Environmental Issues: CFCs and Ozone Depletion

Spec Mapping — OCR H432 Module 4.2.2(f) — CFCs and ozone depletion, covering the role of chlorofluorocarbons (CFCs) and related haloalkanes in catalytic destruction of stratospheric ozone; homolytic fission of the weakest C-X bond (typically C-Cl) by UV-C radiation to give a chlorine radical; the propagation chain $Cl\bullet + O_3 \rightarrow ClO\bullet + O_2$ and $ClO\bullet + O \rightarrow Cl\bullet + O_2$ ; the catalytic regeneration of Cl•; the development of HCFC and HFC replacements and their reduced ozone-depleting potential at the cost of greenhouse warming; international policy context — the Montreal Protocol (1987) and the Kigali Amendment (2016) — paraphrased as scientific milestones (refer to the official OCR H432 specification document for exact wording).

The story of CFCs and the ozone layer is one of the great case studies in modern environmental chemistry — and a startling demonstration of how a property that makes a molecule industrially useful (chemical inertness) can make it environmentally disastrous (atmospheric persistence). Chlorofluorocarbons were celebrated through the 1930s-70s as non-toxic, non-flammable, chemically unreactive refrigerants, aerosol propellants and foam-blowing agents — vastly safer than the ammonia and sulfur dioxide they replaced in domestic refrigerators. Because their C-F and C-Cl bonds resisted hydrolysis, oxidation, and photolysis by tropospheric ultraviolet, CFCs survived for decades after release and slowly diffused into the stratosphere, where short-wavelength UV-C finally fractured their weakest bond. The released chlorine radical entered a catalytic cycle that destroyed up to ~10⁵ ozone molecules each before being scavenged. The 1985 discovery of a springtime Antarctic ozone hole, the 1987 Montreal Protocol that phased out CFC production, and the slow stratospheric recovery now observed are widely held up as the single most successful piece of global environmental policy. This lesson develops the stratospheric chemistry — what ozone does, why it matters, why CFCs reach the stratosphere intact, the radical chain mechanism that destroys ozone, the catalytic regeneration of chlorine that magnifies a single CFC molecule into thousands of lost O₃, and the chemistry of the replacement HCFC, HFC and HFO families. Mario Molina and F. Sherwood Rowland's 1974 paper (Nobel Prize 1995, shared with Paul Crutzen) is the foundational reference, paraphrased here.

Key Mechanism: the radical chain destruction of ozone has three phases. (i) Initiation: UV-C photolysis of a CFC homolytically breaks the C-Cl bond, releasing a chlorine atom radical Cl•. (ii) Propagation: Cl• attacks ozone giving ClO• + O₂; ClO• then reacts with a free O atom (from photolysis of O₂ or O₃) regenerating Cl• and giving a second O₂. (iii) Termination: occasionally two radicals meet (Cl• + Cl•, ClO• + NO₂, etc.) closing the chain. The net effect is the catalytic conversion 2 O₃ → 3 O₂, with chlorine recycled. A single Cl• can turn over ~10⁵ ozone molecules before termination.

The ozone layer and why we need it

Ozone (O₃) is the triatomic allotrope of oxygen, with two O-O bonds of bond order 1.5 (resonance between two equivalent Lewis structures). It exists everywhere in the atmosphere at trace levels, but concentrates in the stratosphere (15-35 km altitude) at peak densities around 10 ppm, forming a layer 3 mm thick if compressed to ground-level pressure.

The reason ozone matters is its absorption of high-energy ultraviolet. UV light is divided by wavelength:

Band	Wavelength / nm	Energy	Effect on biology
UV-A	320-400	Lowest	Tans, ages skin
UV-B	280-320	Medium	Causes sunburn, skin cancer, DNA damage
UV-C	<280	Highest	Lethal to most life if it reached the surface

The natural Chapman cycle establishes a steady-state ozone concentration in the stratosphere:

$O_2 \xrightarrow{UV\text{-}C, <242\,nm} 2\,O\bullet$ $O\bullet + O_2 + M \rightarrow O_3 + M$ $O_3 \xrightarrow{UV\text{-}B, 200\text{-}320\,nm} O_2 + O\bullet$

Each photolysis of O₂ or O₃ converts a UV photon into heat and chemical energy of bond rearrangement. The net effect is to attenuate UV-C to essentially zero by the time sunlight reaches the lower stratosphere, and to attenuate UV-B by ~90 % before reaching the troposphere. The ozone layer is a chemical UV shield. Without it, surface UV-B fluxes would rise by an order of magnitude, increasing rates of skin cancer (melanoma), cataracts, immune suppression, and damage to phytoplankton at the base of the marine food chain.

