Air Pollutants and Sources Questions and Answers

Answer: B

The correct sequence is 60 PPBV, 400 PPMV, 1.9 PPMV, 600 PPBV for O₃, CO₂, CH₄ and CO.

Carbon dioxide is by far the most abundant of the four, about 400 parts per million by volume (it passed 400 ppm around 2015). Methane is about 1.9 ppm. Ozone and carbon monoxide are trace gases measured in parts per billion: urban ozone of the order of 60 ppb, and urban carbon monoxide a few hundred ppb, such as 600 ppb.

Each wrong option misplaces CO₂: the first gives 400 ppm to ozone, the third gives it to methane, and the fourth gives 600 ppb to ozone and only 1.9 ppm to carbon monoxide while leaving methane at 40 ppb, far below its real level.

Answer: B

The secondary pollutant is \(\mathrm{O_{3}}\), ozone.

A secondary pollutant is not emitted directly but forms in the atmosphere from reactions between primary pollutants. Ground-level ozone forms when nitrogen oxides and volatile organic compounds react in sunlight, which is why it peaks on sunny afternoons in photochemical smog.

Sulphur dioxide, carbon monoxide and nitric oxide are primary pollutants, released directly from combustion in vehicles, power plants and industry.

Answer: C

A 3.8 µm particle falls in the coarse mode.

Atmospheric aerosols fall into modes by size. The nucleation (Aitken) range covers freshly formed particles below about 0.1 µm. The accumulation mode runs from about 0.1 to 2 µm. Particles above about 2 to 2.5 µm, such as soil dust, sea salt and pollen, make up the coarse mode; this boundary is why PM_2.5 is used for fine particles.

Aitken and nucleation are the names for the smallest particles, which are around a thousand times smaller than 3.8 µm. A particle has one size, so it cannot be in all the modes at once.

Answer: C

The key gives dung cake.

Ammonia from burning comes from the nitrogen in the fuel. Cattle dung is rich in nitrogen from undigested protein and urea, much richer than wood, so dung-cake fires emit more ammonia than the other fuels.

Wood has a low nitrogen content, petrol contains almost no nitrogen (its nitrogen oxides come from air in the engine, not ammonia), and plastic waste is mostly carbon and hydrogen, apart from nitrogen-containing polymers such as nylon.

Answer: B

The major species is NO\(_{2}\).

Nitrogen dioxide is the principal nitrogen oxide in polluted air. Nitric oxide from combustion is quickly oxidised to NO\(_{2}\), which is a regulated criteria pollutant, irritates the lungs, and drives the formation of ozone, smog and acid rain.

N\(_{2}\)O is a greenhouse and ozone-depleting gas but not the main local air pollutant, N\(_{2}\)O\(_{5}\) is a short-lived night-time intermediate, and HNO\(_{3}\) is a product of NO\(_{2}\) oxidation that falls out as acid deposition.

Answer: B

The natural source is volcanic eruptions.

Volcanoes release large amounts of sulfur dioxide from magma, both in eruptions and by continuous degassing. A large eruption can inject SO₂ into the stratosphere, where it forms sulfate aerosols that cool the climate for a year or more, as after Mount Pinatubo in 1991.

The other three options are man-made sources. Coal-fired power plants and industrial combustion are the largest human sources of SO₂, because coal and heavy oil contain sulfur. Automobiles also emit some, from sulfur in fuel. The question asks for a natural source, so only volcanic eruptions fit.

Answer: A

The products are carbon dioxides, nitrogen oxides and sulphur oxides.

The question, which says 'pulverised soil', clearly means pulverised coal, as the phrase 'organic coaly matter' shows. When coal burns, its carbon is oxidised to carbon dioxide, its organically bound nitrogen to nitrogen oxides (fuel NOx) and its sulphur to sulphur oxides, mainly SO₂. This is why coal-fired power plants are major sources of all three.

Copper and zinc are not part of the organic matter of coal; any trace metals end up in the fly ash, not as products of oxidising carbon, nitrogen and sulphur. The options naming copper or zinc oxides do not answer what the question asks.