APES 9.1-9.2: Stratospheric Ozone and Global Climate Change

Ozone depletion

Ozone depletion includes both a long-term decrease in total stratospheric ozone and the much larger seasonal reduction in polar stratospheric ozone known as the ozone hole.

Ozone-depleting substances (ODS)

ODS are human-produced compounds that can release chlorine or bromine in the stratosphere and catalytically destroy ozone. Important examples include CFCs, HCFCs, and halons.

Why can CFCs reach the stratosphere?

CFCs are chemically unreactive in the troposphere, so they are not readily destroyed before being transported upward by atmospheric mixing. In the stratosphere, higher-energy ultraviolet radiation can break their bonds.

Stratospheric ozone formation

UV radiation photodissociates oxygen gas: $\mathrm{O_2 + h\nu \rightarrow 2O}$. The oxygen atoms then react with oxygen molecules: $\mathrm{O + O_2 \rightarrow O_3}$.

Ozone-oxygen cycle

Ozone absorbs UV radiation and decomposes: $\mathrm{O_3 + h\nu \rightarrow O_2 + O}$. The oxygen atom can then react with another ozone molecule: $\mathrm{O + O_3 \rightarrow 2O_2}$.

How does ultraviolet radiation initiate CFC-driven ozone depletion?

Stratospheric UV radiation photodissociates a CFC, releasing a chlorine radical. For example, $\mathrm{CFCl_3 + h\nu \rightarrow Cl\cdot + \cdot CFCl_2}$.

Catalytic chlorine cycle for ozone destruction

A chlorine radical reacts with ozone to form chlorine monoxide: $\mathrm{Cl\cdot + O_3 \rightarrow ClO\cdot + O_2}$. Then $\mathrm{ClO\cdot + O_3 \rightarrow Cl\cdot + 2O_2}$, giving the net reaction $\mathrm{2O_3 \rightarrow 3O_2}$.

Why is chlorine a catalyst in ozone depletion?

Chlorine is regenerated after participating in the reaction cycle, so it is not consumed in the net reaction. A single chlorine atom can destroy many thousands of ozone molecules before being removed into a reservoir compound.

How do reservoir species affect stratospheric chlorine?

Compounds such as hydrogen chloride, $\mathrm{HCl}$, and chlorine nitrate, $\mathrm{ClONO_2}$, temporarily sequester reactive chlorine and reduce ozone destruction. UV radiation or reactions on particle surfaces can later convert these reservoirs back into reactive chlorine species.

Why is bromine especially important in ozone depletion?

Bromine radicals are more effective than chlorine radicals at destroying ozone on a per-atom basis. However, atmospheric bromine concentrations are much lower than chlorine concentrations.

Why do fluorine- and iodine-containing compounds generally contribute less to stratospheric ozone depletion?

Fluorine atoms rapidly form strongly bonded hydrogen fluoride, while most iodine-containing organic compounds react or break down in the lower atmosphere. Consequently, relatively little iodine reaches the stratosphere in a reactive form.

Which forms of oxygen participate in the ozone-oxygen cycle?

The cycle involves atomic oxygen, $\mathrm{O}$; diatomic oxygen, $\mathrm{O_2}$; and ozone, $\mathrm{O_3}$.

Which ultraviolet wavelengths are most strongly absorbed by atmospheric oxygen and ozone?

Nearly all UVC is absorbed by $\mathrm{O_2}$ or $\mathrm{O_3}$, and the ozone layer absorbs most UVB. UVA is absorbed much less and therefore makes up most UV radiation reaching Earth's surface.

Why does ozone depletion increase surface UVB exposure?

Ozone absorbs most atmospheric UVB radiation. When the stratospheric ozone column decreases, a larger fraction of UVB passes through the atmosphere and reaches Earth's surface.

Dobson unit (DU)

A Dobson unit measures the total column amount of ozone above a location. Ozone-hole studies commonly compare column ozone values in DU with historical or pre-depletion levels.

What is the Antarctic ozone hole?

It is a region of strongly reduced ozone in the Antarctic stratosphere, most pronounced during the Antarctic spring. Ozone levels there have fallen to roughly one-third of pre-depletion values in severe years.

Why is Antarctic ozone depletion strongest during spring rather than winter?

During winter, extremely low temperatures form polar stratospheric clouds, but the polar region lacks sunlight. When sunlight returns in spring, it drives photochemical reactions that convert chlorine reservoirs into reactive chlorine and chlorine monoxide.

Polar stratospheric clouds (PSCs)

PSCs are clouds that form in the extremely cold polar stratosphere. Their particle surfaces convert relatively unreactive chlorine reservoir compounds into reactive species, greatly accelerating ozone destruction when sunlight returns.

