The stratospheric ozone layer is a region of the stratosphere (about 10–50 km up) with high concentrations of O3 that absorbs most of the Sun’s biologically harmful ultraviolet-B (UV-B) radiation. It was crucial to the evolution and continued survival of life by protecting DNA and organisms from damaging UV. If the layer thins (ozone depletion), more UV-B reaches Earth, increasing skin cancer and cataracts in humans and harming plants, phytoplankton, and ecosystems. Major causes include anthropogenic chlorine- and bromine-containing gases (CFCs, halons) and natural processes like polar stratospheric clouds that drive Antarctic ozone loss. International action (Montreal Protocol) has reduced emissions and begun recovery. This topic shows up on the APES exam under Unit 9—know causes, effects (human health + ecosystem), and policy responses. Review the Topic 9.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef), the unit page (https://library.fiveable.me/ap-environmental-science/unit-9), and practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Ozone in the stratosphere (the “ozone layer”) protects life by absorbing most of the Sun’s harmful ultraviolet-B (UV-B) radiation before it reaches Earth’s surface. O3 molecules absorb UV photons and undergo photodissociation (O3 → O2 + O), which removes the energy of those rays; the free oxygen atoms then recombine to reform ozone, so the layer self-regulates. Less stratospheric ozone means more UV-B reaches you, increasing risks like skin cancer and cataracts and harming ecosystems (photosynthesis, plankton). This is why scientists track ozone depletion (e.g., Dobson units) and why the Montreal Protocol targeted CFCs and halons that release chlorine radicals that break down O3. For AP exam review, focus on the photodissociation mechanism, health/ecosystem effects, and human causes (CFCs)—see the Topic 9.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef) and the Unit 9 overview (https://library.fiveable.me/ap-environmental-science/unit-9). Practice questions are at (https://library.fiveable.me/practice/ap-environmental-science).

CFCs (chlorofluorocarbons) are human-made compounds once widely used as refrigerants, aerosol propellants, and in foam production. They’re stable in the lower atmosphere, so they drift up into the stratosphere. There, strong UV light breaks CFC molecules apart (photodissociation), releasing chlorine atoms (Cl·). Those chlorine radicals catalyze ozone (O3) loss: one Cl· reacts with O3 to form ClO· and O2, then ClO· reacts with a free O atom to regenerate Cl· and produce more O2. Because the chlorine is recycled, a single Cl atom can destroy many ozone molecules. Polar stratospheric clouds and cold Antarctic conditions speed this process, creating the ozone hole and increasing UV-B at Earth’s surface (risking skin cancer and cataracts). For more AP-aligned review, see the Topic 9.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef) and Unit 9 overview (https://library.fiveable.me/ap-environmental-science/unit-9).

Antarctica gets the ozone “hole” because of a unique combination of cold, isolation, and seasonal sunlight. In winter a strong polar vortex isolates the air above Antarctica so chlorine from CFCs builds up. Extremely low stratospheric temperatures form polar stratospheric clouds (PSCs); reactions on PSC surfaces convert stable chlorine compounds into reactive chlorine radicals. When sunlight returns in the Antarctic spring, those chlorine radicals rapidly catalyze ozone photodissociation, causing dramatic seasonal depletion. The Arctic has weaker vortices and warmer stratospheric temps, so PSC formation and isolation are less extreme—less ozone loss. Key CED terms: chlorofluorocarbons (CFCs), polar stratospheric clouds, Antarctic spring, atmospheric chlorine radicals, and increased UV-B reaching the surface. This is exactly what the AP question set expects you to explain (see the Topic 9.1 study guide for a focused review: https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef). For extra practice, try problems at https://library.fiveable.me/practice/ap-environmental-science.

“Good” ozone is in the stratosphere (about 10–50 km up). It forms the ozone layer that absorbs most harmful UV-B radiation, protecting DNA and lowering risks of skin cancer and cataracts (CED EK STB-4.A & STB-4.A.3). Stratospheric ozone can be depleted by CFCs and halons: chlorine radicals from CFC breakdown catalyze ozone photodissociation, especially on polar stratospheric clouds, creating the Antarctic ozone hole (EK STB-4.A.2). “Bad” ozone is in the troposphere (near the surface). It’s a secondary pollutant formed from NOx + VOCs in sunlight and is a respiratory irritant, damages plants, and acts as a short-lived greenhouse gas (useful AP vocab: tropospheric ozone, photochemical smog). On the exam, be ready to explain both roles and causes (STB-4 learning objective). For a focused review, check the Topic 9.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Without the stratospheric ozone layer to absorb much of UV-B radiation, more high-energy ultraviolet rays hit Earth’s surface. For people that means higher rates of skin cancer (including melanoma) and more cataracts and other eye damage, because UV-B damages DNA and proteins in cells (CED EK STB-4.A.3). Ecologically, increased UV-B harms phytoplankton and other primary producers, lowering marine food-web productivity and reducing crop yields by damaging plant DNA and photosynthetic tissues. It also accelerates the breakdown of materials like plastics and paints, and can increase mutation rates in many organisms. On the AP exam, link this to causes and consequences of ozone depletion (EK STB-4.A.1–3) and mention human sources like CFCs/halons. For a focused review, see the Topic 9.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef) and more unit resources at (https://library.fiveable.me/ap-environmental-science/unit-9). For practice, try questions at (https://library.fiveable.me/practice/ap-environmental-science).

