AP Syllabus focus:
‘Ozone depletion can be reduced by replacing ozone-depleting chemicals with substitutes that do not destroy the ozone layer.’
Replacing ozone-depleting chemicals is a practical way to protect the stratospheric ozone layer. AP Environmental Science emphasizes what gets replaced, what replaces it, and how chemical choice reduces ozone destruction.
What is being replaced (and where it was used)
Ozone-depleting substances (ODS) were widely used because they were stable, nonflammable, and effective in many products. Major historical uses included:
Refrigerants in refrigerators, freezers, and air conditioners
Foam-blowing agents for insulation and packaging foams
Aerosol propellants in sprays
Industrial solvents for cleaning electronics and metals
Some ODS persist long enough to reach the stratosphere, where they can contribute to ozone destruction.
Ozone-depleting substance (ODS): A chemical (often containing chlorine or bromine) that can reduce stratospheric ozone by releasing reactive halogen atoms after breakdown by UV radiation.
How replacement reduces ozone depletion
Replacement reduces ozone depletion by shifting away from chemicals that can deliver chlorine- or bromine-containing radicals to the stratosphere. Effective substitutes are designed to:
Contain no chlorine or bromine, or
Break down more readily in the lower atmosphere, reducing the chance they reach the stratosphere
This aligns with the core syllabus idea: ozone depletion can be reduced by replacing ozone-depleting chemicals with substitutes that do not destroy the ozone layer.
What counts as an effective substitute
“Substitute” can mean a different chemical, a redesigned technology, or a different process that eliminates the need for the original chemical. Common ozone-safer pathways include:
Alternative refrigerants (selected to avoid ozone destruction)
Pump sprays or compressed air systems instead of ozone-depleting aerosol propellants
Water-based or non-halogenated solvents for cleaning
Alternative foam production methods that do not require ozone-depleting blowing agents
Key criteria used to choose replacements
When selecting replacements, decision-makers typically balance multiple constraints:
Zero (or near-zero) ozone depletion potential
Performance (cooling capacity, insulation quality, cleaning effectiveness)
Human health and safety (toxicity, flammability, exposure risk)
Compatibility with existing equipment (materials, lubricants, seals)
Energy efficiency during use (important for overall environmental impact)
Cost and availability (manufacturing, maintenance, supply chains)
Ozone depletion potential (ODP): A relative measure of how much a substance can destroy stratospheric ozone compared with a reference chemical (commonly CFC-11), where higher values indicate greater ozone-depleting ability.
Implementation in the real world: more than swapping a chemical
Replacement succeeds best when combined with management practices that prevent legacy ODS from escaping:
Leak detection and repair in refrigeration and air-conditioning systems
Recovery and recycling of old refrigerants during servicing
Proper end-of-life handling (capturing refrigerants before disposal)
Retrofitting or redesign of equipment so substitutes can be used safely and effectively
Because many ODS were used in long-lived equipment (e.g., building chillers, insulation), reducing emissions often requires both substitution and containment.
Common misconceptions to avoid
Replacing an ODS is not only about “a new product”; it often involves new servicing practices and equipment changes.
“Ozone-safe” means the substitute does not destroy the ozone layer; it does not automatically mean the substitute is impact-free in every other environmental category.
FAQ
ODP is estimated using atmospheric modelling and laboratory data on:
Expected halogen release (if any)
Atmospheric lifetime and transport to the stratosphere
Ozone-destruction chemistry once breakdown occurs
It is then expressed relative to a reference substance (commonly CFC-11).
Retrofitting can require changes to seals, lubricants, and sometimes compressors because substitutes may differ in pressure, temperature behaviour, or material compatibility.
Technician training and certification can also be required for safe handling.
Some substitutes have different safety profiles (e.g., flammability) or operating pressures, so systems may need redesigned ventilation, sensors, or stronger components to meet building and safety codes.
They may be:
Reclaimed to meet purity standards for limited essential uses
Stored as hazardous materials
Destroyed using approved high-temperature or chemical destruction processes to prevent atmospheric release
Rules often combine:
Phase-out schedules for ODS production/import
Servicing requirements (leak repair and recovery)
Product standards that restrict ODS-containing equipment
Reporting and compliance checks across supply chains
Practice Questions
State two ways replacing ozone-depleting chemicals can reduce stratospheric ozone depletion. (2 marks)
Any two of:
Substitute chemicals that contain no chlorine/bromine so they cannot generate ozone-destroying radicals (1).
Use chemicals that break down in the lower atmosphere and are less likely to reach the stratosphere (1).
Replace ODS-based products/technologies (e.g., aerosol propellants, solvents, refrigerants) with ozone-safe alternatives, reducing ODS emissions (1).
Explain how a programme to replace ODS in refrigeration can reduce ozone depletion, and describe three criteria used to select an appropriate substitute. (6 marks)
Explains reduction pathway (max 3):
Phasing out ODS refrigerants lowers emissions of ozone-depleting chemicals (1).
Capturing/recovering old refrigerant during servicing/disposal prevents release (1).
Retrofitting/replacing equipment enables use of non-ozone-depleting refrigerants, avoiding future ODS leaks (1).
Three selection criteria (max 3; 1 each):
Very low/zero ODP (1).
Suitable performance and energy efficiency for cooling demand (1).
Safety (low toxicity and/or manageable flammability risk) (1).
Compatibility with existing system materials/lubricants (1).
Cost/availability for large-scale adoption (1).
