Helena Joyce
The stratosphere, a region of the atmosphere above the troposphere, is home to the ozone layer, which protects the Earth from harmful ultraviolet radiation. Certain pollutants from the surface can reach the stratosphere and deplete the ozone layer, increasing the amount of UV radiation that reaches the Earth's surface. One such pollutant is chlorofluorocarbons (CFCs), which are man-made chemicals used in various industrial processes, including refrigeration, air conditioning, and aerosol propellants. Other sources of stratospheric pollution include aviation exhaust and volcanic eruptions, which release particles and gases that can alter the balance of sunlight and heat in the atmosphere. To protect the ozone layer and mitigate the effects of stratospheric pollution, global efforts have been implemented to reduce emissions of ozone-depleting substances.
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What You'll Learn
Chlorofluorocarbons (CFCs)
CFCs were first synthesized in 1928 by Thomas Midgley Jr. of General Motors, who was searching for a safer, non-toxic alternative to the toxic gases used in early refrigerators, such as ammonia, sulphur dioxide, and chloromethane. CFCs quickly became the coolant of choice in large refrigeration and air-conditioning systems due to their non-toxic and non-flammable properties. CFCs also found applications in aerosol sprays, blowing agents, and solvents.
However, in 1974, chemists F. Sherwood Rowland and Mario Molina discovered that CFCs could deplete the Earth's atmospheric ozone layer, which is crucial for blocking the sun's damaging ultraviolet rays. This initiated an environmental effort to phase out CFCs and replace them with safer alternatives, such as hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs).
When CFCs are released into the atmosphere, they can gradually rise to the stratosphere, where they undergo photodissociation by ultraviolet (UV) radiation, releasing chlorine atoms. These chlorine atoms then catalyze the destruction of ozone molecules, leading to ozone depletion. This depletion allows more harmful UV radiation to reach the Earth's surface, posing risks to human health, ecosystems, and various organisms, including potential damage to plants, marine life, and even plastics.
The discovery of the harmful effects of CFCs on the ozone layer led to worldwide regulations and the enactment of the Montreal Protocol, which aimed to reduce emissions and phase out the use of ozone-depleting substances. While NASA reported in 2018 that the ozone layer had begun to recover, research in 2019 pointed to an alarming increase in CFCs, indicating unregulated use in certain regions.
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Aviation exhaust
The stratosphere is a layer of Earth's atmosphere, sitting just above the troposphere. It is of particular interest to scientists because of its protective ozone layer and its role in influencing climate.
Commercial aircraft typically cruise at altitudes within the lower stratosphere, and their exhaust contains small particles and sulfur dioxide gas. Air traffic has been steadily increasing, leading to a 23% increase in sulfur dioxide emissions between 2014 and 2018.
NOAA scientist Charles Brock, a co-author of a paper on the subject, stated:
> While more research will be needed to prove a direct link between stratospheric aerosol imbalance and aviation, this study clearly shows human activity has created substantially different conditions in the lower stratosphere of the Northern and Southern Hemispheres.
The increase in global aviation, combined with the planned increase in rocket launches for satellite constellations and space exploration, could lead to significant new stratospheric pollution.
The implications of this are important, as stratospheric aerosols can influence how much heat is absorbed by the atmosphere. Even though these particles are tiny, they can alter the balance between sunlight reaching the Earth's surface and heat escaping back into space.
Flying in the stratosphere is challenging due to the extreme conditions, including thin air, low temperatures, powerful atmospheric waves, and increased UV radiation. Aircraft must be carefully designed and equipped with special materials and components to withstand these conditions.
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Volcanic eruptions
During volcanic eruptions, gases such as sulfur dioxide, water vapor, and carbon dioxide are released into the atmosphere. Sulfur dioxide, in particular, can rise to the stratosphere and convert into sulfuric acid aerosols. These aerosols create a haze of tiny droplets that reflects and blocks incoming solar radiation, causing a cooling effect on the Earth's surface. This cooling effect can last for months or even years, depending on the size of the eruption.
