Comprehensive Notes on Stratospheric Ozone Depletion

Layers of the Atmosphere

  • The atmosphere consists of several layers including the mesosphere, stratosphere, and troposphere.
  • The stratosphere contains the ozone layer, which is crucial for absorbing UV radiation from the sun.

Stratospheric Ozone Depletion (Topic 9.1)

  • Enduring Understanding (STB-4): Local and regional human activities can have global impacts.
  • Learning Objective (STB-4.A): Explain the importance of stratospheric ozone to life on Earth.
  • Essential Knowledge (STB-4.A.1): The stratospheric ozone layer is vital for the evolution, health, and survival of life on Earth.
  • Essential Knowledge (STB-4.A.2): Stratospheric ozone depletion is caused by anthropogenic factors (e.g., chlorofluorocarbons (CFCs)) and natural factors (e.g., ice crystal melting in the Antarctic spring).
  • Essential Knowledge (STB-4.A.3): Decreased stratospheric ozone increases UV rays reaching Earth, leading to skin cancer and cataracts in humans.

Tropospheric Ozone

  • Tropospheric ozone (O_3) is a secondary pollutant formed when sunlight interacts with primary pollutants.
  • It is the main anthropogenic ingredient in photochemical smog.
  • Ground-level ozone is a greenhouse gas and a toxic pollutant to plants and animals.

Stratospheric Ozone

  • Stratospheric ozone (O_3) protects Earth's living organisms from harmful UV light.
  • Before the ozone layer, life existed only in oceans deep enough to shield organisms from UV radiation, yet shallow enough for photosynthesis.

Good Ozone vs. Bad Ozone

  • Stratospheric ozone is "good" because it shields living things from the sun's ultraviolet radiation.
  • Ground-level ozone is "bad" as it triggers health problems, especially for children, the elderly, and people with lung diseases like asthma.

Global Warming vs. Ozone Depletion

  • Global warming occurs in the troposphere.
  • Ozone depletion occurs in the stratosphere.

Key Points to Remember

  • Tropospheric ozone is a harmful air pollutant that damages plants and human health.
  • Stratospheric ozone absorbs some types of ultraviolet radiation.

Ozone Composition

  • An ozone molecule consists of three oxygen atoms bonded together (O_3).
  • Ozone is formed from atomic oxygen (O) and diatomic oxygen (O_2).

Solar Radiation

  • The sun radiates energy at different wavelengths, including UV-A, UV-B, and UV-C.
  • UV radiation can damage the tissues and DNA of living organisms.

UV-C Radiation

  • UV-C has the shortest wavelength and highest energy.
  • The ozone layer absorbs most UV-C rays, preventing them from reaching Earth's surface.

UV-A Radiation

  • UV-A has the longest wavelength and lowest energy.
  • Very little UV-A is absorbed by the ozone layer.
  • UV-A rays are the most common and cause skin aging and wrinkling.
  • Tanning salons use UV-A and UV-B rays.

UV-B Radiation

  • Exposure to UV-B radiation increases the risk of skin cancer and cataracts and suppresses the immune system.

UV-B Impact on Life

  • UV-B harms plant cells, reducing their ability to convert sunlight into usable energy.
  • This can harm entire biological communities.
  • Loss of phytoplankton (microscopic algae) can deplete fisheries.

Protecting Ourselves from UV Radiation

  • Use sunscreens and sunblocks (avoid sprays).
  • Stay out of the sun.
  • Wear sunglasses with UV protection.
  • Protect skin with clothing.
  • Keep babies under 6 months out of the sun; sunscreen is not recommended for infants.

Malignant Melanoma

  • A type of skin cancer occurring in pigmented areas like moles.
  • It can spread to other organs within months.
  • UV-B radiation is largely responsible for this deadliest skin cancer.

Basel Cell Carcinoma

  • The most common skin malignancy, usually caused by excessive sunlight or tanning lamps.
  • It develops slowly and rarely metastasizes.
  • Nearly 100% curable if diagnosed early and treated properly.

Tanning Parlors

  • A study found more skin cancer cases are due to indoor tanning than lung cancer cases are due to smoking.

Lag Effect

  • There's a significant lag time between exposure to harmful UV radiation and subsequent skin cancer development.

Increasing Skin Cancer Risk

  • Sunburns with blistering in childhood significantly increase the risk of developing melanoma later in life.

Ozone Shield

  • The ozone layer in the stratosphere is called the ozone shield because it absorbs most hazardous UV radiation.
  • If this radiation reached Earth, it would harm or kill most living things.

Stratospheric Ozone Formation

  • O_2 + UV-C \rightarrow O + O (occurs to a few oxygen molecules).
  • O + O2 \rightarrow O3
  • O3 + UV-B \text{ or } UV-C \rightarrow O2 + O
  • Ozone formation and breakdown in sunlight is a continuous cycle.
  • Under normal conditions, the amount of ozone in the stratosphere remains constant.

CFCs (Chlorofluorocarbons)

  • The first CFC was discovered by Thomas Midgley, Jr., a GM chemist, in 1930.
  • There is a family of "useful" CFCs.
  • Considered safe (nontoxic and nonflammable) and essential to modern life.
  • Producing them was a multi-billion dollar industry.

CFC Uses

  • CFCs were used as refrigeration and air conditioning coolants.
  • Also used as aerosol propellants, fumigants, blowing foams (insulation, cushions, Styrofoam), and solvents.

