13.1 Stratospheric ozone chemistry
3 min read•july 24, 2024
The stratospheric shields Earth from harmful UV radiation. It forms through a delicate balance of reactions called the , where oxygen molecules split and recombine. This protective shield is crucial for life on our planet.
, caused by human-made chemicals like , threatens this vital layer. These substances release chlorine in the , catalyzing ozone destruction. The effects are far-reaching, impacting ecosystems, agriculture, and human health worldwide.
Stratospheric Ozone Chemistry
Ozone formation in stratosphere
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- Chapman cycle describes and destruction through series of reactions
- Formation of ozone occurs in two steps:
- of molecular oxygen splits O₂ into atomic oxygen
- Atomic oxygen reacts with molecular oxygen forming ozone
- (M is collision partner like N₂)
- of molecular oxygen splits O₂ into atomic oxygen
- Destruction of ozone happens via:
- Photolysis of ozone breaks O₃ into O₂ and O
- Atomic oxygen reacts with ozone producing two O₂ molecules
- Photolysis of ozone breaks O₃ into O₂ and O
- Net reaction results in null cycle maintaining steady-state ozone concentration
- Solar radiation drives cycle with UV light initiating photolysis reactions
- Ozone layer forms protective shield absorbing harmful UV radiation (UVB, UVC)
CFCs and ozone depletion
- CFCs synthetic compounds containing chlorine, fluorine, and carbon used in refrigerants and aerosols
- CFCs transport to stratosphere due to long atmospheric lifetimes (50-100 years)
- UV radiation in stratosphere photolyzes CFCs releasing chlorine atoms
- Chlorine catalyzes ozone destruction through cycle:
- One chlorine atom destroys thousands of ozone molecules before removal
- Other ozone-depleting substances include halons (fire extinguishers), carbon tetrachloride (solvents), methyl chloroform (industrial cleaning)
- Bromine compounds (halons) 40-100 times more destructive than chlorine
Effects of ozone depletion
- Increased UV radiation reaching Earth's surface causes numerous impacts
- Environmental effects include:
- Damage to phytoplankton disrupts marine food chains
- Reduced crop yields decrease agricultural productivity (soybeans, wheat)
- Accelerated degradation of materials like plastics and paints
- Human health impacts encompass:
- Increased risk of skin cancer (melanoma, basal cell carcinoma)
- Cataracts and other eye problems leading to vision impairment
- Suppression of immune system making people more susceptible to diseases
- Polar ozone holes form seasonally:
- Antarctic ozone hole appears during spring (September-November)
- Arctic experiences less severe but significant ozone depletion
Effectiveness of ozone agreements
- (1987) aimed to phase out production of ozone-depleting substances
- 198 countries ratified agreement demonstrating global commitment
- Amendments strengthened protocol:
- London Amendment (1990) added more substances and financial mechanism
- Copenhagen Amendment (1992) accelerated phase-out schedules
- Montreal Amendment (1997) established licensing system for trade
- Beijing Amendment (1999) included bromochloromethane
- Effectiveness shown through:
- 98% reduction in CFC production and consumption since 1986
- Decline in atmospheric concentrations of ozone-depleting substances
- Projected recovery of ozone layer by mid-21st century (2050-2070)
- Challenges remain:
- Addressing HCFCs and HFCs as replacements with high global warming potential
- Combating illegal production and trade of banned substances (CFC-11)
- Montreal Protocol serves as model for addressing other global environmental issues (climate change)
Key Terms to Review (21)
CFCs: CFCs, or chlorofluorocarbons, are synthetic compounds made up of chlorine, fluorine, and carbon. They were commonly used as refrigerants, propellants in aerosol sprays, and solvents. Their connection to stratospheric ozone chemistry is crucial, as CFCs are known to deplete the ozone layer when released into the atmosphere, leading to increased UV radiation reaching the Earth's surface.
