Stratospheric Ozone Depletion Notes

9.1 Stratospheric Ozone Depletion

Stratospheric Ozone

  • The stratosphere contains approximately 97% of the ozone in the atmosphere, mostly between 9 and 25 miles (15-40 km) above Earth's surface.
  • Most ozone is formed over the tropics, but slow circulation currents carry the majority to the poles.
  • The thickness of the ozone layer varies seasonally, being thicker in the spring and thinner in the fall.

Formation of Stratospheric Ozone:

  • Atomic oxygen can combine with oxygen molecules to form ozone: O + O2 \rightarrow O3
  • Atomic oxygen can combine with ozone to form oxygen molecules: O + O3 \rightarrow 2O2 (back to first step)
  • Ultraviolet radiation (UV) can strike ozone and create oxygen molecules and atomic oxygen.
  • Ultraviolet radiation (UV) strikes an oxygen molecule, creating atomic oxygen: O_2 + uv \rightarrow O + O

Ultraviolet Radiation

  • The sun emits a wide variety of electromagnetic radiation, including infrared, visible, and ultraviolet.
  • Ultraviolet radiation is subdivided into three forms: UVA, UVB, and UVC, listed in order of increasing energy (shorter wavelength).

Causes of Ozone Depletion

  • By 1974, nearly 1 million tons of CFC gases were produced each year, with the largest single source being leakage from air conditioners.
  • The average residence time for CFCs in the environment is 200 years.
  • When a CFC molecule enters the stratosphere, ultraviolet radiation causes it to decompose and produce atomic chlorine (Cl).
  • This atomic chlorine then reacts with the ozone in the stratosphere to produce chlorine monoxide (ClO): Cl + O3 \rightarrow ClO + O2
  • The chlorine atom prevents the formation of ozone by sequestering oxygen atoms to form ClO. The chlorine monoxide then reacts with more ozone to produce even more chlorine in a chain reaction: ClO + O3 \rightarrow Cl + 2O2
  • One chlorine atom released from a CFC can destroy over 100,000 ozone molecules.
  • Much of the destruction of the ozone layer that is occurring now is the result of CFCs that were produced many years ago, since a CFC molecule takes about eight years to reach the stratosphere, and the residence time in the stratosphere for a CFC molecule is over 200 years.
  • Halocarbons (halons) are organic chemical molecules composed of at least one carbon atom with one or more halogen atoms (fluorine, chlorine, bromine, and iodine).
  • They are used in fire extinguishers, soil fumigants and pesticides (e.g., methyl bromide), solvents, and foam-blown insulation.
  • The first synthesis of halocarbons was achieved in the early 1800s. However, production began accelerating when their useful properties as solvents and anesthetics, and their uses in plastics and pharmaceuticals, were discovered.
  • A large amount of naturally occurring halocarbons are created by forest fires and other forms of biomass burning, volcanic activities, and marine algae, which produce millions of tons of methyl bromide annually.
  • Bromine, which is found in much smaller quantities than chlorine, is about 50 times stronger than chlorine in its effect on stratospheric ozone depletion.

Effects of Ozone Depletion

  • During the onset of the 1998 Antarctic spring, a hole three times the size of Australia (over 3,500 miles [5,600 km] in diameter) developed in the ozone layer over the South Pole.
  • Harmful effects of increased UV radiation include:
    • A reduction in crop production
    • A reduction in the effectiveness of the human body's immune system
    • A reduction in the growth of phytoplankton and the cumulative effect on food webs
    • Climatic changes
    • Cooling of the stratosphere
    • Deleterious effects on animals (since they don't wear sunglasses or sunscreen)
    • Increases in cataracts
    • Increases in mutations, since UV radiation causes changes in the DNA structure
    • Increases in skin cancer
    • Increases in sunburns and damage to the skin (premature aging)

UVA

  • UVA is closest to blue light in the visible spectrum and is the form of ultraviolet radiation that usually causes skin tanning.
  • UVA radiation is 1,000 times less effective than UVB in producing skin redness, but more of it reaches Earth's surface than UVB.
  • Birds, reptiles, and bees can see UVA since many fruits, flowers, and seeds stand out more strongly from the background in ultraviolet wavelengths. Many birds have patterns in their plumage (feathers) that are not visible in the normal spectrum (white light) but become visible in ultraviolet. The urine of some animals is also visible in the UVA spectrum.

UVB

  • UVB causes blistering sunburns and is associated with skin cancer.

UVC

  • UVC is found only in the stratosphere and is largely responsible for the formation of ozone.

Ozone Depletion

  • The ozone (O_3) layer is a belt of naturally occurring ozone gas that sits between 9 and 19 miles (15-30 km) above Earth and serves as a shield from the harmful ultraviolet B radiation emitted by the sun.
  • Ozone is a highly reactive molecule and is constantly being formed and broken down in the stratosphere.

Causes of Ozone Depletion

  • Thinning of the ozone layer was first discovered over Antarctica in 1986 due to the presence of extremely cold stratospheric clouds.
  • Ozone depletion occurs seasonally (in the winter) and is due to the presence of both natural and human-made compounds that contain halogens (bromine, chlorine, fluorine, or iodine).
  • Measurements indicate that the ozone over the Antarctic has decreased as much as 60% since the late 1970s, with an average net loss of about 3% per year worldwide.
  • The main cause of degradation of the ozone layer is the presence of chlorofluorocarbons (CFCs) and halocarbons (halons), very stable compounds that, as a result of their chemical stability (persistence), are able to reach the stratosphere—there are no natural reservoirs of CFCs or halons in nature.
  • First manufactured during the 1920s, chlorofluorocarbons are nonflammable chemicals that contain atoms of carbon, chlorine, and fluorine. They are used in the manufacture of aerosol sprays, as blowing agents for foam and packing materials, as solvents, and as refrigerants.