`color{green}(★)` Environmental pollution is taking place day by day because of the effect of undesirable changes in our surroundings that have harmful effects on plants, animals and human beings.

`color{green}("★ 𝐀 𝐬𝐮𝐛𝐬𝐭𝐚𝐧𝐜𝐞, 𝐰𝐡𝐢𝐜𝐡 𝐜𝐚𝐮𝐬𝐞𝐬 𝐩𝐨𝐥𝐥𝐮𝐭𝐢𝐨𝐧, 𝐢𝐬 𝐤𝐧𝐨𝐰𝐧 𝐚𝐬 𝐩𝐨𝐥𝐥𝐮𝐭𝐚𝐧𝐭.")` Pollutants can be solid, liquid or gaseous substances present in greater concentration than in natural abundance and are produced due to human activities or due to natural happenings.

`color{brown}("★ 𝐉𝐔𝐒𝐓 𝐅𝐎𝐑 𝐂𝐔𝐑𝐈𝐎𝐔𝐒")`

`color{green}(★)` An average human being requires nearly 12-15 times more air than the food. So, even small amounts of pollutants in the air become significant compared to similar levels present in the food.

`color{green}(★)` Pollutants can be degradable, like discarded vegetables which rapidly break down by natural processes.

`color{green}(★)` On the other hand, pollutants which are slowly degradable, remain in the environment in an unchanged form for many decades. For example, substances such as dichlorodiphenyltrichloroethane (DDT), plastic materials, heavy metals, many chemicals, nuclear wastes etc., once released into the environment are difficult to remove. These pollutants cannot be degraded by natural processes and are harmful to living organisms.

`color{red}(" ★ 𝐈𝐧 𝐭𝐡𝐞 𝐩𝐫𝐨𝐜𝐞𝐬𝐬 𝐨𝐟 𝐞𝐧𝐯𝐢𝐫𝐨𝐧𝐦𝐞𝐧𝐭𝐚𝐥 𝐩𝐨𝐥𝐥𝐮𝐭𝐢𝐨𝐧,")` `color{red}("𝐩𝐨𝐥𝐥𝐮𝐭𝐚𝐧𝐭𝐬 𝐨𝐫𝐢𝐠𝐢𝐧𝐚𝐭𝐞 𝐟𝐫𝐨𝐦 𝐚 𝐬𝐨𝐮𝐫𝐜𝐞 𝐚𝐧𝐝 𝐠𝐞𝐭 𝐭𝐫𝐚𝐧𝐬𝐩𝐨𝐫𝐭𝐞𝐝 𝐛𝐲 𝐚𝐢𝐫 𝐨𝐫 𝐰𝐚𝐭𝐞𝐫")` `color{red}("𝐨𝐫 𝐚𝐫𝐞 𝐝𝐮𝐦𝐩𝐞𝐝 𝐢𝐧𝐭𝐨 𝐭𝐡𝐞 𝐬𝐨𝐢𝐥 𝐛𝐲 𝐡𝐮𝐦𝐚𝐧 𝐛𝐞𝐢𝐧𝐠𝐬.")`

`color{green}(★)` The atmosphere that surrounds the earth varies in thickness with heights.

`color{green}(★)` There are concentric layers of air or regions and each layer has different density.


`color{green}("★ 𝐓𝐑𝐎𝐏𝐎𝐒𝐏𝐇𝐄𝐑𝐄:")`

`color{green}(★)` The lowest region of atmosphere in which the human beings along with other organisms live is called troposphere.

`color{green}(★)` It extends up to the height of ~ 10 km from sea level.

`color{green}(★)` Troposphere is a turbulent, dusty zone containing air, much water vapour and clouds.

`color{green}(★)` This is the region of strong air movement and cloud formation.

`color{green}("★ 𝐒𝐓𝐑𝐀𝐓𝐎𝐒𝐏𝐇𝐄𝐑𝐄:")`

`color{green}(★)` Above the troposphere, between 10 and 50 km above sea level lies stratosphere.

`color{green}(★)` The stratosphere, on the other hand, contains dinitrogen, dioxygen, ozone and little water vapour.

`color{green}(★)` Atmospheric pollution is generally studied as tropospheric and stratospheric pollution.

`color{green}(★)` `color{green}("𝐈𝐌𝐏𝐎𝐑𝐓𝐀𝐍𝐂𝐄 𝐎𝐅 𝐎𝐙𝐎𝐍𝐄 𝐋𝐀𝐘𝐄𝐑 :")` The presence of ozone in the stratosphere prevents about `99.5 %` of the sun’s harmful ultraviolet (UV) radiations from reaching the earth’s surface and thereby protecting humans and other animals from its effect.

