1-9 Ozone depletion

APES 2024

The stratospheric ozone layer exists roughly

The stratospheric ozone layer exists roughly

45-60 kilometers above Earth

Ozone has the ability to

absorb ultraviolet radiation and protect life on Earth

The ultraviolet (UV) spectrum is made up of 3 increasingly energetic ranges:

• UV-A

• UV-B

• UV-C

Formation of Stratospheric Ozone

• UV-C radiation breaks the molecular bond holding an oxygen molecule together:
  O2 + UV-C → O + O

• A free oxygen atom (O) produced in the first reaction encounters an oxygen
  molecule, and they form ozone
  O + O2 → O3

• Both UV-B and UV-C radiation can break a bond in this new ozone molecule:
  O3 + UV-B or UV-C → O2 + O

Caused by anthropogenic factors

such as chlorofluorocarbons (CFC’s)

Caused by natural factors

melting of ice crystals in the atmosphere at the beginning of the Antarctic spring (September)

High-altitude polar stratospheric clouds form during

the dark, frigid winter

Nitric acid in clouds splits

chlorine off of CFCs

A polar vortex (swirling winds) traps

chlorine

UV radiation in September (spring) sunshine dissipates the

clouds and releases the chlorine

Chlorine destroys

the ozone

December’s warmer air shuts down the

polar vertex

Ozone-poor air diffuses, while

ozone-rich air enters

When chlorine is present (from CFCs), it can attach to an oxygen atom in an ozone molecule to form

chlorine monoxide (ClO) and O2:
O3 + Cl → ClO + O2

The chlorine monoxide molecule reacts with a free oxygen atom, which pulls the oxygen from the ClO to produce

free chlorine again:
ClO + O → Cl + O2

A single chlorine atom can catalyze the breakdown of as many as 100,000 ozone molecules until

finally one chlorine atom finds another and the process is stopped

In the process of stratospheric ozone depletion, the ozone molecules are no longer available to absorb incoming

UV-B radiation (As a result, the UV-B radiation can reach Earth’s surface and cause harm to biological organisms)

Exposure to UV rays can cause

skin cancer and cataracts in humans

Ozone depletion can be mitigated by

replacing ozone-depleting chemicals with substitutes that do not deplete the ozone ex. hydroflourocarbons (HFCs) but some HFCs are
greenhouse gases

Montreal Protocol 1987

• 24 nations signed the Montreal Protocol in response to the rapid increase in the depletion of the ozone layer

• The commitment was to reduce CFC production by 50% by the year 2000

• More than 180 countries signed amendments to eliminate CFC production and use in developed nations by 1996

• Concentration of chlorine has decreased and the ozone depletion has shown improvement

CFCs take a long time to break down so

there is still a large amount present