PM2.5, the pollutant reported in air quality bulletins, refers to particles whose diameter is:

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.