Stratospheric Ozone and Human Impacts (APES Unit 9: Global Change)

Stratospheric ozone

Ozone (O3) concentrated in the stratosphere (~15–35 km altitude) that absorbs much of the Sun’s harmful UV radiation and protects life.

Tropospheric ozone

Ozone (O3) near Earth’s surface that is usually an air pollutant and a component of photochemical smog, harmful to lungs and plants.

Ultraviolet (UV) radiation

High-energy sunlight; certain wavelengths (especially UV-B and UV-C) can damage living tissue and DNA if not filtered by the atmosphere.

UV-B

A portion of UV radiation that is strongly absorbed by stratospheric ozone; increased UV-B at the surface raises risks like skin cancer, cataracts, and ecosystem damage.

UV-C

The most energetic UV radiation; nearly all is absorbed by the atmosphere (especially by ozone and oxygen) before reaching Earth’s surface.

Ozone layer

A region of relatively higher ozone concentration in the stratosphere that functions like a “thin sunscreen,” intercepting damaging UV radiation.

Ozone formation–destruction cycle

Natural stratospheric process where UV splits O2 into O, O combines with O2 to form O3, and UV also breaks O3 back into O2 and O; overall converts UV energy to heat.

Ozone depletion

A long-term reduction in average stratospheric ozone concentration (thinning), allowing more UV-B to reach Earth’s surface.

Ozone hole

A region (classically over Antarctica in spring) where stratospheric ozone levels drop dramatically compared with typical values; not a literal hole with zero ozone.

Chlorofluorocarbons (CFCs)

Stable, human-made halogenated compounds formerly used in refrigerants, aerosol propellants, and foams; persist long enough to reach the stratosphere and drive ozone depletion.

Ozone-depleting substances (ODS)

Chemicals (including CFCs and related halogen-containing compounds) that release reactive halogens in the stratosphere and reduce ozone.

Halons

Halogen-containing compounds (often bromine-based) used historically in fire suppression that can contribute to stratospheric ozone depletion.

Chlorine radical (Cl)

A highly reactive chlorine atom released when UV breaks CFCs in the stratosphere; participates in reactions that destroy ozone.

Catalyst (in ozone depletion)

A substance (like chlorine) that speeds ozone-destroying reactions and is regenerated, allowing one atom to destroy many ozone molecules.

Catalytic ozone-destruction cycle

Reaction sequence where chlorine converts O3 to O2 (forming ClO) and is then regenerated, enabling repeated ozone loss without “using up” the chlorine quickly.

Polar stratospheric clouds (PSCs)

Cloud particles that form in the extremely cold polar stratosphere; their surfaces promote reactions that convert chlorine from storage forms into more reactive forms.

Polar vortex

Strong circulating winds that isolate Antarctic air in winter/spring, limiting mixing with ozone-rich air and intensifying regional ozone depletion.

Antarctic spring sunlight return

Seasonal return of sunlight after polar winter that helps drive rapid chlorine-driven ozone destruction when reactive chlorine has accumulated.

Montreal Protocol

International agreement to phase out production and use of major ozone-depleting substances (especially CFCs), leading to declining ODS concentrations and slow ozone recovery.

Source control

Preventing pollution by limiting emissions at the source; emphasized for ozone depletion because the atmosphere is too vast for practical global cleanup.

Ozone Depletion Potential (ODP)

A relative measure of how strongly a substance depletes stratospheric ozone compared to a reference compound (historically CFC-11); higher ODP means more ozone damage per unit emitted.

HCFCs (hydrochlorofluorocarbons)

CFC substitutes containing hydrogen that tend to break down more in the lower atmosphere, usually giving lower ODP than CFCs, though they can still deplete ozone.

HFCs (hydrofluorocarbons)

CFC substitutes without chlorine that do not directly deplete ozone, but some are potent greenhouse gases (a climate trade-off).

Banked ozone-depleting substances

ODS stored in existing equipment or materials (like older refrigerators/AC systems and foams) that can still be released through leaks or improper disposal.

Refrigerant recovery and recycling

Capturing and reusing refrigerants during servicing and disposal to prevent venting ozone-depleting chemicals to the atmosphere.