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Module Introduction:

We have seen that tropospheric or ground-level pollution has been shown to contribute to a number of problems in the natural world, to exacerbate asthma and breathing difficulties in humans, and to cause cancer.
Now we turn to the effects of certain pollutants in the stratosphere that have a substantial impact on the health of humans and ecosystems.
In the troposphere, ozone is an oxidant that can harm respiratory systems in animals and damage a number of structures in plants.
However, in the stratosphere ozone forms a necessary, protective shield against radiation from the Sun; it absorbs ultraviolet light and prevents harmful ultraviolet radiation from reaching Earth.

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Module 49: Stratospheric Ozone Depletion

Module #49 Review Questions: C, D, D, C, A	Review Essential Knowledge: 9.1, 9.2

Learning Objectives After this module you should be able to: Explain the benefits of stratospheric ozone and how it forms. Describe the depletion of stratospheric ozone. Compare tropospheric and stratospheric ozone. Explain efforts to reduce ozone depletion especially the Montreal Protocol.

Additional Resources to Review
Bozeman: Stratospheric Ozone NOAA: HCFCs

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Essential Knowledge

9.1 Stratospheric Ozone Depletion (Module 49)

The stratospheric ozone layer is important to the evolution of life on Earth and the continued health and survival of life on Earth.
Stratospheric ozone depletion is caused by anthropogenic factors, such as chlorofluorocarbons (CFCs), and natural factors, such as the melting of ice crystals in the atmosphere at the beginning of the Antarctic spring.
A decrease in stratospheric ozone increases the UV rays that reach the Earth’s surface. Exposure to UV rays can lead to skin cancer and cataracts in humans.

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Essential Knowledge

9.2 Reducing Ozone Depletion (Module 49)

Ozone depletion can be mitigated by replacing ozone-depleting chemicals with substitutes that do not deplete the ozone layer. Hydrofluorocarbons (HFCs) are one such replacement, but some are strong greenhouse gases.

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Stratospheric Ozone

Stratospheric ozone: Stratospheric ozone is a naturally-occurring gas that filters the sun's ultraviolet (UV) radiation.
Stratospheric ozone is beneficial to life on Earth.
The stratospheric ozone layer exists roughly 45-60 kilometers above Earth.
Ozone has the ability to absorb ultraviolet radiation and protect life on Earth. UV radiation can be damaging to biological molecules like DNA.
The ultraviolet (UV) spectrum is made up of three increasingly energetic ranges: UV-A UV-B, and UV-C.

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Formation of Stratospheric Ozone

UV-C radiation breaks the molecular bond holding an oxygen molecule together:

O₂ + UV-C → O + O

A free oxygen atom (O) produced in the first reaction encounters an oxygen molecule, and they form ozone.

O + O₂ → O₃

Both UV-B and UV-C radiation can break a bond in this new ozone molecule:

O₃ + UV-B or UV-C → O₂ + O

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Protective Ozone

Stratospheric ozone has a very different impact on life than tropospheric ozone, which acts a respiratory irritant and pollutant.
In the stratosphere, ozone absorbs high energy UV-B and UV-C, protecting sensitive biological molecules and cells in organisms.

UV-A light is the least absorbed by the ozone layer but is also has the longest wavelength and the least amount of energy of the 3 UV types. This makes UV-A far less dangerous to organisms than UV-B or UV-C.

UV-C is the most filtered because it is absorbed to create AND destroy ozone. UV-B is only involved in the destruction of ozone, hence why it is only partly filled. UV-A does not assist with the natural accumulation or destruction of ozone, hence it is the least absorbed.

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Chlorofluorocarbons

Chlorofluorocarbons (CFCs): nontoxic, nonflammable chemicals containing atoms of carbon, chlorine, and fluorine used in the manufacture of aerosol sprays, blowing agents for foams and packing materials, as well as solvents and refrigerants.
CFCs were thought to be safe, but inadvertently damaged the stratospheric ozone layer, by introducing chlorine to the atmosphere.
Extended use of CFC put holes in the ozone layer, allowing harmful UV light to pass through the atmosphere and reach the surface of the Earth.
One Cl atom can destroy as many as 100,000 ozone particles.

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Breakdown of Stratospheric Ozone

When chlorine is present (from CFCs), it can attach to an oxygen atom in an ozone molecule to form chlorine monoxide (ClO) and O₂:

O₃ + Cl → ClO + O₂

The chlorine monoxide molecule reacts with a free oxygen atom, which pulls the oxygen from the ClO to produce free chlorine again:

ClO + O → Cl + O₂

A single chlorine atom can catalyze the breakdown of as many as 100,000 ozone molecules until finally one chlorine atom finds another and the process is stopped.
In the process, the ozone molecules are no longer available to absorb incoming UV-B radiation.
As a result, the UV-B radiation can reach Earth’s surface and cause harm to biological organisms.

