Stratospheric ozone practice questions.

The greatest thinning of the ozone layer is measured over Antarctica in spring, not over the industrial regions where ozone-depleting substances are released. This is because

• ozone-depleting substances were manufactured almost entirely in the southern hemisphere
• the stratosphere above Antarctica is warmer in spring than anywhere else on Earth
• ice clouds form in the winter vortex and returning sunlight then frees chlorine
• the ozone layer above the poles lies far closer to the ground than it does elsewhere

Answer: ice clouds form in the winter vortex and returning sunlight then frees chlorine

Ozone-depleting substances are stable and mix globally, so the pattern is set by conditions rather than by where the gases were emitted: polar stratospheric clouds forming in the extreme winter cold convert reservoir compounds into forms that spring sunlight splits into reactive chlorine.

A single chlorine atom released into the stratosphere can destroy many thousands of ozone molecules. The reason is that

• each chlorine atom bonds permanently to several ozone molecules at once
• chlorine is far more abundant in the stratosphere than ozone itself is
• ultraviolet radiation converts every chlorine atom into many new atoms
• the chlorine atom is regenerated at the end of each reaction cycle

Answer: the chlorine atom is regenerated at the end of each reaction cycle

Chlorine behaves as a catalyst: it takes an oxygen atom from ozone, gives it up again and emerges unchanged, ready to repeat the cycle. That is why very small concentrations of chlorofluorocarbons had such a disproportionate effect.

Ozone is described as beneficial in the stratosphere but as a pollutant in the troposphere. Which statement accounts for this?

• the molecule is identical; only its altitude and therefore its effect differ
• stratospheric ozone is chemically a different molecule from tropospheric ozone
• tropospheric ozone is emitted directly by vehicles, stratospheric ozone is not
• stratospheric ozone is a secondary pollutant and tropospheric ozone a primary one

Answer: the molecule is identical; only its altitude and therefore its effect differ

O3 is the same molecule in both layers: at 20-30 km it absorbs UV-B before it reaches living tissue, while at ground level it is a secondary pollutant formed photochemically from vehicle exhaust that irritates lungs and damages leaves.

A sustained increase in UV-B reaching the sea surface is most likely to lower the productivity of a coastal ecosystem by

• raising sea surface temperature enough to bleach the corals
• damaging the DNA of phytoplankton in the surface waters
• increasing the solubility of oxygen in the surface layer
• preventing light of every wavelength from reaching the seabed

Answer: damaging the DNA of phytoplankton in the surface waters

UV-B penetrates several metres of clear water and damages the genetic material of the producers at the base of the food chain, so gross primary productivity falls and the reduced energy flow passes up through every trophic level.

Extremely low winter temperatures over Antarctica allow polar stratospheric clouds to form. Their role in ozone depletion is that they

• absorb ultraviolet radiation that would otherwise split ozone
• carry ozone-depleting substances upward from the troposphere
• reflect sunlight and so cool the polar stratosphere further
• provide surfaces on which stored chlorine becomes reactive

Answer: provide surfaces on which stored chlorine becomes reactive

Most stratospheric chlorine is locked in unreactive reservoir compounds. Reactions on the ice particle surfaces convert it into forms that sunlight can split into free chlorine atoms, which is why the destruction begins when the Sun returns in spring rather than during the cold dark of winter.

Hydrofluorocarbons were introduced as refrigerants to replace ozone-depleting compounds, and were later brought under international control themselves. The reason is that they

• turned out to deplete stratospheric ozone just as fast
• are potent greenhouse gases although they spare the ozone layer
• break down into chlorine atoms in the lower atmosphere
• proved far too costly for manufacturers to produce at a large scale

Answer: are potent greenhouse gases although they spare the ozone layer

Solving one atmospheric problem created another: these compounds contain no chlorine or bromine, so they leave ozone alone, but they absorb longwave radiation strongly. It is a standard illustration that a substitute has to be assessed against every impact, not only the one it was chosen to fix.