Ozone depletion & climate: EPA 608 Practice Questions

In which layer of the atmosphere is the ozone that shields the Earth from ultraviolet radiation?

The protective ozone layer sits in the stratosphere, roughly 6 to 30 miles up. Ozone near the ground (troposphere) is a pollutant, not a shield.

Which element released from CFC and HCFC refrigerants is mainly responsible for destroying stratospheric ozone?

Ultraviolet light breaks chlorine atoms off CFC and HCFC molecules, and each free chlorine atom destroys ozone in a repeating chain reaction.

About how many ozone molecules can a single chlorine atom destroy before it is removed from the stratosphere?

Because chlorine acts as a catalyst and is not used up, one atom can break down on the order of 100,000 ozone molecules.

Which health effect is linked to increased ultraviolet (UV-B) radiation reaching the ground because of ozone depletion?

More UV-B raises the rates of skin cancer and eye cataracts and can weaken the immune system; it also harms crops and marine life.

Why does an HCFC such as R-22 have a lower ozone depletion potential than a CFC such as R-12?

The hydrogen atom makes an HCFC less stable, so most of it breaks down before reaching the stratosphere. It still contains chlorine, so its ODP is low but not zero.

Which of these refrigerants has an ozone depletion potential (ODP) of zero?

R-32 is an HFC and contains no chlorine, so its ODP is zero. R-12 is a CFC, and R-22 and R-123 are HCFCs.

CFC molecules are heavier than air. How do they still reach the stratosphere?

CFCs are so stable that they last for decades, giving atmospheric mixing time to carry them up to the stratosphere, where UV light breaks them apart.

Ozone depletion potential (ODP) values are measured relative to which reference substance?

ODP compares a substance's ozone-destroying ability with CFC-11 (R-11), which has an ODP of 1.0. Global warming potential uses carbon dioxide as its reference.

Global warming potential (GWP) compares a gas's heat-trapping effect with that of which gas?

Carbon dioxide has a GWP of 1. A refrigerant with a GWP of 675 traps about 675 times as much heat as the same mass of CO2 over the reference period.

Which refrigerant family has the highest ozone depletion potential?

CFCs contain chlorine and are very stable, so they have the highest ODP. HCFCs are lower, and HFCs and HFOs contain no chlorine.

Compared with R-134a, what is the main environmental advantage of R-1234yf?

Both have zero ODP, but R-1234yf (an HFO) has a GWP of about 1 to 4, while R-134a has a GWP of about 1,430. R-1234yf is mildly flammable (A2L).

Volcanoes and sea spray release chlorine. Why do they contribute little to stratospheric ozone loss compared with CFCs?

Natural chlorine compounds are water soluble and are washed out of the lower atmosphere by rain, while insoluble, stable CFCs survive long enough to reach the stratosphere.