Chapman Cycle: The Chapman Cycle, also known as the ozone-oxygen cycle, describes the natural process of ozone formation and destruction in the stratosphere. It involves a series of photochemical reactions driven primarily by ultraviolet (UV) radiation from the sun, which plays a crucial role in maintaining the balance of ozone levels essential for protecting life on Earth from harmful UV radiation.
Chlorofluorocarbons (CFCs): Chlorofluorocarbons (CFCs) are a group of man-made organic compounds that contain chlorine, fluorine, carbon, and hydrogen. These compounds were commonly used as refrigerants, propellants in aerosol sprays, and solvents. Their connection to stratospheric ozone chemistry is critical, as they have been identified as major contributors to ozone layer depletion through the release of chlorine atoms when they are broken down by ultraviolet radiation in the stratosphere.
Cl: Cl, or chlorine, is a halogen element with the atomic number 17 that plays a significant role in the chemistry of stratospheric ozone depletion. Chlorine is known for its ability to react with ozone (O₃) in the stratosphere, contributing to the breakdown of this crucial protective layer in the atmosphere. Its compounds, particularly chlorofluorocarbons (CFCs), have been major contributors to ozone layer thinning due to their stability and resistance to decomposition in the lower atmosphere.
Clean Air Act: The Clean Air Act is a comprehensive federal law that regulates air emissions from stationary and mobile sources in the United States. It aims to protect public health and the environment by setting and enforcing air quality standards, promoting research, and encouraging state implementation of air quality management plans. This legislation plays a crucial role in addressing both stratospheric ozone depletion and tropospheric air pollution.
CLO: CLO, or Chlorine monoxide, is a reactive intermediate in stratospheric ozone chemistry that plays a crucial role in the depletion of ozone in the atmosphere. It is formed through various reactions involving chlorine compounds, especially those derived from man-made substances like chlorofluorocarbons (CFCs). CLO participates in catalytic cycles that lead to the breakdown of ozone (O₃), significantly impacting the balance of atmospheric chemistry and contributing to ozone layer depletion.
F. Sherwood Rowland: F. Sherwood Rowland was an American chemist best known for his groundbreaking research on the depletion of the ozone layer caused by chlorofluorocarbons (CFCs). His work, particularly in the 1970s, revealed how these chemicals release chlorine in the stratosphere, which catalyzes the breakdown of ozone molecules, leading to significant environmental concerns regarding UV radiation and its effects on living organisms.
Kyoto Protocol: The Kyoto Protocol is an international treaty that commits its parties to reduce greenhouse gas emissions, based on the premise that global warming exists and human-made CO2 emissions have caused it. Adopted in 1997 and entered into force in 2005, this protocol establishes legally binding obligations for developed countries to limit and reduce their greenhouse gas emissions, playing a crucial role in addressing climate change and protecting stratospheric ozone.
Mario J. Molina: Mario J. Molina was a renowned Mexican chemist who played a pivotal role in understanding the depletion of the ozone layer caused by chlorofluorocarbons (CFCs). His groundbreaking research demonstrated the chemical processes that lead to ozone depletion in the stratosphere, significantly impacting environmental policy and international agreements aimed at protecting the ozone layer.
Montreal Protocol: The Montreal Protocol is an international treaty designed to phase out the production and consumption of substances that deplete the ozone layer. It was adopted in 1987 and has been pivotal in regulating ozone-depleting chemicals like chlorofluorocarbons (CFCs) to protect the stratospheric ozone layer, which shields the Earth from harmful ultraviolet radiation. This treaty has not only helped in restoring ozone levels but also influenced policies related to air quality and climate change.
Nitrous Oxide (N₂O): Nitrous oxide (N₂O), commonly known as laughing gas, is a colorless and non-flammable gas with a slightly sweet odor, often used in medical applications and as an industrial propellant. It is also recognized as a significant greenhouse gas that affects the stratospheric ozone layer, contributing to ozone depletion through its chemical reactions in the atmosphere.
