Air Pollutants and Sources Questions and Answers

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. Chlorofluorocarbons are chemically inert in the troposphere, which is what made them attractive as refrigerants and propellants, and their atmospheric lifetimes run to many decades - CFC-12 to about a hundred years.

They are removed only by photolysis in the stratosphere, which is also where they release the chlorine that destroys ozone. Emissions therefore continue to act long after production ceased, and the ozone layer is expected to recover only towards the middle of this century despite the Montreal Protocol's success.

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. An emission factor relates the mass emitted to a unit of activity - grams per vehicle kilometre, kilograms per tonne of fuel, kilograms per head of livestock - and those activity data are already collected for quite other purposes.

So an inventory is built by multiplying recorded activity by tabulated factors, which is the only tractable approach when a city contains hundreds of thousands of emitters. Measurement is reserved for major point sources and for validating the factors themselves.

Answer: C

Correct answer: 1, 2, 3, 4

The chain runs from emission to observed peak. Traffic emits oxides of nitrogen and volatile organic compounds (1), chiefly during the morning rush. Sunlight then drives the radical chemistry that converts nitric oxide to nitrogen dioxide without consuming ozone (2).

Ozone and secondary aerosol accumulate through the day as that chemistry proceeds (3), and because the air mass is moving while it reacts, concentrations peak in the afternoon downwind of the city (4). That displacement is characteristic - suburban and rural sites downwind often record higher ozone than the city centre that emitted the precursors.

Answer: D

Correct answer: 1, 2, 4, 3

The order follows the method. Sources are first classified as point, line or area (1), because the classification determines what activity data are needed and how each will be modelled. Activity data are then collected for each category (2) - fuel consumed, vehicle kilometres, livestock numbers.

Emission factors are applied to those data (4) to convert activity into emitted mass, and the results are finally aggregated to totals by pollutant and by area (3). The aggregation must come last, since it is the sum of everything computed before it.

Answer: A

Correct answer: 1, 2, 3, 4

The fractions are nested, each contained within the one before. Total suspended particulate (1) is the broadest, covering everything an inlet will collect, up to tens of micrometres. PM10 (2) is the fraction below 10 micrometres aerodynamic diameter, which can penetrate the thoracic region.

PM2.5 (3) is the finer respirable fraction reaching the alveoli, and ultrafine particles below 0.1 micrometres (4) are finer still, contributing almost nothing to mass but dominating particle number and able to cross into the bloodstream.

Answer: B

Correct answer: 1, 2, 3, 4

The chain begins with emission of sulphur dioxide and nitrogen oxides from combustion (1). In the atmosphere these oxidise, in gas phase and in cloud droplets, to sulphuric and nitric acid (2).

Because that transformation takes hours to days, the air mass carries the acids over long distances meanwhile (3), and they are finally deposited in rain, snow or as dry deposition far from the source (4). The delay between emission and deposition is precisely what makes acid deposition a transboundary problem requiring international agreement rather than local control.

Answer: C

The compounds are too stable to break down in the troposphere, which is precisely what allows them to reach the stratosphere intact. One chlorine atom then destroys many thousands of ozone molecules before it is removed.

Answer: D

Correct answer: Only three

Three pairs are right. Sulphur dioxide comes from combustion of sulphur-bearing coal and oil; carbon monoxide from incomplete combustion in engines and stoves; and ammonia from livestock manure and fertiliser.

The fourth is wrong. Ground level ozone is not emitted by anything - it is a secondary pollutant formed in the atmosphere when oxides of nitrogen and volatile organic compounds react in sunlight. Vehicles emit its precursors, which is a different statement with quite different implications for control.

Answer: A

Correct answer: Only three

Three pairs are right. PM2.5 is associated principally with cardiopulmonary disease, penetrating deep into the lung and entering the circulation; chlorofluorocarbons deplete stratospheric ozone; and sulphur dioxide is a principal cause of acid deposition.

The fourth is wrong. Lead is a neurotoxin, damaging cognitive development in children irreversibly; it plays no part in acid deposition, which arises from the oxidation of sulphur and nitrogen oxides to strong acids.

Answer: B

Correct answer: Only three

Three pairs are right. An industrial stack is a point source with a known location, height and gas conditions; a busy road is a line source; and a residential district is an area source represented by an emission density.

The fourth is wrong. A power station is the archetypal point source - a single tall stack whose plume can be modelled directly - not an area source. The classification is not about the size of the emission but about whether it issues from one identifiable location.