Air Pollutants and Sources Questions and Answers

Answer: C

Correct answer: Sulphur dioxide and oxides of nitrogen

Acid deposition arises when sulphur dioxide and oxides of nitrogen are oxidised in the atmosphere to sulphuric and nitric acid. Because the transformation takes hours to days, the acids travel hundreds or thousands of kilometres before being deposited in rain, snow or as dry deposition - which is what makes acid rain an international rather than a local problem.

Carbon monoxide and methane do not form strong acids; lead and mercury are toxic metals with quite different pathways; and ozone and volatile organic compounds belong to the photochemical smog problem.

Answer: D

Correct answer: A single identified stack

A point source is a single discrete emission location whose position, height, gas flow and temperature are all known - typically an industrial or power station stack. Inventories treat it individually, and dispersion models can compute its plume directly.

A road carrying traffic is a line source, and a residential district or agricultural region is an area source, represented by an emission density per unit area because the individual emitters are too numerous to enumerate. The classification determines both how the emission is estimated and how it is modelled.

Answer: A

Multiplying the factor by the activity level gives an emission estimate without measuring every source, which is how national inventories are built. Emission factors of this kind carry considerable uncertainty when applied to a different plant from the one measured.

Answer: B

Correct answer: Escape other than through a stack

Fugitive emissions escape from equipment and operations rather than passing through a stack or vent - leaking valves, flanges and pump seals, evaporation from open tanks and lagoons, dust raised from stockpiles and haul roads, and losses during loading.

They are difficult to quantify because there is no single flow to measure, so they are estimated from leak surveys and emission factors rather than from stack monitoring. In refineries and chemical plants they can rival or exceed the stack emissions of volatile organic compounds.

Answer: C

Concentrations in an unvented kitchen far exceed anything found outdoors, and women and young children are exposed for hours each day. Improved stoves and clean fuels address it far more effectively than any ambient standard could.

Answer: D

Correct answer: Is a neurotoxin harming children

Lead was removed from petrol because it is a neurotoxin, and children are the most vulnerable: exposure in early childhood produces irreversible loss of cognitive function with no identified threshold below which it is safe. Tetraethyl lead put it directly into urban air, and blood lead levels in city populations fell sharply once it was withdrawn.

Lead does not cause acid rain and is not a greenhouse gas. It does poison catalytic converters, which reinforced the case for removal, but the driver was the harm to health.

Answer: A

Correct answer: Windblown mineral dust

Windblown mineral dust from arid and semi-arid regions is the largest natural contributor to global particulate loading - the Sahara alone lifts hundreds of millions of tonnes a year, some of it carried across the Atlantic. Sea spray is the second large natural source.

Volcanic eruptions inject enormous quantities but episodically, so their long-term average contribution is smaller, and forest fires are similarly intermittent and seasonal. The options naming sea spray or fire "only" also exclude the other natural sources, which no single category accounts for.

Answer: B

Its global warming potential over twenty years is many times that of carbon dioxide, and it also contributes to background tropospheric ozone. Its relatively short lifetime makes controlling it unusually effective in the near term.

Answer: C

Correct answer: High ozone, formed in sunlight

Photochemical smog is an oxidising, sunlight-driven phenomenon. Oxides of nitrogen and volatile organic compounds from traffic react through radical chains to produce ozone, peroxyacyl nitrates and secondary organic aerosol, with concentrations building through the day and peaking in the afternoon downwind of the city.

It is the opposite of the older London-type smog, which was reducing rather than oxidising: high sulphur dioxide and smoke from coal burning, accumulating in cold damp stagnant conditions. Both are called smog, but their chemistry, meteorology and controls are quite different.

Answer: D

Correct answer: Agriculture

Agriculture dominates ammonia emissions in most countries, typically contributing 80 to 90 per cent - from livestock manure during housing, storage and spreading, and from the volatilisation of nitrogen fertilisers after application.

Ammonia matters for air quality beyond its own effects because it is the principal atmospheric base: it neutralises sulphuric and nitric acid to form ammonium sulphate and ammonium nitrate, which are major components of fine particulate. Rural ammonia therefore contributes directly to urban PM2.5, and controlling one requires attention to the other.