Ozone Layer and Acid Rain: Causes, Effects, Treaties

Two classic pollution topics sit together in every syllabus because both are caused by gases released far from where the harm appears. Ozone depletion is a global problem of the stratosphere, and acid rain is a regional problem of the lower atmosphere. Exams ask which gas causes what, which treaty deals with which substance, and which unit or date belongs where.

Ozone: good above, bad below

Ozone is a molecule of three oxygen atoms, written O3, and it is pale blue, pungent and highly reactive. About nine tenths of it lies in the stratosphere, roughly fifteen to thirty five kilometres above the surface, where ultraviolet rays split oxygen molecules and the free atoms join other oxygen molecules to form ozone. This layer absorbs most of the harmful ultraviolet radiation of the sun and is called good ozone. At ground level, ozone is a secondary pollutant: it is not emitted directly but formed when sunlight acts on nitrogen oxides and volatile organic compounds from vehicles and industry, and it is the chief constituent of photochemical smog, irritating the eyes and lungs and damaging crops. The thickness of the ozone column is measured in Dobson units.

The ozone hole and what causes it

A severe seasonal thinning of ozone over Antarctica was reported by British scientists in 1985 and is called the ozone hole. It appears in the Antarctic spring, because the extreme cold of the polar winter forms polar stratospheric clouds and the polar vortex traps the air, so that reactive chlorine builds up and destroys ozone rapidly when sunlight returns. The substances responsible are called ozone depleting substances.

A single chlorine atom released from these compounds can destroy a very large number of ozone molecules before it is removed, which is why small quantities do so much damage. The consequences of a thinner layer are more ultraviolet radiation at the surface, and therefore skin cancer, cataract and weaker immunity in people, damage to phytoplankton at the base of the marine food chain, and lower yields of sensitive crops.

The treaties

• The Vienna Convention for the Protection of the Ozone Layer was adopted in 1985 as a framework agreement
• The Montreal Protocol on Substances that Deplete the Ozone Layer was adopted in 1987 and is the operating treaty that fixes the phase out schedules
• The Kigali Amendment of 2016 added hydrofluorocarbons, which do not harm ozone but are powerful greenhouse gases, to the substances to be phased down
• The sixteenth of September is observed as the International Day for the Preservation of the Ozone Layer, marking the day the Montreal Protocol was signed

Acid rain

Rain is naturally slightly acidic, close to a pH of 5.6, because carbon dioxide dissolves in it to form weak carbonic acid. Rain is called acid rain when its pH falls below that level. The cause is sulphur dioxide and oxides of nitrogen, which are released when coal and oil are burnt in thermal power stations, smelters, refineries and vehicles. In the atmosphere they are oxidised and dissolve in water droplets to give sulphuric acid and nitric acid, which fall as wet deposition in rain, snow and fog, or settle as dry deposition of acidic particles. The term acid rain was used by the chemist Robert Angus Smith in the nineteenth century after studying the air of industrial Manchester.

What acid rain does

• Lakes and streams turn acidic, fish eggs fail to hatch and sensitive species disappear
• Soil loses calcium and magnesium by leaching and releases aluminium, which harms roots
• Forests are damaged, with the crowns of trees thinning and growth slowing
• Marble and limestone buildings corrode because the acid attacks calcium carbonate; the yellowing and pitting of the marble of the Taj Mahal is called marble cancer
• Metal structures, bridges and pipelines corrode faster, and monuments lose their carved detail

The remedies are cleaner fuel and cleaner burning. Flue gas desulphurisation units remove sulphur dioxide from power station chimneys, low sulphur coal and oil reduce it at source, catalytic converters and tighter vehicle emission standards cut nitrogen oxides, and affected lakes are sometimes treated with lime to restore their pH. Because the pollutants travel hundreds of kilometres with the wind, acid rain often falls in a country that did not produce the gases, which is why it is treated as a transboundary problem.