Air Pollution and Air Quality: Pollutants, AQI and Laws

The correct answer is C, Peroxyacetyl nitrate. A secondary pollutant is never emitted from a source; it is manufactured in the atmosphere itself when primary pollutants react with one another or with sunlight. Peroxyacetyl nitrate, usually written PAN, is formed in sunlight from oxides of nitrogen and unburnt hydrocarbons, and along with ground-level ozone it is the standard example of a secondary pollutant. Option A is wrong because sulphur dioxide comes straight out of chimneys that burn sulphur-bearing coal and oil. Option B is wrong because carbon monoxide is released directly by the incomplete burning of fuel, chiefly in vehicle engines. Option D is wrong because particulate matter is emitted directly by combustion, construction, road dust and industry, which makes all three of them primary pollutants. The quickest test for such a question is to ask whether the substance can come out of a chimney or an exhaust pipe as it is; if it can, it is primary.

The correct answer is B, Ozone. Photochemical smog, also called Los Angeles smog, forms in warm, dry, sunny weather when oxides of nitrogen from vehicle exhaust and unburnt hydrocarbons react under ultraviolet light. Its chief oxidising components are ground-level ozone, peroxyacetyl nitrate and aldehydes, which is why it is described as oxidising smog and why it irritates the eyes and the throat. Option A is wrong because sulphur dioxide is the marker of classical or London smog, which is reducing and forms in cold, humid air. Option C is wrong because methane is a greenhouse gas from paddy fields, cattle and landfills, not a smog component. Option D is wrong because carbon dioxide drives global warming but is not a constituent of smog. Note the reversal that papers like to set: ozone in the stratosphere shields the earth from ultraviolet radiation, while the same gas at ground level is a harmful secondary pollutant.

The correct answer is A, combines with haemoglobin to form carboxyhaemoglobin. Carbon monoxide binds to the haemoglobin of the blood roughly two hundred times more readily than oxygen does, and the carboxyhaemoglobin so formed cannot carry oxygen. Tissues are therefore starved of oxygen, which causes headache, giddiness and, in a closed room with a burning coal fire or a running engine, unconsciousness and death. Option B is wrong because it is sulphur dioxide and the oxides of nitrogen that dissolve and oxidise to give sulphuric and nitric acid in rain. Option C is wrong because chlorofluorocarbons and halons deplete stratospheric ozone. Option D is wrong because settling in the alveoli describes fine particulate matter such as PM2.5, not a gas. Remember that carbon monoxide is colourless and odourless, which is why it gives no warning of its presence, and that a well-ventilated room is the simplest protection against it.

The correct answer is C, Sulphur dioxide. Sulphur dioxide, released when coal and oil containing sulphur are burnt, is oxidised in the atmosphere to sulphur trioxide, which dissolves in water droplets to give sulphuric acid. The oxides of nitrogen give nitric acid in the same way, and together they bring rain whose pH falls below about 5.6, the value of normal rain made mildly acidic by dissolved carbon dioxide. Option A is wrong because carbon monoxide is a poison that acts on haemoglobin and does not form an acid in rain. Option B is wrong because methane is a greenhouse gas and not acidic. Option D is wrong because argon is a noble gas, chemically inert, and takes no part in such reactions at all. Acid rain lowers the pH of lakes and soils, releases toxic aluminium from soil minerals and corrodes limestone and marble buildings, which is why it is treated as a pollution problem that crosses borders.

The correct answer is B, Sulphur dioxide. Sulphur dioxide from industry and traffic around Agra is oxidised in the air and comes down as sulphuric acid, which attacks the calcium carbonate of marble. The stone loses its polish, turns yellow and develops a pitted, flaking surface, and the popular name for this decay is marble cancer. It was concern over this damage that led to restrictions on polluting industry in the Taj Trapezium Zone. Option A is wrong because carbon dioxide makes rain only mildly acidic and is the driver of global warming, not of this corrosion. Option C is wrong because chlorofluorocarbons deplete the ozone layer. Option D is wrong because ground-level ozone harms lungs, crops and rubber rather than marble. The same acid attack explains the decay of other limestone and marble monuments, so a question naming any such building and asking for the gas responsible is almost always answered by sulphur dioxide.

The correct answer is C, 2.5 micrometres or less. Particulate matter is graded by the diameter of the particles measured in micrometres, so PM10 means particles of ten micrometres or less and PM2.5 means particles of 2.5 micrometres or less, which is about thirty times finer than a human hair. This fine fraction matters most for health because the nose and throat cannot filter it out; it travels deep into the lungs, settles in the alveoli and can pass into the bloodstream. Option A is wrong because a millimetre is a thousand times larger than a micrometre, a size no longer suspended in air. Option B is wrong because a nanometre is a thousand times smaller. Option D is wrong because a centimetre-sized particle is not particulate matter at all.

