Ozone Layer and Acid Rain: Causes, Effects, Treaties

The correct answer is A, all of the options. Each of the other three statements is correct on its own, which is exactly why the combined option is the answer. Option B is a true statement rather than the answer: ozone has three oxygen atoms, is unstable and is a strong oxidising agent, which is why it is used to disinfect water. Option C is also true: the ozone in the stratosphere absorbs most of the ultraviolet radiation from the sun and protects living things from skin cancer and cataract. Option D is true as well, but for ozone at ground level, where it is formed as a secondary pollutant when sunlight acts on nitrogen oxides and hydrocarbons from vehicles, and it is the main component of photochemical smog. Since B, C and D are each correct, no single one of them can be the answer and option A must be chosen. This good and bad character of ozone is the point being tested.

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.

The correct answer is B, sulphur dioxide and oxides of nitrogen. These gases come from burning coal and oil in thermal power stations, smelters and vehicles, are oxidised in the atmosphere and dissolve in water droplets to form sulphuric acid and nitric acid, which come down as acid rain. Option A is wrong because carbon dioxide and methane are greenhouse gases; carbon dioxide does dissolve to form weak carbonic acid, but that only makes normal rain slightly acidic at a pH near 5.6 and is not what is meant by acid rain. Option C is wrong because carbon monoxide is a poisonous gas that does not form a strong acid, and ozone at ground level is an irritant in smog rather than a source of acidity. Option D is wrong because chlorofluorocarbons and halons destroy stratospheric ozone and are the concern of the Montreal Protocol, a separate problem from acid rain.

The correct answer is A. Marble is calcium carbonate, and sulphur dioxide in polluted air, along with the sulphuric acid of acid rain, reacts with it to form calcium sulphate, so the stone loses its shine, turns yellow and becomes pitted and flaky. This damage is called marble cancer, and it is discussed most often in connection with the Taj Mahal at Agra, which is why industries around it are regulated. Option B is wrong because the lung disease caused by inhaling stone dust is silicosis, a different subject altogether. Option C is wrong because algal growth is a biological stain that can be cleaned and is not what the term means. Option D is wrong because thermal cracking is a physical weathering process common to many rocks and has nothing to do with pollution. Remember that any monument of marble or limestone is vulnerable, since both are calcium carbonate.

The correct answer is C, 16 September. The day marks the signing of the Montreal Protocol on Substances that Deplete the Ozone Layer in 1987 and is observed every year with a theme announced by the United Nations. Option A is wrong because the fifth of June is World Environment Day, which marks the opening of the Stockholm Conference of 1972. Option B is wrong because the twenty second of April is Earth Day. Option D is wrong because the second of December is observed in India as National Pollution Control Day, in memory of the Bhopal gas tragedy of 1984. Environment sections regularly place these four dates in one question, so learn them as a set along with the twenty second of May for biological diversity and the second of February for wetlands.

The correct answer is B, hydrofluorocarbons. Hydrofluorocarbons were adopted as substitutes for chlorofluorocarbons in refrigeration and air conditioning because they do not harm the ozone layer, but they turned out to be very powerful greenhouse gases, so the Kigali Amendment of 2016 brought them under the Montreal Protocol with a schedule for reducing their use. Option A is wrong because chlorofluorocarbons were covered by the original protocol of 1987 and their production has already been phased out under it. Option C is wrong because sulphur dioxide, the main cause of acid rain, is controlled by national air quality laws and emission standards, not by this treaty. Option D is wrong because methyl bromide, used to fumigate soil, is also part of the original list of ozone depleting substances. The point being tested is that Kigali deals with a climate gas through an ozone treaty.