The ozone layer, a crucial shield in Earth's stratosphere, protects us from harmful UV radiation. It's made of ozone molecules, with peak concentrations at 20-25 km altitude. Understanding its composition and structure is key to grasping its vital role in our atmosphere.

Ozone forms through complex reactions involving UV light and oxygen. It's constantly created and destroyed by natural processes and human-made chemicals. This delicate balance maintains the layer's protective function, safeguarding life on Earth from dangerous UV exposure.

Ozone Layer Composition and Structure

Composition of stratospheric ozone layer

• Located in the stratosphere, approximately 15-35 km above Earth's surface
• Ozone ($O_3$) is a triatomic molecule composed of three oxygen atoms bonded together
• Region of relatively high ozone concentration compared to other parts of the atmosphere (troposphere, mesosphere)
• Ozone concentrations in the stratosphere typically range from 2 to 8 parts per million (ppm) by volume
• Peak ozone concentration occurs at an altitude of about 20-25 km, known as the "ozone layer"

Ozone formation and destruction processes

• Ozone formation in the stratosphere occurs through the Chapman mechanism:
  1. Molecular oxygen ($O_2$) absorbs ultraviolet (UV) radiation with wavelengths less than 242 nm, leading to the dissociation of $O_2$ into two oxygen atoms: $O_2 + hν (λ < 242 nm) → O + O$
  2. The oxygen atoms quickly react with molecular oxygen to form ozone: $O + O_2 + M → O_3 + M$, where $M$ is a third molecule (usually nitrogen or oxygen) that absorbs the excess energy
• Ozone destruction occurs through various catalytic cycles involving:
  • Nitrogen oxides ($NO_x$)
  • Hydrogen oxides ($HO_x$)
  • Chlorine/bromine oxides ($ClO_x$/$BrO_x$)
• Example of a catalytic cycle involving chlorine:
  1. $Cl + O_3 → ClO + O_2$
  2. $ClO + O → Cl + O_2$
  • Net result: $O_3 + O → 2O_2$, destruction of ozone
• Ozone destruction also occurs through photodissociation: $O_3 + hν (λ < 320 nm) → O_2 + O$

Ozone layer's protective function

• Acts as a natural shield, absorbing most of the Sun's harmful ultraviolet (UV) radiation
  • Particularly effective against UV-B (280-315 nm) and UV-C (100-280 nm)
• UV-B radiation can cause various detrimental effects on human health:
  • Skin cancer (melanoma, squamous cell carcinoma)
  • Cataracts
  • Immune system suppression
• UV radiation can also harm plants:
  • Reduces crop yields (wheat, rice, soybeans)
  • Disrupts ecosystems by damaging phytoplankton and other sensitive organisms
• Without the ozone layer, UV radiation reaching Earth's surface would be significantly higher, making life on land difficult or impossible

Factors affecting stratospheric ozone

• Solar radiation: Higher levels of solar UV radiation lead to increased ozone production
• Atmospheric circulation patterns:
  • Brewer-Dobson circulation transports ozone from the tropics to the poles
  • Results in higher ozone concentrations at mid and high latitudes (ozone "hole" over Antarctica)
• Seasonal variations:
  • Ozone concentrations generally higher in the spring and lower in the fall
  • Changes in atmospheric circulation and solar radiation
• Anthropogenic factors:
  • Emissions of ozone-depleting substances (ODS) like chlorofluorocarbons (CFCs) and halons
  • Can lead to the destruction of ozone in the stratosphere
• Natural factors:
  • Volcanic eruptions can inject sulfur compounds into the stratosphere
  • Enhances ozone depletion by providing surfaces for heterogeneous chemical reactions