9.1 & 9.2 - Stratospheric Ozone Depletion and Reducing Ozone Depletion

Question 1

Stratospheric Ozone & Life on Earth

Answer 1

Ozone in the stratosphere absorbs UV-C and much of UV-B radiation
- 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 stratospheric ozone:
- Prevention of skin cancer & cataracts
- UV-B & C mutate DNA (skin cancer) & cause oxidative stress in eyes (cataracts)
- Remember: tropospheric = respi. Irritant, damaging to plant tissue & precursor to photochemical smog

Question 2

How Ozone Absorbs UV-B & UV-C

Answer 2

UV-C breaks 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 & break down of O3 in stratosphere absorbs all UV-C & much UV-B radiation (protecting org. on earth)

Question 3

Anthropogenic Ozone Depletion

Answer 3

CFCs (chlorofluorocarbons) are a primary anthropogenic (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. chlorine atom bonds to one of the oxygen atoms of ozone (O3) converting it into oxygen (O2)
- Free O atom then bonds to O from chlorine monoxide to form O2 and free Cl atom to go break down more O3
- One single Cl atom persists in atm. for 50-100 years and can destroy up to 100,000 ozone molecules

Question 4

Natural Ozone Depletion

Answer 4

Antarctica spring melt forms polar stratospheric clouds (PSC)
- Clouds made of water & nitric acid (HNO3) that can only form in consistent -1000 F temp. range found above antarctica
- In presence 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))

Question 5

Reducing Ozone Depletion

Answer 5

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)