CFCs are stable in the lower atmosphere, so they diffuse up into the stratosphere. There UV radiation breaks CFC molecules apart (photodissociation), releasing chlorine atoms (Cl·). Those chlorine radicals catalyze ozone destruction in a simple cycle: Cl· + O3 → ClO· + O2, then ClO· + O → Cl· + O2. The net result is O3 + O → 2 O2, and the Cl· is free to repeat the cycle—one chlorine atom can destroy many ozone molecules. Polar stratospheric clouds (especially in Antarctic spring) help convert reservoir chlorine compounds into reactive Cl·, speeding depletion and creating the Antarctic ozone hole. Less stratospheric ozone means more UV-B reaches Earth, increasing risks like skin cancer and cataracts (CED EK STB-4.A.2–A.3). For a focused review, see the Topic 9.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef). For unit overview and extra practice, check Unit 9 (https://library.fiveable.me/ap-environmental-science/unit-9) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Ice crystals in polar stratospheric clouds (PSCs) drive Antarctic ozone loss by converting “safe” reservoir chlorine into reactive chlorine that destroys O3. On PSC ice surfaces, reactions turn HCl and ClONO2 into Cl2 (and other reactive forms). When Antarctic spring returns, sunlight photodissociates Cl2 into chlorine radicals (Cl·). Those radicals catalytically break O3 into O2 (one Cl can destroy many ozone molecules), causing the seasonal ozone hole. The CED highlights PSCs and ice-surface chemistry as the natural factor that, combined with anthropogenic CFCs (the source of the chlorine), produces large depletion (EK STB-4.A.2). Remember: the key steps are heterogeneous reactions on ice, formation of Cl2, springtime sunlight → Cl radicals, and catalytic ozone destruction. For review, see the Topic 9.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef) and the Unit 9 overview (https://library.fiveable.me/ap-environmental-science/unit-9). Practice AP-style questions are at (https://library.fiveable.me/practice/ap-environmental-science).

Stratospheric ozone absorbs most ultraviolet-B (UV-B) radiation from the sun. When human-made chemicals like CFCs break down in the stratosphere (creating chlorine radicals), they destroy ozone molecules so less UV-B is filtered out (EK STB-4.A.2, STB-4.A.3). More UV-B at Earth's surface means more DNA damage in skin cells, which increases risks of skin cancers (basal cell, squamous cell, and melanoma) and also raises cataract risk. That causal link—ozone depletion → more UV-B reaching surface → higher skin cancer and eye disease rates—is what the CED expects you to explain (STB-4.A). For review, check the Topic 9.1 study guide on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef) and broader Unit 9 resources (https://library.fiveable.me/ap-environmental-science/unit-9). Want practice applying this to an FRQ? Try problems at Fiveable Practice (https://library.fiveable.me/practice/ap-environmental-science).

Most common CFCs persist a long time—typically decades to over a century. Many CFCs have atmospheric lifetimes around 20–100+ years. They usually take a few years (roughly 2–5 years) to be mixed up into the stratosphere, and once there UV light breaks them apart to release chlorine atoms. Those chlorine radicals then catalytically destroy ozone, repeatedly, so a single CFC molecule can do a lot of damage over decades. That long lifetime is why stratospheric ozone depletion was a global problem and why recovery takes many decades even after CFC emissions dropped (the Montreal Protocol aims for ozone recovery over the 21st century). This ties directly to EK STB-4.A.2 and other Topic 9.1 concepts—review the Topic 9.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef) and Unit 9 overview (https://library.fiveable.me/ap-environmental-science/unit-9) for AP-relevant details and practice.

