Environment & Ecology Quiz: Ozone Layer and Acid Rain

Each of the other three statements is correct on its own, which is exactly why the combined option is the answer.

A primary pollutant is released directly from a source, while a secondary pollutant is formed in the atmosphere when primary pollutants react.

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.

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 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.

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.

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.

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.

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.

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.