Air Pollutants and Sources Questions and Answers

Answer: C

Correct answer: It means exactly 2.5 micrometres physically

This is the false statement on two counts. PM2.5 is a fraction with aerodynamic diameter below 2.5 micrometres, not particles of exactly that size; and the criterion is aerodynamic, not physical, so it combines size, shape and density rather than measuring the particle's dimensions.

The other three are true: it is defined by aerodynamic diameter below 2.5 micrometres, it penetrates to the alveolar region, and its residence time of days to weeks is what allows the long-range transport that makes fine particulate a regional problem.

Answer: D

Correct answer: Raising the peak flame temperature

Raising the peak flame temperature increases thermal NOx rather than reducing it. The Zeldovich mechanism has a high activation energy, so its rate climbs steeply above about 1300 degrees Celsius - temperature is the single most powerful variable.

The other three all lower that peak. Staged combustion avoids any zone running at the stoichiometric maximum, flue gas recirculation dilutes the charge with inert products that absorb heat, and reducing excess air within safe limits lowers the flame temperature while still ensuring complete combustion.

Answer: A

Correct answer: It is a strong stratospheric ozone depleter

Ammonia has nothing to do with stratospheric ozone. It is highly soluble and reactive, so it is removed within the lower troposphere in hours to days and never reaches the stratosphere - unlike the inert chlorofluorocarbons, which survive to be photolysed there.

The other three are all genuine reasons it matters. It arises mainly from agriculture, it neutralises sulphuric and nitric acid to form ammonium sulphate and nitrate, and those salts contribute a substantial share of fine particulate mass, including in cities far from any farm.

Answer: B

Correct answer: Unworkable, since no source emits ozone

The proposal cannot work, because an emission limit presupposes an emission and ozone is a secondary pollutant - formed entirely in the atmosphere from oxides of nitrogen and volatile organic compounds in sunlight. There is no stack or exhaust from which ozone issues to be limited, for a power station or a vehicle alike.

Ozone is regulated instead by an ambient standard on the concentration measured in the air, together with emission limits on its precursors. The precursor route is complicated by the non-linear chemistry linking the two.

Answer: C

Correct answer: It occurs hundreds of kilometres away

Acid deposition is a long-range phenomenon. Sulphur dioxide and nitrogen oxides must first be oxidised to sulphuric and nitric acid, which takes hours to days, and the air mass travels throughout - so deposition typically occurs hundreds or even thousands of kilometres downwind of the source.

That displacement is what makes it a transboundary political problem rather than a local one, and it was the basis of the international conventions negotiated to address it. It is a tropospheric process, not a stratospheric one, and carbon monoxide plays no part.

Answer: D

Correct answer: District domestic heating

District domestic heating is an area source. It comprises thousands of individual chimneys spread across a residential area, none of them individually significant and none practical to enumerate, so an inventory represents it by an emission density per unit area.

The other three are point sources: a power station stack, a cement kiln stack and a refinery process heater stack are each a single identified location with known height, gas flow and temperature, which can be modelled directly as a plume.

Answer: A

Correct answer: It matters less than ambient pollution

This is the false statement. Household air pollution from solid fuel combustion is among the largest environmental health burdens worldwide, responsible for millions of premature deaths a year, and in many rural households it dominates total exposure - often two-thirds or more of the daily dose is acquired during the cooking hours alone.

The other three are true: kitchen concentrations can exceed outdoor levels twentyfold, exposure falls on those present during cooking, and improved stoves, flues and clean fuels are the main remedies.

Answer: B

Correct answer: An ambient concentration measurement

An ambient concentration is the wrong kind of quantity for building an inventory. It records what is in the air after emission, dispersion, transformation and mixing with contributions from everywhere else, so it cannot be worked backwards to any individual source's output.

The other three are exactly what an inventory needs: an emission factor for each activity, together with activity data such as vehicle kilometres travelled and recorded fuel consumption. Ambient measurements are used afterwards, to validate the inventory and the dispersion modelling built on it.

Answer: C

Correct answer: It must be quoted with a time horizon

Global warming potential compares a gas's cumulative radiative effect with that of carbon dioxide over a stated period, so the period is part of the definition. Methane's value is about 28 over a hundred years but around 80 over twenty, because it is a strong absorber with a short atmospheric lifetime.

The choice therefore changes how a methane reduction is valued against a carbon dioxide one, which has real policy consequences. It applies to all greenhouse gases, with carbon dioxide as the reference at unity, and has nothing to do with acidification.

Answer: D

Correct answer: Its residence time allows long transport

Fine particulate stays airborne for days to weeks - too small to settle appreciably under gravity, too large to be removed quickly by coagulation - so the prevailing wind carries it hundreds of kilometres before wet or dry deposition removes it.

The consequence is that a city's measured PM2.5 includes a substantial contribution emitted or formed well outside its boundary, and that local measures alone cannot bring it down. It is emitted and formed everywhere, not only in rural areas, and it is measured throughout the network rather than only at regional sites.