Environmental Chemistry
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Direct answer
Tropospheric pollution begins with a short list of primary pollutants — oxides of carbon, sulphur and nitrogen, hydrocarbons and particulates — whose interactions brew the secondary ones. Carbon monoxide binds haemoglobin roughly 200-250 times more strongly than oxygen, forming carboxyhaemoglobin and starving tissues of oxygen; sulphur dioxide and nitrogen oxides dissolve into rain to give acid rain below pH 5.6, corroding marble through the calcium carbonate-to-sulphate reaction that yellows the Taj Mahal ("marble cancer"). In warm, sunny, traffic-heavy cities, nitrogen dioxide photolyses (NO2 + sunlight → NO + O; O + O2 → O3) and the ozone so formed reacts with hydrocarbons to give PAN (peroxyacetyl nitrate), the eye-watering oxidant of photochemical smog — distinct from the reducing, cool-weather classical London smog. Stratospherically, CFC-released chlorine radicals catalytically shred ozone (Cl + O3 → ClO + O2; ClO + O → Cl + O2), one chain destroying many ozone molecules before the Montreal Protocol (1987) intervened.
What you must remember
- Primary versus secondary: CO, NO, NO2, SO2, hydrocarbons and particulates are primary; PAN, ozone and acid rain are secondary, formed in air after emission.
- CO toxicity: carboxyhaemoglobin formation, affinity commonly quoted at 200-250 times that of oxygen; CO from incomplete combustion does not irritate — it silently asphyxiates.
- Acid rain chemistry: rain below pH 5.6 from H2SO4, HNO3 and HCl; marble cancer: CaCO3 + H2SO4 → CaSO4 + H2O + CO2; mobilises toxic metals in soil, damaging aquatic life, limestone and steel.
- Smog pair: classical (London) smog — reducing, wet, cool, smoke plus sulphur dioxide; photochemical (Los Angeles) smog — oxidising, dry, sunny, ozone plus PAN; PAN spells peroxyacetyl nitrate.
- Ozone hole mechanism: UV splits CFCs in the stratosphere; the chlorine radical cycles through Cl/ClO consuming ozone catalytically; the hole is worst over Antarctica in spring; Montreal Protocol, 1987, curbed CFC production.
- Greenhouse gases: CO2 (about 60 per cent of the contribution, commonly quoted), methane, water vapour, N2O and CFCs trap outgoing infrared; the natural greenhouse effect keeps Earth about 33 °C warmer than it would otherwise be.
- Water pollution metrics: BOD is the oxygen consumed by biodegradable matter in five days at 20 °C (clean water below 5 ppm); COD measures everything oxidisable chemically and always exceeds BOD; fluoride near 1 ppm protects teeth but above about 2 ppm mottles them.
- Soil and pesticide corner: DDT and other non-biodegradable pesticides biomagnify up food chains — the NCERT caution that made biomagnification an exam word.
Tracing one chlorine atom through the stratosphere
Follow a CFC molecule released from an old refrigerator. Inert in the troposphere, it drifts upward until hard ultraviolet cracks a C–Cl bond. The freed chlorine radical meets ozone: Cl + O3 → ClO + O2. ClO then collides with a free oxygen atom and regenerates Cl: ClO + O → Cl + O2. Net: one ozone and one oxygen atom become two ordinary oxygen molecules, chlorine emerging unspent for the next cycle. A single chlorine atom therefore dismantles many thousands of ozone molecules before being locked away as HCl — why parts-per-billion CFC levels thinned the ozone layer, why the Antarctic spring hole became the 1980s' environmental signature, and why the Montreal Protocol (1987) intervened.
Where the exam plants its environmental traps
The chapter rewards NCERT-line precision. The PAN expansion (peroxyacetyl nitrate) belongs to photochemical, oxidising smog; candidates confuse it with the reducing London type. The pH boundary of acid rain is 5.6, not 7 — rain is naturally mildly acidic from dissolved CO2. BOD versus COD: COD is always the larger number, since permanganate or dichromate oxidises non-biodegradable material too. The greenhouse list must include water vapour; CFCs appear on both rosters (greenhouse and ozone depletion) but for different crimes. The CO number (200-250 times haemoglobin affinity) and the fluoride window (about 1 ppm good, above 2 ppm harmful) are the two most quotable figures. And the Taj Mahal line — marble cancer from acid rain's sulphate attack — remains the favourite India-anchored example here.
Frequently asked questions
Why is rain with pH below 5.6 called acid rain?
Rain in equilibrium with atmospheric CO2 is naturally about pH 5.6 from carbonic acid; lower values indicate sulphuric, nitric or hydrochloric acid from pollution.
What is PAN and in which smog does it occur?
Peroxyacetyl nitrate, formed when ozone reacts with hydrocarbon radicals — the signature eye irritant of photochemical (Los Angeles type) smog.
How does one chlorine radical destroy many ozone molecules?
The radical cycles between Cl and ClO (Cl + O3 → ClO + O2; ClO + O → Cl + O2), regenerating itself with each ozone destroyed — catalytic chain destruction.
What is the difference between BOD and COD?
Biochemical oxygen demand measures what microbes consume in five days at 20 °C, while chemical oxygen demand measures total oxidisable matter, so COD always exceeds BOD.
Why does carbon monoxide poisoning starve tissues of oxygen?
CO binds haemoglobin about 200-250 times more strongly than oxygen, forming carboxyhaemoglobin that blocks oxygen transport without warning irritation.