`color{green}(★)` Tropospheric pollution occurs due to the presence of undesirable solid or gaseous particles in the air.

`color{green}(★)` The following are the major gaseous and particulate pollutants present in the troposphere:

1. Gaseous air pollutants: These are oxides of sulphur, nitrogen and carbon, hydrogen sulphide, hydrocarbons, ozone and other oxidants.

2. Particulate pollutants: These are dust, mist, fumes, smoke, smog etc.

`color{green}("𝟏. 𝐆𝐚𝐬𝐞𝐨𝐮𝐬 𝐚𝐢𝐫 𝐩𝐨𝐥𝐥𝐮𝐭𝐚𝐧𝐭𝐬")`

`color{red}(" (𝐚) 𝐎𝐱𝐢𝐝𝐞𝐬 𝐨𝐟 𝐒𝐮𝐥𝐩𝐡𝐮𝐫:")`

`color{green}(★)` Oxides of sulphur are produced when sulphur containing fossil fuel is burnt. It has been reported that even a low concentration of sulphur dioxide causes respiratory diseases e.g., asthma, bronchitis, emphysema in human beings.

`color{green}(★)` The most common species, sulphur dioxide, is a gas that is poisonous to both animals and plants. Sulphur dioxide causes irritation to the eyes, resulting in tears and redness. High concentration of `color{red}(SO_2)` leads to stiffness of flower buds which eventually fall off from plants.

`color{green}(★)` Uncatalysed oxidation of sulphur dioxide is slow. However, the presence of particulate matter in polluted air catalyses the oxidation of sulphur dioxide to sulphur trioxide.

`color{red}(2SO_2 (g) +O_2 (g) → 2SO_3(g))`

The reaction can also be promoted by ozone and hydrogen peroxide.

`color{red}(SO_2 (g) +O_3 (g) → SO_3(g) + O_2 (g))`

`color{red}( SO_2(g) + H_2O_2(l) → H_2SO_4(aq))`



`color{green}(★)` `color{red}("(𝐛) 𝐎𝐱𝐢𝐝𝐞𝐬 𝐨𝐟 𝐍𝐢𝐭𝐫𝐨𝐠𝐞𝐧:")`

`color{green}(★)` Dinitrogen and dioxygen are the main constituents of air.

`color{green}(★)` At high altitudes when lightning strikes, they combine to form oxides of nitrogen.

`color{green}(★)` `color{red}(NO_2)` is oxidised to nitrate ion, `color{red}(NO_3^( −))` which is washed into soil, where it serves as a fertilizer.

`color{green}(★)` In an automobile engine, (at high temperature) when fossil fuel is burnt, dinitrogen and dioxygen combine to yield significant quantities of nitric oxide `color{red}((NO))` and nitrogen dioxide `color{red}(( NO_2 ))` as given below:

`color{red}(N_2 (g) + O_2 (g) overset(1483K)→ 2NO(g))`

`color{green}(★)` `color{red}(NO)` reacts instantly with oxygen to give `color{red}(NO_2)`

`color{red}(2NO (g) + O_2 (g) → 2NO_2 (g))`

`color{green}(★)` Rate of production of `color{red}(NO_2)` is faster when nitric oxide reacts with ozone in the stratosphere.

`color{red}(NO (g) + O_3 (g) → NO_2 (g) + O_2 (g))`

`color{green}("★ 𝐄𝐅𝐅𝐄𝐂𝐓𝐒 𝐎𝐅 𝐎𝐗𝐈𝐃𝐄𝐒 𝐎𝐅 𝐍𝐈𝐓𝐑𝐎𝐆𝐄𝐍:")`

`color{green}(★)` The irritant red haze in the traffic and congested places is due to oxides of nitrogen.

`color{green}(★)` Higher concentrations of `color{red}(NO_2)` damage the leaves of plants and retard the rate of photosynthesis.

`color{green}(★)` Nitrogen dioxide is a lung irritant that can lead to an acute respiratory disease in children.

`color{green}(★)` It is toxic to living tissues also.

`color{green}(★)` Nitrogen dioxide is also harmful to various textile fibres and metals.

`color{red}("(c) 𝐇𝐲𝐝𝐫𝐨𝐜𝐚𝐫𝐛𝐨𝐧𝐬:")` Hydrocarbons are composed of hydrogen and carbon only and are formed by incomplete combustion of fuel used in automobiles.

