6.4 Stratospheric Ozone

Guiding Questions

• How does the ozone layer maintain equilibrium?

• How does human activity change this equilibrium?

Understandings

• The Sun emits electromagnetic radiation across a spectrum ranging from low-frequency radio waves to high-frequency gamma radiation.

• Shorter wavelengths, especially UV radiation, have higher frequencies and therefore possess more energy, posing increased danger to life.

• Stratospheric ozone absorbs UV radiation from the Sun, thereby reducing the amount that reaches Earth's surface and protecting living organisms from harmful effects.

• UV radiation has numerous adverse effects:

  • Reduces photosynthesis in phytoplankton.

  • Causes DNA damage which may lead to mutations and cancers. In humans, exposure causes sunburn, premature skin aging, and cataracts.

• The relative concentration of ozone has remained constant over long periods due to a steady-state equilibrium maintained between ozone formation and destruction processes.

• Human-generated ozone-depleting substances (ODSs) disrupt this equilibrium by accelerating the natural breakdown of ozone.

• Ozone depletion permits increasing amounts of UVB radiation to penetrate the atmosphere, adversely affecting ecosystems and human health.

International Treaties and Actions

• The Montreal Protocol is recognized as an international treaty that regulates the production, trade, and use of chlorofluorocarbons (CFCs) and other ODSs.

• It is considered the most successful example of international cooperation aimed at addressing environmental issues.

• The actions taken in compliance with the Montreal Protocol have successfully prevented the crossing of the planetary boundary for stratospheric ozone depletion.

• ODSs release halogens (e.g., chlorine, fluorine) into the stratosphere, leading to ozone destruction.

• Polar stratospheric ozone depletion predominantly occurs in the spring due to distinct chemical and atmospheric conditions in polar regions.

• Hydrofluorocarbons (HFCs) were developed as replacements for CFCs, as they can be similarly applied but result in much less ozone depletion. However, HFCs are potent greenhouse gases (GHGs) and have been controlled by the Kigali Amendment to the Montreal Protocol.

• Air conditioning units are energy-intensive and traditionally contained ODSs, contributing to GHG emissions.

Electromagnetic Radiation

• Electromagnetic (EM) radiation refers to the complete range of radiation with magnetic and electric fields.

• The Sun primarily emits radiation in the visible portion of the electromagnetic spectrum, with peak emissions occurring here.

• Earth's atmosphere protects its surface from higher energy waves through the presence of gases like water vapor, carbon dioxide, and ozone.

• Infrared radiation serves as radiant heat; although invisible to the human eye, it can be felt as warmth.

• The ecosphere absorbs infrared heat, which drives essential processes in the atmosphere, hydrosphere, and biosphere.

• Visible light consists of a mixture of wavelengths detectable by human eyes, while bees can see ultraviolet light that guides them to nectar sources.

• Photosynthesis in green plants occurs because specific leaf pigments, primarily chlorophyll and carotenoids, absorb certain light wavelengths.

  • Chlorophylls absorb blue and red light while reflecting green light, thus appearing green.

  • Carotenoids absorb violet and blue-green light, which make fruits appear red (e.g., tomatoes) or yellow (e.g., corn).

Effects of Ultraviolet Radiation (UV)

• UV radiation can damage organisms in various ways:

  • UV-A:

  • Comprises 95% of terrestrial UV radiation and has the longest wavelengths.

  • Causes wrinkles and premature skin aging; can penetrate skin to the middle layer.

  • Capable of penetrating glass and clouds.

  • UV-B:

  • Comprises 5% of terrestrial UV radiation with slightly shorter wavelengths than UV-A.

  • Penetrates only the top layer of skin, can cause reddening and burning, and is strongly linked to skin cancer due to DNA damage.

  • Can be blocked by glass.

  • SPF 30 sunscreen deflects UV-B radiation.

  • UV-C:

  • Has the shortest wavelengths (highest frequencies) and is mostly absorbed by the stratospheric ozone, protecting Earth.

  • Can be intentionally used in tertiary water treatment as a disinfectant.

  • Particularly harmful to the eyes and emitted by lasers and old tanning beds.

Damaging Effects of UV Radiation

• Increased exposure to UV radiation has severe consequences:

  • Aquatic ecosystems, critical for biomass production, experience reduced productivity in phytoplankton near the poles due to UV damage.

  • The human immune system can be suppressed by UV exposure.

  • Cataracts form when lens proteins denature, clouding vision and potentially leading to blindness; approximately 15 million people worldwide are estimated to be blind due to cataracts, with 10% attributed to UV exposure (WHOs data).

  • Skin cancers, particularly melanoma, are caused predominantly by UV exposure, with over 1.5 million diagnosed cases and over 120,000 deaths reported globally in 2020.

  • Excessive sun exposure in youth significantly increases skin cancer risk in later life, particularly in Australia and New Zealand, where skin cancer cases have surged.

Protective Measures

• In New Zealand, summer weather reports include burn time isolines.

• Australia promotes the sun safety slogan:

  • "Slip on a shirt; Slop on sunscreen (factor 30+); Slap on a hat; Seek shelter; Slide on the shades (sunglasses)."

• Originally, the slogan was simplified to just “slip, slop, slap” in 1981 but was updated in 2007 to emphasize seeking shade and using sunglasses.

Beneficial Effects of UV Radiation

• Convincing benefits of UV radiation include:

  • Stimulating vitamin D production in animals. Vitamin D deficiency can lead to rickets.

  • Clinical treatment of skin diseases like psoriasis and vitiligo.