The correct answer is D, phasing out of ozone-depleting substances. The Montreal Protocol, adopted in 1987 under the Vienna Convention, commits its parties to phasing out chlorofluorocarbons, halons and other chemicals that destroy stratospheric ozone, and it is regarded as the most successful environmental treaty for that reason. Option A is wrong because greenhouse gas emissions are the subject of the Kyoto Protocol and the Paris Agreement under the climate convention. Option B is wrong because wetlands of international importance are protected under the Ramsar Convention of 1971. Option C is wrong because trade in endangered species is regulated by CITES, the Convention on International Trade in Endangered Species of Wild Fauna and Flora. A note worth carrying into the exam hall is that 16 September is observed as World Ozone Day, marking the day the Montreal Protocol was adopted, and that the Kigali Amendment later brought hydrofluorocarbons within the same treaty.

The correct answer is B, Poor. The National Air Quality Index has six bands: 0 to 50 is Good, 51 to 100 Satisfactory, 101 to 200 Moderate, 201 to 300 Poor, 301 to 400 Very Poor and 401 to 500 Severe. A sub-index is computed for each pollutant for which enough data are available, and the worst of these sub-indices becomes the AQI of the day, the pollutant responsible being named as the prominent pollutant. Option A is wrong because Moderate covers 101 to 200. Option C is wrong because Very Poor begins only at 301. Option D is wrong because Severe is the topmost band, from 401 to 500, at which even healthy people are affected and emergency restrictions are usually imposed. The index is published for eight pollutants, so it is worth memorising the bands as three pairs: fifty and a hundred, two hundred and three hundred, four hundred and five hundred.

The correct answer is A, the Air (Prevention and Control of Pollution) Act, 1981. This Act is the specific law on air pollution. It empowers the Central and State Pollution Control Boards to lay down emission standards for industry and for vehicles, to declare air pollution control areas and to require consent before a polluting plant is set up or operated. Option B is wrong because the Environment (Protection) Act of 1986, passed after the Bhopal gas tragedy, is the umbrella law covering all media of the environment rather than air alone. Option C is wrong because the Water Act of 1974 concerns water pollution, though it is the Act under which the Central Pollution Control Board was first created. Option D is wrong because the Wildlife (Protection) Act of 1972 protects species and habitats.

The correct answer is C, Particulate matter. An electrostatic precipitator gives the dust and ash particles in the flue gas an electric charge as they pass an electrode, and then collects them on plates carrying the opposite charge; the plates are shaken so that the collected ash falls into a hopper. It is the standard device for cleaning the chimney gases of a coal-fired power station and can remove the great majority of the particles. Options A and B are wrong because gases are not charged and collected this way; sulphur dioxide and the oxides of nitrogen are removed instead by a scrubber, in which the gas is washed with water or a lime slurry. Option D is wrong because carbon dioxide is not removed by either device in ordinary practice.

The correct answer is B, inactivates the catalyst in the converter. A catalytic converter holds platinum, palladium and rhodium on a honeycomb support, which oxidise carbon monoxide and unburnt hydrocarbons to carbon dioxide and water and reduce the oxides of nitrogen to nitrogen. Lead compounds from leaded petrol coat the surface of these metals and destroy their catalytic action, a process called catalyst poisoning, after which the converter no longer cleans the exhaust. Option A is wrong because the problem is chemical poisoning, not a rise in temperature. Option C is wrong because lead does not form a solid with carbon monoxide in the exhaust. Option D is wrong because tetraethyl lead was in fact added to petrol to raise its octane number, not to lower it. India therefore phased out leaded petrol before the tighter Bharat Stage norms, since the converter is the main device by which a modern engine meets them.

The correct answer is A, Lichens. A lichen is a partnership of an alga and a fungus and takes its water and nutrients straight from the air, with no protective cuticle and no roots. It therefore absorbs dissolved sulphur dioxide along with moisture and dies even where the concentration is too low to harm most plants, so the disappearance of lichens from tree trunks in a city is read as a sign of sulphur dioxide pollution. Option B is wrong because earthworms indicate the health and moisture of soil. Option C is wrong because honey bees are watched as indicators of pesticide use and of the state of flowering plants. Option D is wrong because water hyacinth, the Bengal terror, indicates nutrient-rich polluted water rather than polluted air. Bio-indicator questions are best answered by asking which organism takes its living directly from the medium named, since that is the one which suffers first when the medium is fouled.