The aerosols formed from volcanic sulfur dioxide also contribute to ozone destruction. While the amount of SO2 released by volcanoes is much less compared to man-made sources, some eruptions can have a disproportionately large impact. For example, the 1980 eruption of Mount St. Helens released approximately 10 million tons of carbon dioxide in just 9 hours.
In addition to gases, volcanic eruptions also emit volcanic ash and dust particles that can reach the stratosphere. The smallest dust particles can stay in the stratosphere for months, blocking sunlight and causing cooling over large areas. However, most of the injected ash falls rapidly from the stratosphere and has little impact on climate change.
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Sulphur dioxide
The pollutant that manages to reach the stratosphere is Sulphur Dioxide (SO2). It is a precursor gas that can be pumped into weather balloons, which then rise into the stratosphere. There, the sulphur dioxide reacts with water vapour to form long-lived sulphate aerosols in the upper atmosphere. These aerosols reflect sunlight and temporarily cool the planet.
Some scientists have proposed intentionally injecting sulphur dioxide into the stratosphere through a process called stratospheric aerosol injection. This process mimics the natural cooling effect of volcanic eruptions and could potentially offset the warming effects of greenhouse gases. However, there are concerns about the potential risks and unknowns of such geoengineering projects.
Tropospheric-sourced sulphur dioxide has been a subject of debate for decades due to its role in maintaining background stratospheric aerosols. Recent in situ measurements at the tropical tropopause suggest that the contribution of sulphur dioxide to stratospheric aerosols may be negligible, challenging previous assumptions.
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Nitrogen oxides (NOx)
NOx can be produced by both human activities and natural processes. Human activities, such as the combustion of fossil fuels and the use of nitrogen-fixing plants in agriculture, contribute significantly to NOx emissions. Natural processes, like lightning, also produce NOx, particularly in areas near the equator during the summer months.
The vertical distribution of NOx in the atmosphere varies with its source. Lightning-produced NOx is typically found at altitudes greater than 5 km, while combustion and biogenic (soil) NOx are usually found near their sources at near-surface elevations, where they can have the most significant health impacts.
NOx emissions have been linked to health and environmental concerns. In addition to contributing to smog and acid rain, NOx plays a role in ozone depletion and has a high global warming potential. Efforts to reduce NOx emissions, such as vehicle and transportation standards, are crucial to improving air quality and mitigating the negative impacts of this pollutant.
While NOx is a significant pollutant, it is important to note that it does not reach the stratosphere. The stratosphere is the layer of the atmosphere located about 9 to 18 miles (15 to 30 km) above the Earth's surface. Pollutants like chlorofluorocarbons (CFCs) released from industrial processes can reach the stratosphere and cause ozone depletion, but NOx typically remains in the troposphere, contributing to ground-level ozone formation and associated health issues.
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Frequently asked questions
The stratosphere is the region of the atmosphere above the troposphere, which is the layer closest to the Earth's surface. Ozone is a pollutant in the troposphere, but it is essential in the stratosphere as it protects life on Earth from harmful UV radiation. However, human activities have damaged this protective shield by releasing pollutants that deplete the ozone layer. These pollutants include oxides of nitrogen (NOx) and volatile organic compounds (VOCs) emitted by cars, power plants, industrial boilers, refineries, and chemical plants.
Tropospheric or ground-level ozone is a harmful air pollutant and a major component of smog. Exposure to ground-level ozone can worsen respiratory conditions such as bronchitis and emphysema, trigger asthma, and permanently damage lung tissue. It is particularly dangerous for children, the elderly, and people with lung or cardiovascular diseases.
Atmospheric particles, also known as aerosols, are microscopic bits of liquid or solid material suspended in the air. Aerosols can be emitted directly or transported from the lower atmosphere, such as soot or dust. They can also form in the atmosphere through the condensation of gases, producing particles as small as 3-12 nanometers in diameter. These tiny particles can alter the balance between sunlight reaching the Earth's surface and heat escaping back into space.
