Q.Which of the following is a secondary air pollutant?

Leaded petrol releases lead compounds in vehicle exhaust; lead is a cumulative neurotoxin, making it the most harmful metal pollutant of automobile exhaust among the options given.

Before the switch to unleaded petrol, tetraethyl lead was added to petrol as an anti-knocking agent. When burnt, it released lead particulates into the air through vehicle exhaust. Lead is a heavy metal that accumulates in the body (bones, blood, soft tissue) and is especially harmful to the nervous system, causing anaemia, kidney damage, and impaired mental development, part …

Fine particulate matter of diameter 2.5 micrometres or less (PM2.5) is flagged by pollution-control bodies as most harmful because such small particles bypass the body's natural filtering and reach deep into the lungs and even the bloodstream.

Airborne particulate matter is classified by size — coarser particles (like PM10, up to 10 micrometres) are trapped more effectively in the nose and upper airway, while fine particles of 2.5 micrometres or smaller (PM2.5) can travel deep into the alveoli of the lungs and cross into the bloo …

CNG combusts more completely than diesel, so it emits far fewer pollutants, making it a cleaner automobile fuel choice.

Compressed Natural Gas (CNG) is largely methane, and it burns almost completely in an engine, producing mainly carbon dioxide and water vapour with only trace amounts of other pollutants. Diesel, by contrast, undergoes comparatively incomplete combustion, releasing significant quantities of unburnt hydrocarbons, carbon monoxide, oxides of nitrogen, sulphur compounds, and soot/particulate matter — all major contributors to urban air pollution and respiratory illness. CNG is also cheaper than diesel/petrol and, being a gas supplied through metered pumps, is far less prone to adulteration or pilferage compared to liquid diesel.

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Secondary pollutants are formed in the atmosphere by reactions between primary pollutants, unlike primary pollutants which are released directly.

Primary pollutants (e.g., CO, CO2, and particulate aerosols) are emitted directly into the atmosphere from a source (vehicle exhaust, industry, etc.). Secondary pollutants are NOT directly emitted but are formed in the atmosphere when primary pollutants react with each other or with atmospheric constituents, often in the presence of sunlight. PAN (peroxyacetyl nitrate) is formed by the photochemical reaction of hydrocar …

The Bhopal gas tragedy was caused by the leakage of methyl isocyanate (MIC) gas.

On the night of 2–3 December 1984, a highly toxic gas, methyl isocyanate (MIC), leaked from the Union Carbide India Limited pesticide manufacturing plant in Bhopal, Madhya Pradesh. MIC is an intermediate used in the manufacture of the pesticide carbaryl (Sevin). Water entered a storage tank of MIC, triggering an uncontrolled exothermic reaction that released a dense cloud of the gas over the surrounding city. It caused immediate and long-term respiratory, ocular and other health damage, and remains one of the world's worst industrial disasters — a case study of the human cost of air pollution and industrial hazard mismanagement. …

CNG stands for Compressed Natural Gas, which is predominantly methane.

Compressed Natural Gas (CNG) is natural gas — composed mainly of methane (CH4) — that has been compressed to less than 1% of its volume at standard atmospheric pressure for use as vehicle fuel. It burns more cleanly than petrol or diesel, producing fewer particulates, unburnt hydrocarbons and toxic emissions, which is why many Indian cities adopted it (e.g., for public transport) as a pollution-cont …

Delhi's winter reliance on air filters (A) is correctly explained by heavy fossil-fuel combustion releasing hydrocarbons and other pollutants (R).

During Delhi winters, cold, still air traps vehicular and industrial emissions close to the ground (a temperature-inversion effect), sharply raising particulate matter and hydrocarbon pollution; this is precisely why residents increasingly rely on indoor air filters/purifiers to protect themselves, making the assertion a real, well-documented public-health response. The reason correctly identifies the underlying cause (fossil-fuel-derived hydrocarbon emissions) …

Unlike SO2, CFCs and aerosols, which are released as harmful contaminants, CO2 is a natural atmospheric gas essential for photosynthesis, so it is not classed as a conventional air pollutant among these options.

An air pollutant is any substance introduced into the atmosphere (usually by human activity) that has a harmful effect on living organisms or the environment. Sulphur dioxide (SO2) is released by burning fossil fuels and causes respiratory irritation and acid rain. Chlorofluorocarbons (CFCs) are synthetic industrial/refrigerant chemicals that deplete the stratospheric ozone layer. Aerosols (fine solid/liquid particles suspended in air, e.g. from combustion, spray propellants) directly cause respiratory harm and haze. Carbon dioxide (CO2), by contrast, is a naturally occurring atmospheric gas that plants require for p …

Lead in fuel permanently deactivates the metal catalyst inside a catalytic converter, so unleaded fuel is essential for it to keep working.

A catalytic converter fitted in a vehicle's exhaust system uses metal catalysts, commonly platinum and palladium, to convert harmful pollutant gases — carbon monoxide (CO), unburnt hydrocarbons, and nitrogen oxides (NOₓ) — into comparatively harmless carbon dioxide, water and nitrogen. Leaded petrol/diesel contains tetraethyl lead as an anti-knocking agent; if such fuel is used, the lead coats and 'poisons' the surface of the platinum/palladium catalyst, permanently destroying its catalytic activity. This is why only lead-free (unleaded) fuel is recommend …