuel) & Nitric oxide (NO). When sufficiently high levels, a chain reaction starts:

NO₂(g)→ NO(g) + O(g)

O(g) + O₂ (g) → O₃ (g); Reversible reaction

NO(g) + NO₃(g) → NO₂(g) + O₂(g) = Rapid reaction: as soon as ozone forms, it reacts.

• NO₂ is brown gas & at sufficiently high levels, can contribute to haze. O₃ is toxic; both O₃ & NO₂ are strong oxidising agents & can react with un-burnt polluted air:

3CH₄ + 2O₃ → 3CH₂=O + 3H₂O

Form formaldehyde, Acrolein & Peroxy Acetyl nitrate (PAN) = Volatile Organic compounds (VOC).

The main components of photochemical smog are ozone, nitric oxide, acrolein, formaldehyde & peroxyacetyl nitrate (PAN)

Effect of Photochemical smog:

• Ozone & PAN act as a powerful eye irritant.
• Ozone & nitric oxide irritate the nose & throat;
• High concentration causes headache, chest pain, dry throat, cough & difficulty in breathing.
• Photo- smog leads to cracking of rubber, extensive damage to plant life, corrodes metals, stones, buildings, rubber & painted surfaces.

How to control Photochemical Smog:

• Control the primary precursors: NO₂ & hydrocarbons AND
• Secondary precursors: O₃ & PAN.
• Catalytic converters used in automobiles: prevent release of Nitrogen oxide & HCs;
• Certain plants, such as Pinus, Juniperus, Quercus, Pyrus & Vitis, can metabolise nitrogen oxide, and the plantation could help.

Initiatives taken to control Photochemical Smog:

• Gothenburg protocol: aim to abate acidification, eutrophication, and ground-level ozone. Part of the convention on long-range transboundary air pollution.

Stratospheric Pollution:

• Ozone layer: Ozone is generally present in the lower stratosphere at the height of 15-35km. It absorbs and reflects the harmful ultraviolet (UV) radiations (l= 225nm) coming from the sun. These radiations can cause skin cancer(melanoma).
• The photochemical mechanisms It is only 3 mm thick.

Creation of the Ozone layer

O₂ (g) → O(g) + O(g);

(In the presence of UV light)

O(g) + O₂ (g) → O₃ (g);

(Rapid reaction in the presence of UV light, creating fast moving Oxygen molecule)

Thus, it absorbs UV light, both during production and during its decomposition.

Further, Ozone is generally unstable and slowly decomposes to a slow-moving O₂, spontaneously scattering UV light back to the space. Thus, a dynamic equilibrium exists between the production and decomposition of O₃.

The Ozone layer thus formed has only small amounts of ozone. If all the ozone in the atmosphere is brought under atmospheric pressure at the ground level, the total thickness of the layer would be less than 3mm.

Ozone layer depletion:

In the 1970s, it was observed that the ozone layer had been depleted. The main reason behind this depletion was the leakage of Chloro-Floro-Carbons (CFCs) used in refrigeration processes (refrigerators and air conditioners).

These were being marketed under the trade name ‘Freons’. These are non-reactive, non-flammable, non-toxic organic molecules. These were also used in the production of plastic foam & electronic industry for cleaning computer parts.

CFCs spontaneously create Chlorine free radicals (Cl•) that reduce the Ozone layer.

CF₂Cl₂ (g)    →      Cl•(g) + CF₂Cl (g)

Cl• (g) + O₃ (g) → ClO•(g) + O₂ (g)

ClO•(g) + O (g) → Cl•(g) + O₂ (g);

Cl radicals are continuously regenerated due to the presence of CFCs and cause the breakdown of the ozone cycle.

Ozone Hole:

In the 1980s, atmospheric scientists in Antarctica reported the depletion of the ozone layer. An Ozone hole over the north and the south poles was reported.

Effects of depletion: More UV filters into the troposphere. Leads to:

• Ageing of skin, cataracts, sunburn, skin cancer
  • Killing of many phytoplankton (marine plants): Damage fish productivity.
  • Plant proteins get easily affected by UV radiations: harmful mutation of cells.
  • Increases evaporation of surface water through stomata of the leaves & decreases soil moisture.
  • Damage paints and fibres, causing them to fade faster.

International Efforts on Ozone Depletion

Vienna Convention, 1985: It acts as a framework for international efforts to protect the ozone layer. These are laid out in the accompanying Montreal Protocol. It was agreed upon at the 1985 Vienna Conference and entered into force in 1988 (Montreal in 1989).It does not include legally binding reduction goals for the use of CFCs. It is one of the most successful treaties of all. In terms of universality. It has been ratified by 197 states (all UN members as well as the Niue, Holy See and the Cook Islands) as well as the European Union. Montreal Protocol, 1989: It sets targets for phasing out production of numerous Ozone Depleting Substances (ODSs). It is legally binding to all signatories. Ratified in 1997.Ratified by 141 nations. It bounds 35 industrialised countries to reduce their emissions by the year 2012 to 5% less than prevalent in the year 1990Under it, the production and consumption of key ODSs like chlorofluorocarbon (CFCs), Methyl Chloroform, CTC halons and Methyl Bromide have been phased out globally. It has been one of the most successful International agreements; Kofi Annan Says:  “Perhaps the single most successful international agreement to date has been the Montreal Protocol.”Studies show that the Ozone hole in Antarctica is slowly recovering. & will come to 1980s level by 2050-2070.

Green Chemistry

Green Chemistry: is a production process that would bring about minimum pollution or deterioration to the environment. The utilisation of the existing knowledge base for reducing chemical hazards along with developmental activities is the foundation of green chemistry.

Suppose reactants are fully converted into useful environmentally friendly products by the environment-friendly medium. Then, there would be no chemical pollutants introduced into the environment.

• Care must be taken to choose starting materials that can be converted into end products with a yield of approximately up to 100% by arriving at optimum conditions of synthesis (Temp, Pressure, etc.)
• Yves Chauvin, Institut Français du Pétrole, Rueil-Malmaison France, Robert H. Grubbs California Institute of Technology (Caltech), and Richard R. Schrock (MIT), Cambridge, won the 2005 Nobel Prize in chemistry for the development of metathesis method in organic synthesis– development of revolutionary environmentally friendlier polymers.
• It has tremendous commercial potential in the pharmaceuticals, biotech & foodstuff production industries.

Green Chemistry in day-to-day life:

1. Dry cleaning of cloths: Tetrachloroethene was earlier used as a solvent for dry cleaning – it contaminates groundwater & suspected carcinogenic and is now replaced by liquified CO₂ with a suitable detergent.
   1. H₂O₂ is used these days to bleach clothes – It gives better results & less water
2. Bleaching of paper: Chlorine gas was used earlier. H₂O₂ with a suitable catalyst is now used.
3. Synthesis of Chemicals: Ethanol is now commercially prepared by one-step oxidation of ethene in the presence of an ionic catalyst in an aqueous medium with a 90% yield.