Polar vortex

The polar vortex is a strong circulation of stratospheric winds that isolates and traps cold polar air. In Antarctica, it helps maintain conditions favorable for PSC formation and concentrates ozone-destroying reactions.

How does denitrification intensify polar ozone depletion?

PSC particles convert nitrogen dioxide into nitric acid, which can sediment out of the stratosphere. Removing nitrogen compounds prevents reactive chlorine monoxide from being reconverted into the reservoir compound $\mathrm{ClONO_2}$.

Why does the Antarctic ozone hole eventually close?

As spring ends, temperatures rise and the polar vortex breaks down. PSCs disappear, and ozone-rich air containing nitrogen compounds flows in from lower latitudes, shutting down the enhanced depletion cycle.

How does Arctic ozone depletion differ from Antarctic ozone depletion?

Arctic depletion is more variable because the Arctic stratosphere is generally warmer and its vortex is less stable. In especially cold Arctic winters and springs, ozone loss can approach about 30 percent, but it is usually less severe than the Antarctic ozone hole.

Why is there generally no major tropical ozone hole?

Halogen-containing compounds do not usually release most of their chlorine and bromine until they have been transported to higher latitudes and the stratosphere. Thus, substantial tropical depletion has not generally developed.

How can volcanic eruptions affect ozone?

Large eruptions can inject particles and sulfur-containing material into the atmosphere. These surfaces can promote heterogeneous chemical reactions that increase ozone destruction, although the effect varies spatially and temporally.

What evidence connects CFCs to ozone depletion?

Measurements show that ozone depletion increased as atmospheric halocarbon concentrations increased. Chemistry-transport models reproduce observed ozone patterns by combining atmospheric measurements, transport data, and known photochemical reaction rates.

Chemistry-transport model

A chemistry-transport model combines atmospheric motion, measured chemical concentrations, and reaction-rate data to predict how compounds and reaction products move and react in the atmosphere.

Why does ozone depletion cool the stratosphere?

Ozone absorbs UV radiation and converts some of that energy into thermal energy. Less ozone means less UV absorption and therefore less stratospheric heating.

Montreal Protocol

The Montreal Protocol is an international agreement adopted in 1987 to phase out the production of major ozone-depleting substances, including CFCs and halons. The resulting decline in stratospheric chlorine and bromine has stabilized ozone levels and initiated recovery.

Why do CFC emissions have effects lasting for decades?

CFC molecules are chemically stable and can persist for roughly a century in the atmosphere. They also take several years to reach the stratosphere, so emissions continue influencing ozone long after release.

Very short-lived substances (VSLS)

VSLS are ozone-depleting compounds that break down in less than about six months. Most are natural bromine-containing substances produced by marine organisms, while a smaller fraction is human-produced.

What is the relationship between ozone depletion and skin cancer?

More surface UVB increases DNA damage, including pyrimidine-dimer formation, which can cause replication and transcription errors. Basal and squamous cell carcinomas are strongly associated with UVB exposure; a commonly cited estimate is that a 1% long-term ozone decrease could raise their incidence by about 2%.

How does UVB radiation damage DNA?

UVB can cause adjacent pyrimidine bases in DNA to form covalent dimers. These lesions interfere with accurate DNA replication and transcription and can contribute to mutations and cancer.

Why is the effect of ozone depletion on melanoma difficult to quantify?

Melanoma is associated with UV exposure, but its relationship to specific UV wavelengths and exposure patterns is complex and not fully understood. Lifestyle and other confounding factors also make the effect of ozone loss difficult to isolate.

How can ozone depletion affect the eyes?

Increased UV exposure is associated with cortical cataracts, in which the lens becomes opaque. Ozone depletion is therefore expected to increase the risk of UV-related eye damage.

How can increased surface UV affect ecosystems?

Excess UV can reduce photosynthesis and growth and damage cellular structures in plants and aquatic microorganisms. These effects can propagate through soil microbes, insects, wildlife, and broader ecosystems.

How can ozone depletion increase health risks in the lower atmosphere?

More UV radiation at Earth's surface can promote formation of tropospheric ozone. Ground-level ozone is a harmful oxidant and an air pollutant that can damage respiratory tissues.

How does ozone depletion affect vitamin D production?

Increased UV exposure can increase skin production of vitamin D, a potential benefit. However, it occurs alongside harmful effects such as sunburn, DNA damage, skin cancer, and cataracts.

What is the expected long-term consequence of regulating CFCs and related ODS?

Because atmospheric chlorine and bromine decline slowly, ozone recovery takes many decades. Under current regulations, the ozone layer is projected to return approximately to pre-1980 conditions during the middle or latter part of the twenty-first century.