Air Pollutants and Sources Questions and Answers

Answer: A

Correct answer: Indoors, at many times the outdoor level

The risk lies indoors. An unvented biomass stove can raise kitchen PM2.5 to a thousand micrograms per cubic metre or more, twenty times a typical outdoor level, and the exposure falls on whoever is present - usually women and young children.

Good ambient air quality is no protection, because the ambient monitoring network cannot see the kitchen at all. This is why household air pollution is counted as one of the largest environmental health burdens worldwide, and why the remedies are improved stoves, flues and clean fuels rather than anything in the ambient regime.

Answer: B

Correct answer: Fugitive losses from valves and tanks

Fugitive emissions are the usual explanation for this pattern. Volatile organic compounds escape from leaking valve stems, pump seals and flanges, from tank breathing and working losses, and during loading - none of which passes through a stack, so none appears in stack monitoring however rigorous.

At a refinery these can rival or exceed the stack emissions of the same compounds. They are found by leak detection surveys and controlled by improved seals, floating roofs and vapour recovery rather than by anything applied at the stack.

Answer: C

Correct answer: Whether the factors fit the local sources

The dominant uncertainty is whether national emission factors represent the district's actual sources. A factor is an average over some assumed population of plant, fuel quality, maintenance standard and operating régime, and a district whose vehicles are older, whose fuel is dirtier or whose boilers are differently fired may depart from it by a factor of two or more.

The arithmetic contributes nothing and the definitions and units are matters of convention. Where a factor is critical, the remedy is local measurement to derive or validate one.

Answer: D

Correct answer: secondary

A secondary pollutant is one formed in the atmosphere by reaction between other substances rather than emitted from any source. Ozone is the classic example, formed from oxides of nitrogen and volatile organic compounds in sunlight; ammonium sulphate and nitrate aerosol and sulphuric acid in rain are others.

The distinction has a direct policy consequence: a secondary pollutant cannot be regulated by an emission limit on itself, because nothing emits it. Control must act on the precursors, and the chemistry between them is often non-linear.

Answer: A

Correct answer: aerodynamic

The aerodynamic diameter is defined as the diameter of a sphere of unit density having the same settling velocity as the particle in question, so it folds size, shape and density into one number.

That number is what governs how the particle settles, how it deposits in the respiratory tract and how it is captured in a sampler or control device - which is why PM10 and PM2.5 are defined in these terms rather than by physical size. A dense compact grain and a light fluffy agglomerate of the same physical diameter behave quite differently, and only the aerodynamic value predicts either.

Answer: B

Correct answer: fugitive

Fugitive emissions escape from valves, flanges, pump seals, tanks, transfer points, stockpiles and haul roads rather than passing through a stack or vent.

Because there is no single flow to measure, they are estimated from leak detection surveys and emission factors rather than from monitoring, which makes them one of the more uncertain parts of an inventory. They are also frequently large - at refineries and chemical works, fugitive losses of volatile organic compounds can exceed the stack emissions of the same substances.

Answer: C

Correct answer: factor

An emission factor relates the mass of a pollutant emitted to a unit of activity - kilograms per tonne of fuel burnt, grams per vehicle kilometre, kilograms per head of livestock per year.

It is the basic tool of inventory compilation, because the activity data are usually recorded already for other purposes, so emissions can be estimated for hundreds of thousands of sources without measuring any of them. An inventory is the resulting compilation, and a standard is a limit on emissions or ambient concentration - quite different things.

Answer: D

Correct answer: photochemical

Photochemical smog is the sunlight-driven, oxidising type: oxides of nitrogen and volatile organic compounds from traffic react through radical chains to produce ozone, peroxyacyl nitrates and secondary organic aerosol, peaking in the afternoon and downwind of the city.

It contrasts with sulphurous, or London-type, smog, which is reducing rather than oxidising - sulphur dioxide and smoke from coal burning accumulating in cold, damp, stagnant winter conditions. The two have opposite chemistry, occur in opposite weather, and need entirely different controls.

Answer: A

Correct answer: Only their precursors can be controlled

Ozone and ammonium nitrate aerosol are both formed in the atmosphere, so no source emits either and no emission limit can be written for them. Control must act on the precursors - oxides of nitrogen and volatile organic compounds for ozone; ammonia and nitric acid, itself from nitrogen oxides, for the aerosol.

That makes both harder to manage than a primary pollutant. The chemistry is non-linear, the precursors often come from different sectors and different jurisdictions, and the response to a given reduction cannot be assumed proportional.

Answer: B

Correct answer: Emission is proportional to fuel sulphur

Check the ratios: doubling the sulphur from 0.5 to 1.0 per cent doubles the emission from 5.0 to 10.0 t/day, and 3.5 per cent gives seven times the 0.5 per cent figure. The relation is strictly proportional.

The reason is that sulphur in the fuel oxidises almost completely, so essentially all of it leaves as sulphur dioxide. Note also the factor of two between the sulphur and \( \mathrm{SO_2} \) columns, which is the ratio of molar masses - 64 for \( \mathrm{SO_2} \) against 32 for sulphur. This proportionality is what makes fuel sulphur limits so effective a control.