The Dark Side of CFCs

  • Sherwood Rowland and Mario Molina's research in 1974 indicated CFCs were lowering stratospheric ozone concentration.
  • Their call to ban CFCs shocked the scientific and CFC industry (worth $28 billion a year).
  • They were later awarded the Nobel Prize.

Rowland and Molina’s Research Conclusions

  • CFCs remain in the troposphere because they are insoluble and chemically unreactive.
  • Over 11-20 years, these heavier-than-air chemicals are lifted into the stratosphere via convection, drift, and air mixing.

Chlorine in the Stratosphere

  • CFCs introduce chlorine (Cl) into the stratosphere when CFC molecules break down under high-energy UV radiation.
  • O3 + Cl \rightarrow ClO + O2
  • ClO + O \rightarrow Cl + O_2

Chlorine as a Catalyst

  • Chlorine starts and ends as a free Cl atom.
  • Ozone and a free oxygen atom are converted into two oxygen molecules.
  • A catalyst aids in a reaction without being used up.
  • Chlorine is a catalyst.

Chlorine’s Impact

  • A single chlorine atom can catalyze the breakdown of up to 100,000 ozone molecules.
  • Ozone molecules are no longer available to absorb incoming UV-B radiation.
  • More UV-B radiation reaches Earth, harming biological organisms.

Other Ozone-Depleting Compounds (ODCs)

  • Halons and hydrobromofluorocarbons (HBFCs) are used in fire extinguishers.
  • Methyl bromide is a widely used fumigant in agriculture.
  • Hydrogen chloride was emitted by the space shuttle program.
  • Carbon tetrachloride, methyl chloroform, n-propyl bromide, and hexachlorobutadiene are cleaning solvents.

Researchers' Observations

  • In the mid-1980s, researchers noticed stratospheric ozone in Antarctica decreasing each year since 1979.
  • Global ozone concentrations had decreased by more than 10%.
  • Depletion was greatest at the poles but occurred worldwide.

Dobson Units

  • A Dobson unit (DU) is the basic measure used in ozone research.
  • Ozone layer thickness is expressed in Dobson units, measuring its physical thickness if compressed in Earth’s atmosphere.

Ozone Loss Seasonality in Antarctica

  • Ozone depletion occurs from roughly August through November (late winter through early spring in the Southern Hemisphere).
  • This depletion created an area of severely reduced ozone concentrations over Antarctica, called the “ozone hole” (actually a thinning).

Ozone Loss in the Arctic

  • Ozone depletion occurs over the Arctic in January through April, but it is less severe and varies more year to year.
  • It does not usually cause a “hole” as in the Antarctic.

Causes of the Antarctic Ozone Hole

  • Each winter in the Southern Hemisphere, steady winds blow in a circular pattern over the Earth’s poles.
  • This creates a polar vortex: a huge swirling mass of very cold air isolated from the rest of the atmosphere until the sun returns.

The Formation of ClO

  • Water droplets in clouds enter this frigid air stream and form tiny ice crystals that collect CFCs and other ODCs on their surfaces.
  • This sets up conditions for the formation of ClO, which is most responsible for seasonal ozone loss over the Antarctic.

Effects at the End of Winter

  • Huge masses of ozone-depleted air flow northward and linger over parts of Australia, New Zealand, South America, and South Africa.
  • This raises biologically damaging UV-B levels in these areas by 3-10%, and in some years as much as 20%.

Increase in UV-B Radiation

  • Besides adverse impacts on ecosystem productivity, significant increases in skin cancers have been recorded, especially in countries near the Antarctic ozone hole like Chile and Australia.

Chlorine Reduction Process

  • CFCs are not easily removed from the stratosphere, and ozone depletion has reached record levels in some recent years.
  • However, with the leveling off of chlorine concentrations, stratospheric ozone depletion should decrease in subsequent decades, along with an eventual decrease in skin cancer cases.

Effects of Ozone Depletion

  • Human Health:
    • Worse sunburns
    • More eye cataracts and skin cancers
    • Immune system suppression
  • Food and Forests:
    • Reduced yields for some crops
    • Reduced seafood supplies due to reduced phytoplankton
    • Decreased forest productivity for UV-sensitive tree species
  • Climate Change:
    • While in the troposphere, CFCs act as greenhouse gases
  • Wildlife:
    • Increased eye cataracts in some species
    • Decreased populations of aquatic species sensitive to UV radiation
    • Reduced populations of surface phytoplankton
    • Disrupted aquatic food webs
  • Air Pollution and Materials:
    • Increased acid deposition
    • Increased photochemical smog
    • Degradation of outdoor paints and plastics

Reducing Exposure to UV Radiation

  • Stay out of the sun, especially between 10 A.M. and 3 P.M.
  • Do not use tanning parlors or sunlamps.
  • Wear protective clothing and sunglasses that protect against UV-A and UV-B radiation when in the sun.
  • Be aware that overcast skies do not protect you.
  • Do not expose yourself to the sun if you are taking antibiotics or birth control pills.
  • Use sunscreen with a protection factor of at least 15 when in the sun.
  • Examine your skin and scalp at least once a month for moles or warts that change in size, shape, or color and sores that do not heal. Consult a doctor immediately if you observe any of these signs.