Key Distinction: Ozone in the stratosphere is "good ozone" — it shields life from harmful UV. Ozone at ground level (in photochemical smog from vehicle exhaust + sunlight) is "bad ozone" — a respiratory irritant and component of smog. Same molecule, opposite consequences by altitude.

CFCs — what they are and why we used them

CFCs are alkanes (typically methane, ethane) with all hydrogens replaced by chlorine and fluorine. The most common examples and their industrial uses:

Formula	Common name	Industrial use
CCl₃F	CFC-11 (trichlorofluoromethane)	Foam blowing for insulation
CCl₂F₂	CFC-12 (dichlorodifluoromethane)	Refrigerant, aerosol propellant
CClF₃	CFC-13 (chlorotrifluoromethane)	Specialty refrigerant
C₂Cl₃F₃	CFC-113 (1,1,2-trichloro-1,2,2-trifluoroethane)	Electronics cleaning solvent
CCl₂FCClF₂	CFC-114	Specialty refrigerant

CFCs were developed by Thomas Midgley Jr. at General Motors in 1928 as a replacement for ammonia, sulfur dioxide and propane in refrigerator compressors. The desirable property profile:

Non-toxic — unlike ammonia (toxic) and SO₂ (toxic and corrosive).
Non-flammable — unlike propane and butane.
Chemically inert — no reaction with food, plastics, lubricants, water, or atmospheric oxygen.
Convenient boiling points — typically -10 to -30 °C, ideal for vapour-compression cycles.
Cheap and easy to synthesise from CCl₄ + HF over an SbF₃ catalyst.

By 1974, global CFC production exceeded 800,000 tonnes per year. The same chemical inertness that made them so useful was about to become the reason for their atmospheric persistence.

How CFCs reach the stratosphere

When a CFC leaks from a refrigerator, fire extinguisher or spray can, it disperses into the troposphere (0-15 km). Several normal atmospheric loss processes that remove most other organic gases do not work on CFCs:

Tropospheric loss process	Acts on CFCs?	Why not
Hydrolysis in rain droplets	No	CFCs are highly hydrophobic, water-insoluble
Reaction with •OH radical	No	No C-H bond for H-abstraction
Reaction with O₃	No	C-F and C-Cl bonds are too strong
Photolysis by tropospheric UV-A	No	UV-A photons are too low-energy to break C-Cl
Dry/wet deposition	No	Vapours, not aerosols
Reaction with NOₓ	No	Chemically inert

With essentially no tropospheric sink, CFCs accumulate. Atmospheric mixing — driven by tropical convection, mid-latitude eddies, and the Brewer-Dobson stratospheric circulation — slowly transports them upward. Tropospheric lifetimes are 45-100 years for CFC-11, 100+ years for CFC-12. By the time a CFC molecule reaches the upper stratosphere (~30 km), short-wavelength UV-C (λ < 230 nm) photons are abundant enough to provide the ~340 kJ mol⁻¹ needed to break the C-Cl bond homolytically.

flowchart TD
  A[CFC released at ground level] --> B[Diffuses through troposphere<br/>no chemical sink, persistent]
  B --> C[Slow upward transport 10-30 years<br/>by Brewer-Dobson circulation]
  C --> D[Reaches upper stratosphere 25-30 km<br/>UV-C photolysis begins]
  D --> E["Initiation: C-Cl homolysis<br/>CCl2F2 + h-nu &#8594; Cl + CClF2"]
  E --> F[Chlorine radical Cl enters<br/>ozone-destroying chain]

Why C-Cl, not C-F?