  • Sterilization properties that kill pathogenic bacteria; also used as air and water purifiers.

  • Various industrial applications such as lasers, historical document viewing, forensic analysis, and specialized lighting.

Introduction to Ozone

• Oxygen gas is primarily diatomic (O₂) while ozone (O₃) consists of three oxygen atoms.

• Ozone Presence in Atmosphere:

  • Stratospheric Ozone:

  • Acts as protective °"good" ozone, blocking incoming UV radiation from the Sun.

  • Critical for protecting life from damaging UV.

  • Not a contributor to global warming.

  • Tropospheric Ozone:

  • Considered °"bad" ozone as a GHG; concentrations vary and is short-lived.

Ozone Layer

• The ozone layer is a reactive gas predominantly located in the lower stratosphere, with the highest concentration generally found at altitudes between 20 and 40 km (15-20 km at the poles).

• Its thickness is approximately 1-10 ppm of ozone.

• The ozone layer absorbs:

  • 99% of incoming UV-C radiation

  • Most incoming UV-B radiation

  • 5% of UV-A radiation

• The incoming UV-C radiation is most penetrating and damaging.

• The ozone layer maintains a dynamic equilibrium; ozone is perpetually produced from oxygen atoms and simultaneously converted back to oxygen.

• If the rates of ozone formation and depletion become unequal, this balance becomes disrupted.

Ozone Formation and Destruction

• Ozone formation and destruction involves the absorption of UV radiation.

• UV radiation leads oxygen molecules (O₂) to split into two individual oxygen atoms.

• These reactive oxygen atoms can then combine with other oxygen molecules to create ozone (O₃) according to the reaction:
  O + O2 ightarrow O3

• Ozone molecules themselves can absorb UV radiation, resulting in their decomposition back into an oxygen molecule and an oxygen atom:
  O3 + UV ightarrow O2 + O

• The released oxygen atom can subsequently react with another oxygen molecule, reforming ozone:
  O + O2 ightarrow O3

• The role of the ozone layer in absorbing UV radiation is vital; without it, terrestrial life could not exist.

Ozone Depletion - The Ozone Hole

• Since the 1950s, ozone levels have been monitored over Antarctica, with dramatic thinning recognized in the early 1980s, termed the ozone hole.

• Measurements revealed atmospheric ozone levels below 220 Dobson units.

• Ozone depletion peaks annually in the spring (September and October in the southern hemisphere) but the layer recovers in November.

• The ozone hole's area has continued to expand, with thickening observed taking longer and also measured across the Arctic region.

Causative Factors of Ozone Depletion

• Ozone depletion is largely due to chemicals, particularly human-made ODSs, which augment the natural degradation process and disrupt equilibrium.

• Major ODSs include halogenated organic gases such as:

  • CFCs (Chlorofluorocarbons)

  • HCFCs (Hydrochlorofluorocarbons)

  • HFCs (Hydrofluorocarbons)

• Comparison of ODSs:

  • CFCs and HCFCs release chlorine atoms upon exposure to UV radiation, destroying ozone.

  • HFCs are not considered ODSs but are potent GHGs with a shorter life; they can replace CFCs and HCFCs.

Table 1: Summary of Ozone-Depleting Substances

The Mechanism of Ozone Destruction

• Chlorine atoms released from CFCs bond with ozone, causing significant degradation:
  Cl + O3 ightarrow ClO + O2
  ClO + O
  ightarrow Cl + O_2

• A single chlorine atom can destroy thousands of ozone molecules in a repeated chain reaction.

Human Action Against Ozone Depletion

• CFC removal from aerosols is showcased as an individual positive action; petitions and boycotts of CFC products influenced companies to cease production.

• Industry shifts began with companies like Johnson and Sons implementing bans.

• Montreal Protocol (1987):

  • Organized by UNEP to regulate CFCs and other ODSs.

  • Involved 197 countries which agreed to freeze and reduce ODS production by 1986 levels.

  • Substantial amendments were made post-1987.

  • The Montreal Protocol is a benchmark in global cooperation to mitigate environmental issues.

Timeline of CFC Reduction

• 1970s: Recognition of CFC's ozone-depleting characteristics begins.

• 1985: The British Antarctic Survey reports the ozone hole.

• 1987: Implementation of the Montreal Protocol.

• 2006: Largest recorded ozone hole documented by NASA and NOAA.

• 2007: Agreement to phase out HCFCs by 2030.

• 2019: Ratification of the Kigali Amendment targeting HFC production.

Impact and Significance of the Montreal Protocol

• The actions taken through the Montreal Protocol have prevented a global environment becoming uninhabitable by avoiding a widespread ozone hole.

• Factors leading to successful implementation include:

  • The immediate risk to human health raised alarm.

  • Clear visual evidence from satellite imagery made the hazard tangible.

  • Feasible technological alternatives facilitated quick transitions.

• The Protocol serves as a model for global environmental agreements, embodying precautionary principles and adaptive policies.

Current Ozone Status and Challenges

• Despite successful measures, the ozone hole still appears annually above Antarctica due to the long-lived nature of certain ODSs, persisting for 50-150 years post-emission.

• Additional factors affecting ozone levels include natural occurrences, such as volcanic eruptions, and anthropogenic emissions like growing nitrous oxide from fertilizers, which remain unregulated by the Montreal Protocol.

Monitoring and Future Considerations

• Ozone depletion monitoring remains crucial as various layers of threats emerge over time, requiring sustained global commitment and adaptive strategies.