A primary air pollutant is one that is
Answer: A
Sulphur dioxide from a boiler stack and carbon monoxide from a vehicle exhaust are typical examples. Secondary pollutants such as ozone and much fine particulate form later in the air itself and cannot be traced to a single emission point.
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Ground level ozone is classified as a secondary pollutant because it is
Answer: B
Sunlight drives the reaction between the two precursor families, which is why urban ozone peaks in the afternoon and downwind of the city rather than in it. Controlling it means controlling its precursors rather than ozone itself.
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PM2.5 refers to particulate matter with an aerodynamic diameter
Answer: C
The fraction penetrates deep into the alveolar region and is the size class most strongly associated with cardiopulmonary mortality. Its long atmospheric residence time also makes it a regional rather than a local problem.
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Aerodynamic diameter is used to describe a particle because it
Answer: D
It is the diameter of a unit density sphere with the same settling velocity, so two particles of quite different shape and density share it if they behave alike. Deposition in the lung and capture in a control device both follow that behaviour.
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The principal anthropogenic source of sulphur dioxide is
Answer: A
Correct answer: Combustion of sulphur-bearing fuel
Sulphur dioxide comes overwhelmingly from burning fuels that contain sulphur - coal and heavy fuel oil above all, in power stations, industrial boilers and smelters. The sulphur in the fuel oxidises almost completely during combustion, so the emission scales directly with the fuel's sulphur content.
That direct relationship is why fuel switching and desulphurisation are such effective controls. Road transport contributes little now that fuel sulphur limits are tight; fertiliser use releases ammonia rather than sulphur dioxide; and refrigerants are a stratospheric ozone and climate issue.
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Oxides of nitrogen from combustion arise chiefly as
Answer: B
Correct answer: Thermal NOx from atmospheric nitrogen
Most oxides of nitrogen from combustion are thermal NOx, formed when nitrogen and oxygen in the combustion air react at high temperature by the Zeldovich mechanism. The reaction becomes significant above about 1300 degrees Celsius and rises steeply thereafter, which is why the formation depends on flame temperature far more than on the fuel.
Fuel NOx, from nitrogen chemically bound in the fuel, matters for coal and heavy oil but is secondary for gas. The emission leaves the stack largely as nitric oxide, which oxidises to nitrogen dioxide in the atmosphere, so it is not emitted directly as \( \mathrm{NO_2} \).
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Carbon monoxide is emitted principally as a result of
Answer: C
Correct answer: Incomplete combustion
Carbon monoxide is the product of combustion that did not go to completion - insufficient oxygen, inadequate mixing, too short a residence time or too low a temperature leave carbon oxidised only as far as CO rather than \( \mathrm{CO_2} \).
Hence its sources: petrol engines under rich or transient operation, domestic solid fuel stoves, and open burning. Complete combustion produces carbon dioxide and no carbon monoxide, which is why good burner design and adequate excess air are the primary controls. It is a primary pollutant, not formed photochemically.
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Volatile organic compounds are important in air quality principally because they
Answer: D
Correct answer: Act as precursors to photochemical ozone
Volatile organic compounds matter chiefly as precursors. In sunlight they react with oxides of nitrogen through a chain of radical reactions that converts nitric oxide to nitrogen dioxide without consuming ozone, allowing ozone to accumulate - the basis of photochemical smog.
Some individual compounds are also directly toxic or carcinogenic, benzene among them, but the reason the class as a whole is regulated is ozone formation. They are not the bulk of coarse particulate, and it is the chlorofluorocarbons, a separate group, that deplete stratospheric ozone.
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Ozone in the stratosphere and ozone at ground level differ in that
Answer: A
Correct answer: Stratospheric ozone is beneficial
The molecule is identical in both places - \( \mathrm{O_3} \) - but its effect depends entirely on where it is. In the stratosphere it absorbs biologically damaging ultraviolet radiation and is essential, which is why its depletion by chlorofluorocarbons was treated as an emergency.
At ground level the same molecule is a powerful oxidant that damages the respiratory tract, injures vegetation and attacks materials, so it is a regulated pollutant. Hence the phrase "good up high, bad nearby": not both beneficial, not both harmful, and not chemically different.
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Chlorofluorocarbons were phased out under the Montreal Protocol because they
Answer: B
Chlorine released by ultraviolet photolysis catalytically destroys ozone, and a single atom can consume many thousands of molecules. Their long atmospheric lifetime means the recovery of the ozone layer will take several decades.
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