Stratospheric ozone blocks much of the sun’s harmful UV-B radiation, so when ozone is depleted more UV-B reaches Earth. Health effects include: increased risk of skin cancer (UV-B causes DNA damage that can lead to basal cell, squamous cell, and melanoma), more frequent and severe sunburns, suppressed immune function (making infections and some cancers more likely), and higher rates of cataracts and other eye damage from UV exposure. UV can also accelerate skin aging. These outcomes are exactly why the CED links ozone loss to skin cancer and cataracts (EK STB-4.A.3). For AP review, study Topic 9.1 to understand the role of ozone and UV-B, and check Fiveable’s Topic 9.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef). If you want practice questions on this unit, use Fiveable’s APES practice set (https://library.fiveable.me/practice/ap-environmental-science).

Even though the ozone layer sits high in the stratosphere (about 10–50 km up), it matters because it absorbs most of the Sun’s ultraviolet-B (UV-B) radiation before it reaches Earth’s surface. Less stratospheric ozone → more UV-B at ground level. Increased UV-B raises human risks like skin cancer and cataracts (CED EK STB-4.A.3) and damages ecosystems: it can reduce photosynthesis in phytoplankton (base of many food webs), harm plant growth, and disrupt food chains. Ozone loss is driven by CFCs/halons and amplified by polar stratospheric clouds that enable ozone photodissociation (EK STB-4.A.2); the Antarctic ozone hole is a dramatic example. On the AP exam, be ready to connect causal agents (CFCs), processes (photodissociation, chlorine radicals), and biological impacts (skin cancer, cataracts, ecosystem disruption) as specified in STB-4.A. For a clear review, see the Topic 9.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef). More practice questions for exam prep are at (https://library.fiveable.me/practice/ap-environmental-science).

They’re related to the atmosphere but are different problems with different causes and effects. Stratospheric ozone depletion: loss of ozone (O3) in the stratosphere that normally blocks harmful ultraviolet-B (UV-B) radiation. Caused mainly by CFCs and halons breaking down to chlorine radicals that destroy O3, especially over the poles (Antarctic ozone hole). Fewer ozone molecules → more UV-B at the surface → higher risks of skin cancer and cataracts (CED EK STB-4.A.2–A.3). Policy success: the Montreal Protocol reduced CFCs and ozone is slowly recovering (Topic 9.1; study guide: https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef). Greenhouse effect: greenhouse gases (CO2, CH4, N2O, CFCs/HFCs) trap outgoing longwave (infrared) radiation in the troposphere, warming the planet and changing climate patterns (Topics 9.3–9.5). Main human sources are fossil fuel burning, agriculture, and deforestation. Consequences include global temperature rise, sea level rise, altered weather, and ocean acidification. Bottom line: ozone depletion affects UV shielding (health/UV effects) and is driven notably by CFC chemistry; the greenhouse effect traps heat and drives climate change. For unit review see Unit 9 (https://library.fiveable.me/ap-environmental-science/unit-9) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Scientists first noticed ozone loss from measurements—ground-based Dobson spectrophotometers and satellites—that showed huge, unexpected springtime drops over Antarctica in the early 1980s. In 1985 British researchers (Farman et al.) published Dobson data showing a dramatic seasonal “ozone hole.” Atmospheric chemists then connected the timing and location to cumulative emissions of chlorofluorocarbons (CFCs): UV light in the stratosphere breaks CFCs, releasing chlorine atoms that catalytically destroy ozone (Cl + O3 → ClO + O2; ClO can reform Cl, so one Cl atom destroys many O3). Polar stratospheric clouds in Antarctic spring accelerate these reactions, explaining the large seasonal hole. This combination of observational evidence, reaction chemistry, and modeling convinced scientists and led to policy action (Montreal Protocol). For a concise AP-focused review, see the Topic 9.1 study guide on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Ozone loss spikes in Antarctic spring because of a perfect seasonal setup: during the polar winter the stratosphere over Antarctica gets extremely cold and polar stratospheric clouds (PSCs) form. CFCs and other chlorine-containing compounds settle into the stratosphere and on PSC surfaces get converted into reactive chlorine compounds. While it’s still dark, that reactive chlorine stays “locked up.” When sunlight returns at the start of spring, UV light drives photodissociation that frees chlorine radicals (Cl·). Those radicals catalytically destroy ozone very quickly, producing the seasonal ozone hole. This process—cold winter PSC formation + spring sunlight enabling chlorine-driven ozone photodissociation—is why depletion is worst in Antarctic spring (CED EK STB-4.A.2; keywords: PSCs, CFCs, chlorine radicals, ozone photodissociation). For quick review, check the Topic 9.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/stratospheric-ozone-depletion/study-guide/HFK6z9TTzohtCQhNxeef) and more unit resources (https://library.fiveable.me/ap-environmental-science/unit-9).