`color{green}(" ★ 𝐇𝐀𝐑𝐌𝐅𝐔𝐋 𝐄𝐅𝐅𝐄𝐂𝐓𝐒 𝐎𝐅 𝐇𝐘𝐃𝐑𝐎𝐂𝐀𝐑𝐁𝐎𝐍𝐒:")`

`color{green}(★)` Hydrocarbons are carcinogenic, i.e., they cause cancer.

`color{green}(★)` They harm plants by causing ageing, breakdown of tissues and shedding of leaves, flowers and twigs.


`color{green}("(𝐝) 𝐎𝐱𝐢𝐝𝐞𝐬 𝐨𝐟 𝐂𝐚𝐫𝐛𝐨𝐧")`

`color{red}("(𝐢 ) 𝐂𝐚𝐫𝐛𝐨𝐧 𝐦𝐨𝐧𝐨𝐱𝐢𝐝𝐞:")`

`color{green}(★)` Carbon monoxide `color{red}((CO))` is one of the most serious air pollutants. It is produced as a result of incomplete combustion of carbon

`color{green}(★)` It is a colourless and odourless gas, highly poisonous to living beings because of its ability to block the delivery of oxygen to the organs and tissues.

`color{green}(★)` Carbon monoxide is mainly released into the air by automobile exhaust. Other sources, which produce `color{red}(CO)`, involve incomplete combustion of coal, firewood, petrol, etc.

`color{brown}(" ★ 𝐉𝐔𝐒𝐓 𝐅𝐎𝐑 𝐂𝐔𝐑𝐈𝐎𝐔𝐒")`

`color{green}(★)` `color{green}("𝐃𝐨 𝐲𝐨𝐮 𝐤𝐧𝐨𝐰 𝐰𝐡𝐲 𝐜𝐚𝐫𝐛𝐨𝐧 𝐦𝐨𝐧𝐨𝐱𝐢𝐝𝐞 𝐢𝐬 𝐩𝐨𝐢𝐬𝐨𝐧𝐨𝐮𝐬?")`

`color{green}(★)` It binds to haemoglobin to form carboxyhaemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex. In blood, when the concentration of carboxyhaemoglobin reaches about `3–4 %` the oxygen carrying capacity of blood is greatly reduced. This oxygen deficiency, results into headache, weak eyesight, nervousness and cardiovascular disorder. This is the reason why people are advised not to smoke. In pregnant women who have the habit of smoking the increased `color{red}(CO)` level in blood may induce premature birth, spontaneous abortions and deformed babies.

`color{red}("(𝐢𝐢) 𝐂𝐚𝐫𝐛𝐨𝐧 𝐝𝐢𝐨𝐱𝐢𝐝𝐞:")`

`color{green}(★)` `color{green}("𝐒𝐎𝐔𝐑𝐂𝐄𝐒 𝐎𝐅 𝐂𝐀𝐑𝐁𝐎𝐍 𝐃𝐈𝐎𝐗𝐈𝐃𝐄:")` Carbon dioxide `color{red}((CO_2))` is released into the atmosphere by respiration, burning of fossil fuels for energy, and by decomposition of limestone during the manufacture of cement. It is also emitted during volcanic eruptions.

`color{green}(★)` Carbon dioxide gas is confined to troposphere only. Normally it forms about 0.03 % by volume of the atmosphere.

`color{green}(★)` Excess of `color{red}(CO_2)` in the air is removed by green plants and this maintains an appropriate level of `color{red}(CO_2) ` in the atmosphere. Green plants require `color{red}(CO_2)` for photosynthesis and they, in turn, emit oxygen, thus maintaining the delicate balance.

`color{green}(★)` The increased amount of `color{red}(CO_2)` in the air is mainly responsible for global warming.

`color{green}(" ★ 𝐃𝐈𝐒𝐓𝐑𝐈𝐁𝐔𝐓𝐈𝐎𝐍 𝐎𝐅 𝐓𝐇𝐄 𝐒𝐎𝐋𝐀𝐑 𝐄𝐍𝐄𝐑𝐆𝐘")`

`color{green}(★)` About 75 % of the solar energy reaching the earth is absorbed by the earth’s surface, which increases its temperature.

`color{green}(★)` The rest of the heat radiates back to the atmosphere.

`color{green}(★)` Some of the heat is trapped by gases such as carbon dioxide, methane, ozone, chlorofluorocarbon compounds (CFCs) and water vapour in the atmosphere. Thus, they add to the heating of the atmosphere. This causes global warming.

