13.1 Stratospheric ozone chemistry

Stratospheric Ozone Chemistry

The stratospheric ozone layer absorbs most of the Sun's harmful ultraviolet radiation before it reaches Earth's surface. Understanding how ozone forms, how it gets destroyed, and what humans have done about it connects core photochemistry concepts to one of the biggest environmental success stories in modern science.

Ozone formation in stratosphere

The Chapman cycle describes the natural formation and destruction of ozone in the stratosphere through four key reactions. Two build ozone up, and two break it down.

Formation (two steps):

1. UV photons split molecular oxygen into two oxygen atoms:

   • $O_2 + h\nu \rightarrow O + O$
   • This requires short-wavelength UV light ($\lambda < 240 \text{ nm}$), which is only available high in the stratosphere.

2. Each oxygen atom then combines with another $O_2$ to form ozone:

   • $O + O_2 + M \rightarrow O_3 + M$
   • $M$ is a third body (usually $N_2$ or $O_2$) that carries away excess energy. Without it, the newly formed $O_3$ would just fly apart.

Destruction (two steps):

1. Ozone absorbs UV and breaks apart:

   • $O_3 + h\nu \rightarrow O_2 + O$

2. Atomic oxygen reacts with ozone:

   • $O + O_3 \rightarrow 2O_2$

The net result of the full cycle is a steady-state balance: ozone is constantly being created and destroyed, maintaining a roughly stable concentration. Solar UV radiation drives the entire process, and the ozone produced absorbs UVB (280–315 nm) and UVC (100–280 nm), preventing most of it from reaching the surface.

One thing the Chapman cycle alone doesn't explain: it predicts more ozone than we actually observe. That discrepancy is what led scientists to discover catalytic destruction cycles involving species like $Cl$, $Br$, $NO_x$, and $HO_x$.

CFCs and ozone depletion

Chlorofluorocarbons (CFCs) are synthetic compounds containing chlorine, fluorine, and carbon. They were widely used as refrigerants, aerosol propellants, and foam-blowing agents because they're chemically stable and non-toxic at ground level. That very stability is the problem: CFCs don't break down in the troposphere, so they drift intact into the stratosphere over decades (atmospheric lifetimes of 50–100+ years).

Once in the stratosphere, UV radiation photolyzes CFCs and releases free chlorine atoms. For example:

$CCl_2F_2 + h\nu \rightarrow CClF_2 + Cl$

The freed chlorine then catalyzes ozone destruction through a two-step cycle:

1. $Cl + O_3 \rightarrow ClO + O_2$
2. $ClO + O \rightarrow Cl + O_2$

Notice that $Cl$ is regenerated in step 2. This is what makes it a catalytic cycle: a single chlorine atom can destroy roughly 100,000 ozone molecules before it's finally removed (typically by reacting with methane or nitrogen dioxide to form reservoir species like $HCl$ or $ClONO_2$).

Other ozone-depleting substances (ODS) work through similar catalytic mechanisms:

• Halons (used in fire extinguishers) release bromine, which is 40–100 times more effective at destroying ozone per atom than chlorine
• Carbon tetrachloride ($CCl_4$, used in solvents)
• Methyl chloroform ($CH_3CCl_3$, used in industrial cleaning)

Effects of ozone depletion

When the ozone layer thins, more UV radiation reaches Earth's surface. The consequences span ecosystems, agriculture, and human health.

Environmental effects:

• Damage to phytoplankton, which form the base of marine food chains. Even modest UV increases reduce their productivity, with ripple effects through ocean ecosystems.
• Reduced crop yields in UV-sensitive species like soybeans, wheat, and rice
• Accelerated degradation of materials such as plastics, paints, and rubber

Human health impacts:

• Higher rates of skin cancer, including melanoma, basal cell carcinoma, and squamous cell carcinoma. A sustained 1% decrease in ozone corresponds to roughly a 2% increase in skin cancer incidence.
• Increased risk of cataracts and other eye damage
• Suppression of the immune system, making people more vulnerable to infectious diseases

Polar ozone holes are the most dramatic manifestation of depletion. The Antarctic ozone hole forms each spring (September–November) because polar stratospheric clouds (PSCs) that form during the extremely cold polar winter provide surfaces for heterogeneous reactions. These reactions convert reservoir species ($HCl$, $ClONO_2$) back into reactive chlorine, which destroys ozone rapidly when sunlight returns in spring. The Arctic experiences similar but less severe depletion because its stratosphere doesn't get quite as cold.

Effectiveness of ozone agreements

The Montreal Protocol (1987) committed nations to phasing out production and consumption of ozone-depleting substances. It has been ratified by 198 countries, making it the first universally ratified UN treaty.

Successive amendments strengthened the original agreement:

1. London Amendment (1990) — expanded the list of controlled substances and created a financial mechanism to help developing countries comply
2. Copenhagen Amendment (1992) — accelerated phase-out schedules for CFCs and halons
3. Montreal Amendment (1997) — established a licensing system to control international trade in ODS
4. Beijing Amendment (1999) — added bromochloromethane to the controlled list
5. Kigali Amendment (2016) — targeted HFCs, which aren't ozone-depleting but are potent greenhouse gases used as CFC replacements

The results have been substantial:

• 98% reduction in global CFC production and consumption since 1986
• Atmospheric concentrations of most ODS are measurably declining
• The ozone layer is projected to recover to 1980 levels by approximately 2050–2070, depending on latitude

Challenges remain. HCFCs (transitional replacements for CFCs) are still being phased out. Illegal production of banned substances like CFC-11 was detected as recently as 2018, highlighting the need for continued monitoring. And HFCs, while ozone-safe, have global warming potentials thousands of times greater than $CO_2$, which is why the Kigali Amendment was added.

The Montreal Protocol is widely considered the most successful international environmental agreement ever enacted, and it serves as a reference point for how global cooperation can address atmospheric threats.