Which of the following statements about ozone is true?

The correct answer is B, smog. A primary pollutant is released directly from a source, while a secondary pollutant is formed in the atmosphere when primary pollutants react. Photochemical smog is formed when sunlight acts on nitrogen oxides and volatile organic compounds to produce ozone and other irritants, so it is the classic secondary pollutant, and ground level ozone is counted with it. Option A is wrong because carbon monoxide comes straight out of an engine or a stove when fuel burns without enough oxygen. Option C is wrong because carbon dioxide is released directly by combustion and respiration. Option D is wrong because fly ash is the fine particulate matter that leaves the chimney of a thermal power station and is emitted as it is. The same distinction applies to acid rain, where sulphuric and nitric acids are secondary products formed from the primary gases sulphur dioxide and nitrogen oxides.

The correct answer is A, nitrogen oxide. Nitric oxide and nitrogen dioxide, together written as nitrogen oxides, are air pollutants that come mainly from combustion in vehicles and power stations; they help form smog and acid rain but are not counted among the greenhouse gases of the natural atmosphere. The gas of this family that is a greenhouse gas is nitrous oxide, a different compound, so read the name carefully. Option B is wrong as the answer because carbon dioxide occurs naturally from respiration, decay and volcanoes and is the principal greenhouse gas. Option C is wrong because methane is released naturally from wetlands and the digestion of animals and is a strong greenhouse gas. Option D is wrong because ozone occurs naturally in the stratosphere, is listed as a greenhouse gas, and also forms near the ground as a pollutant. The trap here is the similarity between nitrogen oxide and nitrous oxide.

The correct answer is B, stratosphere. About nine tenths of the atmosphere's ozone lies in the stratosphere, roughly fifteen to thirty five kilometres above the surface, where ultraviolet rays split oxygen molecules and the free atoms combine with other oxygen molecules to make ozone. The absorption of ultraviolet radiation there is also why temperature rises with height in this layer. Option A is wrong because the troposphere is the lowest layer, where weather occurs and where ozone appears only as a pollutant in smog. Option C is wrong because the mesosphere lies above the stratosphere, is the coldest layer and is where meteors burn up. Option D is wrong because the thermosphere is far higher, holds the ionosphere that reflects radio waves, and has air too thin for an ozone layer. Keep the order of the layers ready, since papers combine this with a question on where temperature rises or falls with height.

The correct answer is A, Dobson unit. The Dobson unit, named after the scientist who built the instrument for measuring atmospheric ozone, expresses how thick the ozone column would be if all of it were brought down to the surface at standard temperature and pressure; a hundred Dobson units correspond to a layer one millimetre thick. Option B is wrong because the curie is a unit of radioactivity, used for the rate at which a substance decays. Option C is wrong because the decibel measures the loudness of sound and is used in noise pollution standards. Option D is wrong because the angstrom is a unit of length equal to one ten billionth of a metre, used for atoms and for wavelengths of light rather than for a column of gas. Remember the Dobson unit together with the year 1985, when the thinning over Antarctica was reported.

The correct answer is B, substances that deplete the ozone layer. The Montreal Protocol was adopted in 1987 under the Vienna Convention of 1985 and sets the timetable by which countries must stop producing and using chlorofluorocarbons, halons, carbon tetrachloride, methyl chloroform and methyl bromide. Option A is wrong because greenhouse gas emissions are the subject of the United Nations Framework Convention on Climate Change and the agreements under it, although the Kigali Amendment of 2016 did bring hydrofluorocarbons, which are greenhouse gases, into the Montreal Protocol. Option C is wrong because the movement of hazardous waste across borders is governed by the Basel Convention. Option D is wrong because wetlands are covered by the Ramsar Convention. Matching each convention to its single subject is the fastest preparation, since one such question appears in nearly every environment section.