`color{green}(★)` Just as the glass in a greenhouse holds the sun’s warmth inside, atmosphere traps the sun’s heat near the earth’s surface and keeps it warm. This is called natural greenhouse effect because it maintains the temperature and makes the earth perfect for life.

`color{green}("★ 𝐇𝐎𝐖 𝐆𝐑𝐄𝐄𝐍𝐇𝐎𝐔𝐒𝐄 𝐄𝐅𝐅𝐄𝐂𝐓 𝐖𝐎𝐑𝐊𝐒:")`

`color{green}(★)` In a greenhouse, solar radiations pass through the transparent glass and heat up the soil and the plants. The warm soil and plants emit infrared radiations. Since glass is opaque to infrared radiations (thermal region), it partly reflects and partly absorbs these radiations. This mechanism keeps the energy of the sun trapped in the greenhouse. Similarly, carbon dioxide molecules also trap heat as they are transparent to sunlight but not to the heat radiation. If the amount of carbon dioxide crosses the delicate proportion of 0.03 per cent, the natural greenhouse balance may get disturbed.

`color{green}(★)` Carbon dioxide is the major contributor to global warming.

`color{green}(★)` Besides carbon dioxide, other greenhouse gases are methane, water vapour, nitrous oxide, CFCs and ozone.

`color{green}(★)` Methane is produced naturally when vegetation is burnt, digested or rotted in the absence of oxygen. Large amounts of methane are released in paddy fields, coal mines, from rotting garbage dumps and by fossil fuels.

`color{green}(★)` Chlorofluorocarbons (CFCs) are man-made industrial chemicals used in air conditioning etc. CFCs are also damaging the ozone layer.

`color{green}(★)` Nitrous oxide occurs naturally in the environment.

`color{green}(★)` Increase in the global temperature increases the incidence of infectious diseases like dengue, malaria, yellow fever, sleeping sickness etc.

`color{green}(★)` Normally rain water has a pH of 5.6 due to the presence of `color{red}(H^+)` ions formed by the reaction of rain water with carbon dioxide present in the atmosphere.



`color{red}(H_2O (l) + CO_2 (g) → H_2CO_3 (aq))`

`color{red}(H_2 CO_3 (aq) → H^(+) (aq) + HCO_3^(-) (aq))`

`color{green}(★)` When the pH of the rain water drops below 5.6, it is called acid rain. Acid rain refers to the ways in which acid from the atmosphere is deposited on the earth’s surface.

`color{green}(★)` Oxides of nitrogen and sulphur which are acidic in nature can be blown by wind along with solid particles in the atmosphere and finally settle down either on the ground as dry deposition or in water, fog and snow as wet deposition.


`color{green}(★)` Acid rain is a byproduct of a variety of human activities that emit the oxides of sulphur and nitrogen in the atmosphere.

`color{green}(★)` `color{red}(SO_2)` and `color{red}(NO_2)` after oxidation and reaction with water are major contributors to acid rain, because polluted air usually contains particulate matter that catalyse the oxidation.

`color{red}(2SO_2 (g) + O_2 (g) + 2H_2O (l) → 2 H_2SO_4 (aq))`

`color{red}(4NO_2 (g) + O_2 (g) + 2 H_2O (l) → 4 HNO_3 (aq))`

`color{green}(★)` Ammonium salts are also formed and can be seen as an atmospheric haze (aerosol of fine particles). Aerosol particles of oxides or ammonium salts in rain drops result in wet deposition.

`color{green}(★)` `color{red}(SO_2)` is also absorbed directly on both solid and liquid ground surfaces and is thus deposited as dry-deposition.

`color{green}("★ 𝐄𝐅𝐅𝐄𝐂𝐓𝐒 𝐎𝐅 𝐀𝐂𝐈𝐃 𝐑𝐀𝐈𝐍")`

`color{green}(★)` Acid rain is harmful for agriculture, trees and plants as it dissolves and washes away nutrients needed for their growth.

`color{green}(★)` It causes respiratory ailments in human beings and animals.

`color{green}(★)` When acid rain falls and flows as ground water to reach rivers, lakes etc. it affects plants and animal life in aquatic ecosystem.

`color{green}(★)` It corrodes water pipes resulting in the leaching of heavy metals such as iron, lead and copper into the drinking water.

`color{green}(★)` Acid rain damages buildings and other structures made of stone or metal. The Taj Mahal in India has been affected by acid rain.