Ozone depletion: Ozone depletion refers to the thinning of the ozone layer in the Earth’s stratosphere, primarily caused by human-made chemicals like chlorofluorocarbons (CFCs). This phenomenon is critical as the ozone layer acts as a shield, protecting living organisms from harmful ultraviolet (UV) radiation, which can lead to skin cancer, cataracts, and other environmental impacts. Understanding ozone depletion connects to the broader implications of photochemistry in atmospheric processes and highlights the importance of protecting our environment from synthetic pollutants.
Ozone formation: Ozone formation refers to the process through which ozone (O₃) is created in the atmosphere, primarily through photochemical reactions involving oxygen (O₂) and ultraviolet (UV) radiation. This process plays a crucial role in both the stratosphere, where ozone forms a protective layer that absorbs harmful UV radiation from the sun, and in the troposphere, where it can contribute to air pollution and smog. Understanding how ozone is formed helps to grasp its environmental significance and its impact on human health.
Ozone layer: The ozone layer is a region of Earth's stratosphere that contains a high concentration of ozone (O₃) molecules, which absorb most of the Sun's harmful ultraviolet (UV) radiation. This protective layer plays a crucial role in safeguarding life on Earth by filtering out the majority of the Sun's harmful UV rays, thus reducing the risk of skin cancer, cataracts, and other health issues while also protecting ecosystems.
Ozone regeneration: Ozone regeneration refers to the natural process in which ozone (O₃) is formed and replenished in the stratosphere, primarily through photochemical reactions involving solar ultraviolet (UV) radiation. This process is crucial for maintaining the balance of ozone in the stratosphere, which protects life on Earth from harmful UV radiation. Ozone regeneration is influenced by various factors, including the presence of ozone-depleting substances and atmospheric dynamics.
Ozone sonde: An ozone sonde is a type of instrument used to measure the concentration of ozone in the atmosphere, typically deployed via weather balloons that ascend through the stratosphere. These sondes play a crucial role in understanding stratospheric ozone chemistry by providing valuable data on ozone levels at various altitudes, which helps scientists monitor changes and trends related to atmospheric processes and environmental impacts.
Photolysis: Photolysis is the process by which chemical compounds are broken down by the action of light, typically ultraviolet or visible light. This reaction can lead to the formation of free radicals and other reactive species, which can have significant implications for various chemical processes in the atmosphere and in water treatment applications. The process is essential for understanding how pollutants degrade in the environment and how stratospheric ozone is formed and destroyed.
Satellite remote sensing: Satellite remote sensing refers to the use of satellite technology to collect data about the Earth's surface and atmosphere from space. This technology plays a crucial role in monitoring environmental changes, including stratospheric ozone levels, by capturing information on gas concentrations, temperatures, and other atmospheric phenomena through various sensors and imaging techniques.
Stratosphere: The stratosphere is the second layer of Earth's atmosphere, located above the troposphere and below the mesosphere, extending from about 10 to 50 kilometers above sea level. This layer is crucial for stratospheric ozone chemistry, as it contains the ozone layer, which absorbs and scatters the solar ultraviolet radiation, protecting living organisms on Earth from harmful UV rays.
Troposphere: The troposphere is the lowest layer of Earth's atmosphere, extending from the surface to about 8 to 15 kilometers high, depending on geographic location and weather conditions. This layer contains approximately 75% of the atmosphere's mass and is where most weather phenomena occur, making it essential for understanding atmospheric chemistry and pollution's effects on stratospheric ozone.
Uv radiation absorption: UV radiation absorption refers to the process in which ultraviolet (UV) light, a type of electromagnetic radiation, is taken up by molecules, leading to various chemical reactions and changes in energy states. This phenomenon is particularly important in the context of stratospheric ozone chemistry, where ozone molecules absorb significant amounts of UV radiation, protecting living organisms from its harmful effects and playing a critical role in the Earth's atmospheric processes.