# Environmental Chemistry > Environmental chemistry for NEET Chemistry: primary pollutants, photochemical smog PAN, acid rain, ozone depletion by CFCs, BOD COD and biomagnification. - Canonical URL: https://prepelephant.com/topics/neet-ug/chemistry/environmental-chemistry-neet - Exam / course: NEET-UG · Subject: Chemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Environmental Chemistry", PrepElephant, https://prepelephant.com/topics/neet-ug/chemistry/environmental-chemistry-neet ## 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.