APES Unit 9.1 & 9.2 (Stratospheric Ozone)
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 -100oF 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)
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 -100oF 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)
