Stratospheric Ozone & Life on Earth

• Ozone in the stratosphere absorbs UV-C and much of UV-B

  • Without ozone layer, life on land would not be possible since UV-B & C radiation causes significant tissue damage & mutates DNA

  • Human Health benefits of startosphereic ozone:

    • Prevention of skin cancer & cataracts

    • UV-B & C mutate DNA (skin cancer) & cause oxidative stress in eyes (cataracts)

• Tropospheric ozone is bad (respi. Irritant, damaging to plant tissue & prescursor to photochemical smog)

How Ozone Absorbs UV-B & UV-C

• UV-C breaks into O₂ into two free oxygen atoms (2 O)

  • When a free oxygen atom from this rxn combines with an O₂ molecule, ozone (O₃) is formed

  • UV-C also reverses the rxn by breaking ozone (O₃) into O₂, and O, which can then bond with another free O to form O₂

  • Continued formation and breakdown of O₃ in stratosphere absorbs all UV-C & much UV-B radiation (protecting org. On earth)

Antrhopogenic Ozone Depletion

• CFCs (chlorofluorcarbons) are a primary antrhopogeic (human) cause of O₃ breakdown

  • Used as refrigerant chemicals and propellants in aerosol containers (hair spray, febreeze, etc.)

  • UV radiation causes free chlorine atom to separate from CFCs

  • Highly electroneg. Chlroine atom bonds to one of the oxygen atoms of ozoen (O₃) converting it into oxygen (O₂)

  • Free O atom then bonds to O from chlroine monoxide to form O₂ and free CL atom to go break down more O₃

  • One singel Cl atom persists in atm. for 50-100 years and can destroy up to 100000 ozone molecules

Natural Ozone Depletion

• Antartica spring melt forms polar stratospheric clouds (PSC)

  • Clouds made of water & nitric acid (HNO₃) that can only form in consistent -100^oF temp. Range found above antartica 

  • In prescense of PSCs chlorine nitrate (ClONO₂) and hydrochloric acid (HCl) react & give off Cl₂

    • CL₂ is photolyzed (broken by sun) into 2 free Cl atoms

• Remember what Cl atoms due to ozone from CFCs (break O₃ down into O₂ over and over)

Reducing Ozone Depletion

• Main way to reduce anthropogenic O₃ depletion is phasing out & replacing CFCs

  • Montreal Protocol (87’) was a global agreement to phase CFCs out of production in refrigerators, aerosols and other uses

    • Replaced with HCFCs (CFCs with hydrogen added)

    • HCFCs still deplete O₃ and act as GHGs but to a lesser degree than CFCs

    • Not a permanent solution, but a temporary transition option (phase out in dev. Nations after 2020, developing nations have until 2030)

  • Replacement for HCFCs is HFCs (still GHGs but not O₃ depleting since they don’t contain Cl)

    • Replacements for HFCs are HFOs (just HFCs with C-C double bonds that shorten atm. Lifetime & GWP)