Chemistry 2nd Paper · Classes 11-12 · Chapter 1 · 1.10
Air Pollutants.
On an early morning a brown haze hangs low over the city and the nose and eyes sting. Yet no haze came out of the chimney or the exhaust pipe; what came out was colourless NO. So where did the brown layer come from? It was made in the air itself. This note shows which pollutants arrive straight from a source, which are built in the air, and what each does to the body and the environment.
1. Pollutants, primary and secondary
Coal, natural gas, kerosene, diesel and petrol are fossil fuels. Burning them in industry produces the waste gases CO, CO₂, SO₂, SO₃, NO and NO₂, which pollute the air.
When the level of such a waste gas in the air goes above what the environment and the human body can tolerate, it is called an air pollutant.
A pollutant that leaves its source and enters the air unchanged is a primary pollutant. Examples: CO, CO₂, SO₂, NO, NH₃, H₂S, dust, smoke, ash and hydrocarbons.
When a primary pollutant combines with one or more gaseous components of the environment and makes a new pollutant, that is a secondary pollutant. Examples: SO₃, H₂SO₄ vapour, NO₂, N₂O₅ and peroxyacyl nitrates (PANs).
Now sort them yourself. NO₂ and O₃ are the two everyone gets wrong; their whole story is in section 5.
Straight from the source or made in the air?
The organism part that reacts badly, or the structure that is badly damaged, under a pollutant's effect is its receptor (or target). The receptors of acid rain are living things and limestone structures.
A medium of the environment that keeps reacting with a pollutant, absorbing it or reducing its amount, is that pollutant's sink. The sinks of CO₂ are sea water and green plants; the sink of H₂SO₄ vapour is limestone. The two reactions: CO₂ + H₂O → H₂CO₃, and H₂SO₄ + CaCO₃ → CaSO₄ + CO₂ + H₂O.
2. Safe limits
The level above which the environment and living things are harmed is the pollutant's safe limit, or TLV (Threshold Limiting Value). For example CO 0.15 ppm, CO₂ 350 ppm, SO₂ 0.05 ppm.
The highest safe levels given by the WHO: SO₂ 0.05 ppm; SO₃ 0.02 ppm; CO 0.15 ppm; CO₂ 350 ppm; NO 350 ppb; NO₂ 250 ppb; CH₄ 2.0 ppm; H₂S 0.0002 ppm. Watch the units: NO and NO₂ are in ppb, the rest in ppm. Put each gas in the box with its limit.
Which gas, which limit
3. CO and CO₂
CO and CO₂ are both colourless, odourless and poisonous. Incomplete burning of coal and hydrocarbon fuels gives CO; complete burning gives CO₂. Sources: gas manufacturing plants, coal mines, petroleum refineries, blast furnaces and automobile waste gas. As a greenhouse gas, CO₂ causes global warming.
CO is called the silent killer gas. Unnoticed, it goes into the lungs with the breath, is absorbed into the blood, binds the Fe²⁺ of haemoglobin as a ligand, and stops oxyhaemoglobin (HbO₂) from forming.
The two reactions: Hb + O₂ → HbO₂; HbO₂ + CO → HbCO + O₂. CO breaks down oxyhaemoglobin and forms carboxyhaemoglobin (HbCO). With oxygen short, a headache and drowsiness come first; if more CO is absorbed, death follows.
Haemoglobin (Hb) sits in the red blood cells. It is made of a basic protein called globin and a complex compound containing Fe²⁺, ferroprotoporphyrin (heme, formula C₃₄H₃₂FeN₄O₄).
4. SO₂ and SO₃
SO₂ and SO₃ are colourless gases with an acidic smell, and they cause breathing trouble. They form when sulphur, sulphur-containing coal and natural gas, or mineral oil is burnt. Industrial sources: sulphuric acid plants, paper manufacturing plants, thermal power stations, automobile waste gas, and plants extracting Zn, Cu and Pb from metal sulphide ores.
They cause acid rain. They irritate the nose and eyes, inflame the airways, and cause bronchitis and asthma.
5. From NO to photochemical smog
NO is a colourless neutral gas; NO₂ is a brown acidic gas. About 75 crore automobile engines worldwide and old industrial plants burn petroleum fuel, and their waste gas carries NO.
The O₂ of air oxidises NO to NO₂. In dry air and sunlight (hν), NO₂ breaks into a free O atom and NO. The free O atom joins O₂ and forms highly reactive ozone (O₃).
The air over a city. Switch the sources on one by one, then let the sun out.
Which source, which pollutant?
The meter lengths are a model, not measured values. Which gas comes from which source is the book's.
NO, NO₂, O₃ and other pollutants, with burnt hydrocarbons and free radicals, form the mixed pollutant peroxyacyl nitrates, R-CO-O-O-NO₂ (PANs). This mixture makes a brown layer of haze low in the sky of cities in the early morning. That is photochemical smog; smog = smoke + fog. It badly irritates the nose and eyes. Put the steps in order.
From NO to smog
So why is O₃ secondary? Because it does not come straight from a source. In the air, the free O atom that NO₂ gives off in sunlight joins O₂ to make O₃. The book does not name O₃ among its secondary examples; this comes from putting the definition beside the formation steps.
One trap: CFC takes no part in forming photochemical smog. The participants are hydrocarbons, NO, NO₂ and O₃. CFC is not in the book's smog chain.
6. Effects of air pollution
On weather: more greenhouse gases raise the temperature of the earth's surface; suspended pollutants make fog in the morning and afternoon, which makes travel difficult. On health: more dust causes breathing difficulty, too much CO poisons the blood, and photochemical smog badly irritates the nose and eyes.
On the environment: the acidic secondary pollutants SO₃, H₂SO₄ vapour, NO₂ and N₂O₅ mix with rain water and cause acid rain. This lowers the pH of soil and water bodies; fish and aquatic plants die, forests are destroyed, crop yield is hurt and the ecosystem is disturbed.
7. Check yourself
Three questions in the exam's style. Start with the first.
Question 1
Which compound forms from the incomplete burning of fuel in a brick kiln?
In the second, watch the kind of pollutant.
Question 2
Which of the following is a secondary pollutant?
The last one tests your memory.
Question 3
Which substance takes no part in forming photochemical smog?
The whole note on seven cards.
Seven cards
Source: Chemistry 2nd Paper, Classes 11-12 (NCTB), Chapter 1 “Environmental Chemistry”, section 1.10.