Stratospheric Ozone (College Board AP® Environmental Science): Revision Note

What is the stratospheric ozone layer?

• The atmosphere is a 10,000 km layered envelope of mixed gases

• Its layers are based on temperature changes and consist of 78% nitrogen, 21% oxygen and 1% trace gases

• Gravity compresses 99% of the atmosphere to within 40 km of the Earth's surface 

• 50% of the atmosphere is in the first 5.6 km

Atmospheric layers

• The stratosphere is the second layer of Earth's atmosphere and extends roughly 38 km (24 miles) between the troposphere and the mesosphere

• Within the first 3-20 km of the lower stratosphere (15–30 km or 10-20 miles above the Earth's surface) lies the stratospheric ozone layer

• This region of the stratosphere contains the highest concentration of ozone gas

• This layer serves as a natural 'sunscreen' for life on Earth by absorbing the majority of the sun's harmful ultraviolet (UV) radiation before it reaches the planet's surface

Ozone

• Ozone is a molecule composed of three oxygen atoms (O₃)

• Ozone is made naturally in the stratosphere through a two-step reaction

• In the first step, solar ultraviolet radiation splits one ozone molecule (O₃) into one single oxygen atom (O) and an oxygen molecule (O₂)

• In the second step, each oxygen atom (O) collides with another oxygen molecule (O₂) to produce an ozone molecule (O₃)

• The largest ozone production occurs in the tropical stratosphere because this area has the highest levels of insolation

• Under normal conditions, there is a continuous cycle of ozone formation and destruction, which creates a dynamic equilibrium in the stratosphere

• This keeps the concentration of ozone relatively stable over time

• Some stratospheric ozone is transported down into the troposphere, which increases ozone levels at Earth’s surface, particularly in remote, unpolluted regions of the globe

Stratospheric ozone depletion

• Ozone can be destroyed faster than it is created when certain substances reach the stratosphere — this is called stratospheric ozone depletion

• The main culprits are chlorofluorocarbons (CFCs) — synthetic compounds containing chlorine atoms

  • Historically used in aerosol sprays, refrigerants, and foam-blowing agents

• Other halogen-containing compounds also contribute, including halons (used in fire extinguishers, contain bromine) and methyl bromide (used as a soil fumigant)

• In the stratosphere, UV radiation breaks these compounds apart, releasing reactive chlorine (Cl) and bromine (Br) atoms

• These atoms catalyse the destruction of ozone

  • One chlorine atom can destroy over 100,000 ozone molecules before it is removed from the stratosphere

  • Because the atom is regenerated in each cycle, it keeps destroying ozone until it eventually leaves the stratosphere

Natural sources

• Natural processes, such as large volcanic eruptions, have an indirect effect on ozone levels

  • Mt. Pinatubo's 1991 eruption produced large amounts of aerosols (small particles) that increased chlorine's effectiveness at destroying ozone, although the effect was short-lived

• Polar stratospheric clouds (PSCs) form during the long, cold Antarctic winter

  • These clouds consist of tiny ice crystals in the stratosphere

  • Chlorine compounds, mainly man-made CFCs, are trapped within the ice crystals, forming chlorine reservoirs

  • During the Antarctic spring, the returning sunlight melts the ice crystals, releasing and reactivating the chlorine atoms

  • This causes rapid stratospheric ozone depletion and the formation of the 'ozone hole' over Antarctica

• Not all chlorine and bromine sources cause ozone layer depletion

  • Chlorine from pools, factories, sea salt, and volcanoes does not reach the stratosphere

• Because CFCs and halons are stable and do not dissolve in rain, there are no natural processes that remove them from the lower atmosphere

Effects of stratospheric ozone depletion

• Ozone depletion refers to the gradual thinning of the ozone layer in the Earth's stratosphere

• Stratospheric ozone depletion leads to increased levels of harmful ultraviolet (UV) radiation reaching the Earth's surface

• This negatively impacts human health, including

  • Higher UV-B radiation exposure significantly increases the risk of skin cancer, particularly melanoma, the most dangerous form

  • Increased UV radiation can lead to the development of cataracts, clouding of the lens in the eye

  • Excessive UV exposure can weaken the immune system, making individuals more susceptible to infections

  • Chronic exposure to UV radiation accelerates the ageing process of the skin, causing the breakdown of collagen and elastin fibres, leading to wrinkles, sagging skin and the development of age spots

  • Sunburn occurs when the skin is exposed to excessive UV rays that trigger an inflammatory response as a defence mechanism indicating damage to the skin cells

• Ecosystems are damaged, affecting productivity

  • UV radiation can harm phytoplankton, the base of the marine food chain, impacting aquatic ecosystems

  • High UV levels can damage plant tissues, affecting crop yields and plant productivity

  • Disruptions in ecosystems due to UV damage can affect various species and alter food webs

• Clothing and construction materials exposed to UV rays are damaged and degrade

  • Fabrics, plastics, paints and building materials may become brittle, faded or weakened, reducing their durability and lifespan

  • This degradation not only affects the aesthetic appearance of materials but also compromises their structural integrity and functionality