A CFC has both C-F and C-Cl bonds, but UV-C photolysis preferentially breaks the C-Cl bond because:

Bond	Bond enthalpy / kJ mol⁻¹	UV wavelength needed
C-F	484	<247 nm (rare in stratosphere)
C-Cl	338	<354 nm (abundant in stratosphere)

UV-C photons at the typical stratospheric flux carry enough energy to break C-Cl easily, but not enough to break C-F. So the photolysis product is a chlorine radical (not a fluorine radical) plus a chlorofluorocarbon radical. This is critical because chlorine radicals catalyse ozone destruction; fluorine radicals do not (HF formation is the dominant fate of F• in the stratosphere and does not propagate a chain).

The radical chain mechanism

Once a Cl• has been released, it enters a two-step propagation cycle that catalytically converts ozone into ordinary oxygen.

Initiation

$CCl_2F_2 \xrightarrow{UV\text{-}C,\ h\nu < 230\,\text{nm}} \bullet CClF_2 + \bullet Cl$

The single photon-driven homolytic fission of the weakest C-X bond. Note the fish-hook half-arrows in mechanism diagrams (each atom takes one electron).

Propagation step 1

$\bullet Cl + O_3 \rightarrow ClO\bullet + O_2$

The chlorine radical abstracts an oxygen atom from ozone, forming the chlorine monoxide radical (ClO•) and ordinary molecular oxygen. The reaction is exothermic (~163 kJ mol⁻¹) because the C-Cl bond formed (in ClO bond order) and the new O=O bond compensate for the breaking of an O-O bond in ozone.

Propagation step 2

$ClO\bullet + O\bullet \rightarrow \bullet Cl + O_2$

The ClO• radical reacts with a free oxygen atom (abundant in the stratosphere, generated by the parallel UV-driven photolysis of O₂ and O₃ in the Chapman cycle). The product is ordinary O₂ and a regenerated chlorine radical — ready to attack another ozone molecule.

Net catalytic effect

Adding the two propagation steps:

$O_3 + O\bullet \xrightarrow[\text{net, catalysed by Cl}\bullet]{} 2\,O_2$

Chlorine appears in the equation as a catalyst — consumed in step 1, regenerated in step 2, with zero net change. One Cl• can recycle through this cycle ~10⁵ times before it is finally lost to a termination step. This is why CFCs are so destructive: each photolysed molecule releases a chlorine radical that destroys not one but ~100,000 ozone molecules.

flowchart TD
  A[CFC, e.g. CCl2F2] -->|UV-C<br/>initiation| B["Cl radical + CClF2 radical"]
  B --> C["Propagation 1<br/>Cl + O3 &#8594; ClO + O2"]
  C --> D["Propagation 2<br/>ClO + O &#8594; Cl + O2"]
  D --> C
  D --> E["Termination occasionally:<br/>Cl + Cl &#8594; Cl2<br/>ClO + NO2 &#8594; ClONO2"]
  E --> F["Net catalytic effect:<br/>O3 + O &#8594; 2 O2<br/>Cl is catalyst"]

Termination

Eventually the chain is broken by reactions that consume the radicals without regenerating them. Common stratospheric termination steps:

$\bullet Cl + \bullet Cl \rightarrow Cl_2$ $\bullet Cl + ClO\bullet \rightarrow Cl_2 + O$ $ClO\bullet + NO_2 \rightarrow ClONO_2 \text{ (chlorine nitrate reservoir)}$ $\bullet Cl + CH_4 \rightarrow HCl + \bullet CH_3 \text{ (HCl reservoir)}$

The "reservoir species" HCl and ClONO₂ are not permanent sinks — they can be photolysed or heterogeneously processed on polar stratospheric clouds (PSCs) back into reactive Cl•. This is why Antarctic springtime is when the ozone hole opens: cold PSCs at -80 °C catalyse the release of Cl from reservoirs all at once.

Evidence — the Antarctic ozone hole

In 1985 a team at the British Antarctic Survey (Halley Bay station), led by Joe Farman, reported that springtime total-column ozone above Antarctica had declined by ~40 % from the 1970s baseline. The dip occurs every September-October when sunlight returns to the polar stratosphere after the winter darkness and activates the chlorine that has been accumulating on PSCs.