The Cl atom effectively replaces UV light in the natural cycle of ozone creation and destruction, allowing UV light pass through the atmosphere more easily.

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Visualizing Stratospheric Ozone Breakdown

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Environmental Impacts of

Ozone Loss

UV light is high intensity and can be damaging to biological molecules such as DNA and ultimately cause cancer including melanoma.
Depletion of the ozone layer increases the amount of UVB and UVC that reaches the Earth’s surface.
Exposure to UV light can suppress primary productivity in both plants and other photosynthetic organisms such as algae and phytoplankton.

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Depletion of the Ozone Layer

This data from Switzerland shows a generally decreasing trend in ozone concentration from 1970 to 2011.
In 1987, 24 nations signed the Montreal Protocol, agreeing to reduce the use of CFCs by 50% by the year 2000.
These efforts have reduced CFC levels to 5 ppb and should fall to 1 ppb by the year 2100.

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APES Exam Environmental

Legislation: Montreal Protocol (1987)

The Montreal Protocol, finalized in 1987, is a global agreement to protect the stratospheric ozone layer by phasing out the production and consumption of ozone-depleting substances (ODS) such as chlorofluorocarbons (CFCs) and halons.
The stratospheric ozone layer filters out harmful ultraviolet radiation, which is associated with an increased prevalence of skin cancer and cataracts, reduced agricultural productivity, and disruption of marine ecosystems.
The Montreal Protocol has proven to be innovative and successful, and is the first treaty to achieve universal ratification by all countries in the world.

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APES Exam Environmental

Legislation: Montreal Protocol (1987)

Full implementation of the Montreal Protocol is expected to result in avoidance of more than 280 million cases of skin cancer, approximately 1.6 million skin cancer deaths, and more than 45 million cases of cataracts in the United States alone by the end of the century, with even greater benefits worldwide.
The Montreal Protocol’s Scientific Assessment Panel estimates that with implementation of the Montreal Protocol we can expect near complete recovery of the ozone layer by the middle of the 21st century.
Further information on the science of the Stratospheric Ozone Layer can be found on the NASA and NOAA websites, and information on the U.S. domestic implementation of the Montreal Protocol can be found on the EPA website.

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Antarctic Ozone Hole

Anthropogenic chemicals including CFCs generate this impact. The chemistry that leads to their formation involves chemical reactions that occur on the surfaces of cloud particles that form in cold stratospheric layers, leading ultimately to runaway reactions that destroy ozone molecules.
In warmer temperatures fewer polar stratospheric clouds form and they don’t persist as long, limiting the ozone-depletion process.

The Antarctic ozone hole forms during the Southern Hemisphere’s late winter as the returning Sun’s rays start ozone-depleting reactions.
Cold winter temperatures persisting into the spring enable the ozone depletion process, which is why the “hole” forms over Antarctica.

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Hydrochlorofluorocarbons HCFCs

HCFCs are compounds containing carbon, hydrogen, chlorine and fluorine.
The HCFCs have shorter atmospheric lifetimes than CFCs and deliver less reactive chlorine to the stratosphere where the "ozone layer" is found.
Consequently, it is expected that these chemicals will contribute much less to stratospheric ozone depletion than CFCs.
Because they still contain chlorine and have the potential to destroy stratospheric ozone, they are viewed only as temporary replacements for the CFCs.
Current international legislation has mandated production caps for HCFCs; production is prohibited after 2020 in developed countries and 2030 in developing countries.

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Stratospheric vs. Ground-Level Ozone

	Location	Environmental Effects	Significant Chemical Involvement	Current Status
Stratospheric Ozone	Upper to middle stratosphere	Breakdown causes increased UV-B radiation that damages cells, reduces photosynthesis and causes health problems in humans including skin cancer, cataracts and immune system suppression	CFCs introduce Cl which causes the reaction: O₃ + Cl → ClO + O₂	Efforts to reduce ozone depletion through restricting the use of CFCs have been effective but Cl reduction in stratosphere is slow
Tropospheric Ozone	Lower troposphere	Reduces lung function and exacerbates respiratory symptoms, harms plant surfaces and damages materials such as rubber and plastic	A secondary pollutant formed by the combination of sunlight, water, VOCs and NO_x.	In 2013, reported that 46 regions did not comply with maximum allowable ozone concentration in the air of 0.075 ppm over an 8 hour period.

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Module Review:

In this module, we have seen that there is a natural process of ozone formation and ozone destruction in the stratosphere.
As a result, with relatively constant ozone concentrations, harmful ultraviolet radiation is absorbed in the upper atmosphere and does not reach ground level where it could be harmful to plants and animals, including humans.
However, the introduction of human-synthesized chlorofluorocarbons led to increased amounts of chlorine in the stratosphere, leading to destruction of stratospheric ozone.
This caused an increase in UV-B radiation in certain locations on Earth. Since nations signed the Montreal Protocol on Substances That Deplete the Ozone Layer, stratospheric ozone depletion has begun to slow.