APES - 9.1 & 9.2 - Stratospheric Ozone Depletion and Reducing Ozone Depletion

Stratospheric Ozone & Life on Earth

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

How Ozone Absorbs UV-B & UV-C

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)

Anthropogenic Ozone Depletion

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

Natural Ozone Depletion

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)

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)