6.4B Ozone depletion Notes

Dynamic balance of ozone formation and destruction

1. Stratospheric ozone concentrations have remained relatively constant for thousands of years because ozone formation and ozone destruction occur at the same rate.
2. UV radiation naturally splits oxygen molecules (O₂) into individual oxygen atoms (O), which then recombine with O₂ to form ozone (O₃).
3. When an O₃ molecule absorbs UV radiation, it breaks apart into O₂ + O, meaning the destruction process is also natural.
4. The steady state equilibrium exists because ozone destroyed = ozone produced, maintaining a stable ozone concentration under natural conditions.
5. Although the ozone layer fluctuates seasonally and geographically, long-term concentrations remain stable under natural, pre-industrial conditions.

Ozone Formation and Destruction

Formation of Ozone

1. Ozone (O₃) is formed when ultraviolet (UV-C) radiation from the Sun breaks apart molecular oxygen (O₂) in the stratosphere.
2. The freed oxygen atoms (O) then react with other O₂ molecules to form ozone (O₃).

Destruction of Ozone

1. Ozone is naturally destroyed when it absorbs UV radiation, splitting back into an oxygen molecule (O₂) and a free oxygen atom (O).
2. The free oxygen atom can then recombine with another ozone molecule, breaking it down into more oxygen molecules.
3. This cycle continuously occurs, maintaining an equilibrium concentration of ozone in the stratosphere.

Ozone-Depleting Substances (ODSs) and Their Impact on the Ozone Layer

1. Ozone-depleting substances (ODSs) accelerate the destruction of ozone molecules, disrupting the natural equilibrium between ozone formation and breakdown in the stratosphere.
2. This imbalance leads to ozone depletion, especially over polar regions like Antarctica.
3. They include chlorofluorocarbons (CFCs), halons, hydrochlorofluorocarbons (HCFCs), carbon tetrachloride and methyl bromide.
4. These substances were extensively used because they were non-toxic, non-reactive and excellent refrigerants, which made them popular in:
   1. Aerosols
   2. Refrigeration and air-conditioning
   3. Gas-blown insulating foams
   4. Fire-extinguishing systems
   5. Agricultural fumigants

Key ODSs and Their Effects

1. Chlorofluorocarbons (CFCs): Used in refrigeration, air conditioning, and aerosol propellants.
2. Halons: Found in fire extinguishers.
3. Hydrochlorofluorocarbons (HCFCs): Transitional substitutes for CFCs but still have ozone-depleting potential.
4. Methyl bromide: Used as a pesticide but phased out due to its ozone-depleting effects.

How ODSs disrupt ozone equilibrium

1. ODS molecules slowly migrate to the stratosphere because they are chemically stable in the troposphere.
2. In the stratosphere, exposure to strong UV radiation breaks ODS molecules apart.
3. This releases halogen radicals (especially chlorine and bromine) that trigger chain reactions destroying ozone molecules faster than they are formed.
4. One chlorine atom can destroy up to 100,000 ozone molecules before being removed, which massively disrupts equilibrium.

When formation and destruction become unequal

1. When destruction exceeds formation, equilibrium shifts toward ozone loss, resulting in:
   1. Thinning of the ozone layer
   2. Lower average global ozone concentration
   3. Development of regional ozone holes
2. The breakdown of equilibrium is a direct consequence of anthropogenic ODS emissions.

Impacts of Ozone Depletion on Ecosystems and Human Health

1. Why UVB exposure increases

1. As the ozone layer thins, more UVB radiation penetrates the atmosphere and reaches the Earth’s surface.
2. This causes biological, ecological and health-related impacts that extend worldwide.
3. At the poles, seasonal “ozone holes” form each spring, with the most extreme depletion occurring over Antarctica due to stratospheric weather patterns and ODS concentration.

2. Ecological impacts

1. UVB exposure damages plant tissue and decreases photosynthetic rates, reducing plant growth and crop productivity.
2. Marine phytoplankton are particularly vulnerable, leading to:
   1. Reduced primary productivity
   2. Disruption of aquatic food webs
   3. Decline in fish stocks and marine biodiversity
3. UV radiation can kill nutrient-cycling microbes in soil and water, slowing decomposition and altering nutrient availability.

3. Human health impacts

1. Increased UVB exposure causes:
   1. Skin cancer (melanoma and non-melanoma)
   2. Cataracts and eye damage
   3. Premature skin aging
   4. Immune system suppression
2. Increased sun exposure during childhood raises lifetime cancer risk significantly, making UV protection especially important for younger populations.

Formation of the ozone hole

1. The ozone hole refers to a large seasonal reduction of ozone concentration over the poles.
2. The Antarctic ozone hole appears each spring due to:
   1. Very low stratospheric temperatures
   2. Polar stratospheric clouds that support halogen-driven chemical reactions
   3. The return of sunlight, which triggers rapid ozone destruction
3. Although global levels remain lower than natural conditions, the ozone hole has been shrinking gradually due to reductions in ODS emissions.