Mario Molina and F. Sherwood Rowland had predicted exactly this kind of catastrophic chlorine-driven destruction in their 1974 Nature paper (which won the 1995 Nobel Prize in Chemistry, shared with Paul Crutzen for his earlier work on nitrogen-oxide-driven ozone destruction). The 1985 observations transformed the predicted theoretical risk into measured global reality.

The international response was rapid by environmental-policy standards. The Montreal Protocol (1987) committed signatory countries to phase out CFC production by 2000 (later accelerated to 1996 for developed countries). All 198 UN member states are now parties. By the mid-2020s, stratospheric chlorine levels are declining at ~1 % per year, and the Antarctic ozone hole is on a slow recovery trajectory projected to return to 1980 levels around 2060-2070.

Replacements — HCFCs, HFCs and HFOs

The chemical industry developed three families of replacement refrigerants, each less destructive than the last:

Family	Composition	Example	Ozone-depleting potential	Global warming potential	Status
CFC	C, Cl, F (no H)	CFC-12, $CCl_2F_2$	1.0 (reference)	10,900	Banned (Montreal Protocol 1987)
HCFC	C, H, Cl, F	HCFC-22, $CHClF_2$	~0.05	1,810	Phase-out (Montreal Protocol 1992 amend.)
HFC	C, H, F (no Cl)	HFC-134a, $CH_2FCF_3$	0	1,430	Phase-down (Kigali Amendment 2016)
HFO	C, H, F with C=C	HFO-1234yf, $CF_3CF{=}CH_2$	0	<10	Current generation

HCFCs — a stepping stone

Adding a hydrogen to the molecule gives a C-H bond that the tropospheric hydroxyl radical (•OH) can attack:

$HO\bullet + CHClF_2 \rightarrow H_2O + \bullet CClF_2$

This is the same chemistry that limits the atmospheric lifetime of methane and other hydrocarbons. With a tropospheric sink, HCFC molecules survive only a few years (12 for HCFC-22 vs 100+ for CFC-12), and most are destroyed at ground level before reaching the stratosphere. The remaining ~5 % do reach the stratosphere and contribute some Cl•, but the overall ozone-depleting potential is only ~5 % of CFCs.

HFCs — zero ozone depletion, but greenhouse-active

Removing chlorine altogether eliminates the radical-chain mechanism. HFC photolysis in the stratosphere produces only fluorine radicals, which react rapidly with stratospheric water vapour and methane to give HF — a stable reservoir that does not catalyse ozone loss:

$\bullet F + H_2O \rightarrow HF + HO\bullet$ $\bullet F + CH_4 \rightarrow HF + \bullet CH_3$

HFCs therefore have zero ozone-depleting potential. However, their multiple C-F bonds give them strong infrared absorption (Lesson 9: the IR-spectroscopic basis of greenhouse warming), so they are potent greenhouse gases. One kilogram of HFC-134a in a refrigerator traps as much infrared as ~1,400 kg of CO₂ over a 100-year horizon. The 2016 Kigali Amendment to the Montreal Protocol commits signatories to phase down HFC use by 80-85 % by 2047.

HFOs — the current generation

The newest replacements (HFO-1234yf, HFO-1234ze) include a C=C double bond, which dramatically increases reactivity with tropospheric •OH and shortens the atmospheric lifetime to days-weeks. Both ozone-depleting potential and global warming potential are essentially zero. HFOs are now standard in new automotive air conditioning units in Europe and the US.

flowchart LR
  A[1930s-80s<br/>CFCs] -->|Mario Molina, Rowland 1974<br/>predict ozone destruction| B[1985 Antarctic<br/>ozone hole discovered]
  B --> C[1987 Montreal Protocol<br/>CFCs phased out by 1996]
  C --> D[1990s-2010s<br/>HCFCs, HFCs deployed]
  D --> E[2016 Kigali Amendment<br/>HFCs phased down by 2047]
  E --> F[2020s onward<br/>HFOs, CO2, NH3, hydrocarbons]

Lesson 7: Environmental Issues — CFCs and Ozone Depletion