6.4D Stratospheric ozone depletion and ODSs (HL) Notes

How ODSs cause ozone depletion

1. Ozone-depleting substances (ODSs) such as CFCs, HCFCs, carbon tetrachloride and methyl chloroform are stable in the troposphere but break down under UV radiation in the stratosphere.
2. When ODS molecules reach the stratosphere, UV radiation causes halogens (especially chlorine and fluorine) to be released.
3. These halogens form highly reactive radicals, which catalytically destroy ozone (O₃).
4. One chlorine radical can destroy thousands of ozone molecules because it is regenerated rather than consumed during reactions.
5. This reduces the concentration of ozone in the stratospheric ozone layer, lowering protection against UV-B radiation.

Major ODSs and Their Impact

The catalytic breakdown of ozone

1. Chlorine radicals initiate a two-step cycle that converts ozone (O₃) into oxygen (O₂).
2. The radical is continuously regenerated and therefore continues destroying ozone as long as it remains in the stratosphere.
3. CFCs are especially harmful because they have long atmospheric lifetimes (often greater than 100 years) and spread globally.

Polar Stratospheric Ozone Depletion in Spring

Why ozone depletion is strongest at the poles

1. The most severe depletion occurs in Antarctica during Southern Hemisphere spring (September–November).
2. Extremely low winter temperatures (as low as −90 °C) allow the formation of polar stratospheric clouds (PSCs) made of frozen water vapour and nitric acid.
3. These clouds provide active surfaces where inactive chlorine compounds (for example, chlorine nitrates) convert into highly reactive forms.

Role of the polar vortex and volcanic aerosols

1. A polar vortex forms over Antarctica during the winter, isolating air masses and trapping ODSs and reactive halogens.
2. This isolation prevents dilution with air from lower latitudes, increasing the concentration of ozone-destroying chemicals.
3. Volcanic aerosols reaching the stratosphere provide additional reactive surfaces, intensifying ozone loss during PSC formation.

Why Antarctic ozone depletion is stronger than Arctic

1. The Antarctic polar vortex is stronger, larger and more stable than the Arctic vortex.
2. Temperatures are lower for a longer duration, enabling more PSC formation.
3. Therefore, greater chlorine activation and greater seasonal ozone depletion occur over Antarctica.

Why depletion peaks in spring

1. During winter, halogens accumulate in PSCs but little ozone is destroyed due to lack of sunlight.
2. At the beginning of spring, sunlight returns and UV radiation reaches the stratosphere, triggering rapid halogen-driven ozone destruction.
3. The ozone hole shrinks during summer once PSCs disappear and halogens convert back to inactive forms.

HFCs as Substitutes for CFCs and the Kigali Amendment

Why HFCs were originally adopted

1. As CFCs were phased out under the Montreal Protocol, hydrofluorocarbons (HFCs) became replacements because they do not significantly deplete ozone.
2. HFCs were used in the same applications as CFCs: aerosols, refrigeration, foam blowing and air-conditioning systems.

Environmental challenges of HFCs

1. Although HFCs do not break down ozone, they have very high global warming potentials (GWP).
2. Some HFCs can trap thousands of times more heat than CO₂.
3. Leakage from old refrigerators, air conditioners and insulating foams releases HFCs into the atmosphere.

The Kigali Amendment

1. The Kigali Amendment (2016) is an update to the Montreal Protocol that specifically targets HFC phase-down.Its goals include:
   1. Reducing the production and consumption of HFCs by more than 80 percent by 2050.
   2. Avoiding up to 0.5°C of additional global warming by 2100.
2. Different phase-down schedules apply to:
   1. High-income countries (earlier and faster reductions)
   2. Lower-income countries (later deadlines, more gradual phase-down)
3. The amendment encourages transition to low-GWP alternatives, such as hydrofluoroolefins (HFOs), ammonia, carbon dioxide and some hydrocarbons.

Air Conditioning, ODSs, Energy Use and Climate Solutions

Environmental impacts of air conditioning

1. Air conditioning (AC) is used to control indoor temperature and humidity for comfort and safety.
2. Its environmental impacts include:
   1. High electricity use, often generated from fossil fuels, which increases carbon dioxide emissions.
   2. Leakage of refrigerants (CFCs, HCFCs, HFCs) from units, which can contribute to:
   3. Ozone depletion (for older ODS refrigerants)
   4. Global warming (for high-GWP refrigerants)
3. Global demand for cooling is rapidly increasing, especially in hot and urban regions and in emerging economies.

Refrigerants in AC units and their evolution

1. Traditional AC systems used refrigerants such as CFCs and HCFCs, which:
   1. Depleted the ozone layer
   2. Often had high GWP
2. After the Montreal Protocol:
   1. Many systems shifted to HFCs, which solved the ozone problem but worsened the climate problem.
3. Newer systems now explore:
   1. HFOs, which have low ozone depletion potential and lower GWP
   2. Natural refrigerants, such as ammonia, carbon dioxide and certain hydrocarbons
4. However, some alternatives involve trade-offs, such as:
   1. Flammability (hydrocarbons)
   2. Toxicity (ammonia)
   3. Higher operating pressures (carbon dioxide)

Alternatives and strategies to reduce reliance on AC

1. Passive building design: insulation, roof reflectivity, double glazing, shading and cross-ventilation reduce heat gain.
2. Urban greening and rewilding: vegetation cools air through shade and evapotranspiration.
3. Cool roofs: reflective surfaces reduce urban heat island effects and reduce demand for cooling.
4. Radiative cooling innovations (e.g., SkyCool panels) cool buildings by reflecting sunlight and radiating heat into the atmosphere.