Stratospheric ozone

The ozone layer absorbs all UV-C and most UV-B. UV-A mostly reaches Earth’s surface.

Ozone is a molecule made of three oxygen atoms (O3). In the stratosphere it forms a layer that absorbs harmful UV radiation.

It is located in the stratosphere and its main function is absorbing harmful UV radiation (especially UV-B and UV-C).

Ozone in the stratosphere is protective; ozone at ground level (troposphere) is a pollutant and respiratory irritant.

Stratospheric ozone is beneficial (absorbs UV). Tropospheric ozone is a pollutant (smog) that harms human health and plants.

Examples include higher skin cancer risk, cataracts, and immune suppression.

UV-C is completely absorbed by ozone and oxygen in the atmosphere.

UV-B can reduce phytoplankton productivity and survival, weakening the base of marine food chains and reducing carbon uptake.

The ozone layer is mainly in the stratosphere (roughly 15–35 km altitude).

They are exposed near the surface, can’t escape UV easily, are the base of ocean food webs, and are an important carbon sink.

Increased UV-B exposure leading to more skin cancer/cataracts and reduced productivity or survival of sensitive organisms (e.g., phytoplankton).

UV splits oxygen molecules (O2) into O atoms; an O atom combines with O2 to form O3. Ozone also breaks down naturally, creating a dynamic equilibrium.

UV can degrade plastics, paints, rubber, and building materials faster, shortening product lifespan.

They occur in different layers and have different roles: ozone (stratosphere) absorbs UV; greenhouse effect (troposphere) traps long-wave radiation to warm Earth.

Ozone depletion is mainly caused by ozone-depleting substances (e.g., CFCs) reducing stratospheric ozone, while climate change is driven by greenhouse gases increasing heat trapping.

Ozone depletion is mainly caused by ODS (especially CFCs and halons) releasing chlorine/bromine that catalytically destroys ozone.

ODS are chemicals (e.g., CFCs, halons, some HCFCs) that release chlorine or bromine in the stratosphere and destroy ozone.

The ozone hole is a region of severely depleted ozone in the stratosphere that forms seasonally (mainly over Antarctica) during spring.

CFCs were used in refrigerators/air conditioners, aerosol sprays, and foam/blowing agents.

A strong polar vortex isolates air, extreme cold allows polar stratospheric clouds (PSCs) to form, and returning spring sunlight triggers rapid ozone destruction.

Catalytic means the chlorine/bromine is regenerated and not used up, so it can destroy many ozone molecules repeatedly.

CFCs persist in the atmosphere for decades, so existing CFCs continue reaching the stratosphere and releasing chlorine long after production stops.

A circular wind pattern that isolates Antarctic stratospheric air during winter, helping conditions build up for ozone depletion.

Because chlorine from CFCs acts catalytically: it destroys ozone and is regenerated, so one chlorine atom can destroy many ozone molecules.

It increased in size/severity through the late 20th century, then stabilised and has shown signs of slow recovery since around the early 2000s.

PSCs provide surfaces for chemical reactions that convert chlorine into reactive forms, priming the stratosphere for rapid ozone destruction when sunlight returns.

Cl + O3 → ClO + O2, then ClO + O → Cl + O2. Chlorine is regenerated and can repeat the cycle.

They are stable and can persist long enough to rise to the stratosphere, where UV radiation breaks them down to release reactive chlorine/bromine.

It typically develops in September–October (Southern Hemisphere spring) and then shrinks toward summer.

Clearly explain the catalytic mechanism (or the Antarctic conditions) using a stepwise chain and correct key terms (ODS, chlorine, PSCs, polar vortex, UV).

An international treaty that phases out the production and consumption of ozone-depleting substances (ODS) such as CFCs.

To phase out ozone-depleting substances (ODS) to allow recovery of the stratospheric ozone layer.

Clear scientific consensus, available substitutes, fewer major producers to regulate, strong monitoring, and financial support for developing countries (any two).

Recovery is slow due to long-lived ODS; illegal production/smuggling can occur; and some replacement chemicals have climate impacts.

It achieved near-universal participation and a >99% reduction in many ODS, enabling ozone recovery.

A >99% reduction in many ODS and evidence that ozone depletion has stabilised with signs of slow recovery.

ODS were produced by fewer sectors with clearer substitutes, whereas greenhouse gases come from almost all economic activity and require economy-wide transformation.

A funding mechanism that helped developing countries transition away from ODS by supporting technology transfer and implementation.

They allow targets to be strengthened as science improves and new problems (or substitutes) emerge, keeping policy aligned with evidence.

It added HFCs to the Montreal Protocol. HFCs do not deplete ozone but are powerful greenhouse gases, so phasing them down helps climate mitigation.

Clear science, feasible alternatives, financial support, and universal cooperation can achieve large global environmental improvements.

Many CFCs are powerful greenhouse gases, so reducing them avoided significant additional warming.

Conclude that it was highly effective at reducing ODS and enabling recovery, but note slow timelines, enforcement/replacement issues, and why lessons only partly transfer to climate.

Because many ODS persist in the atmosphere for decades, so existing chemicals continue to release reactive chlorine/bromine.

The Kigali Amendment (2016), which targets HFCs (strong greenhouse gases).