The ozone layer plays a fundamental role in protecting life on Earth, acting as a natural shield against ultraviolet (UV) radiation from the Sun. Understanding its composition, the complex chemical reactions that occur within it, and the factors that influence its stability are key to understanding both its environmental relevance and the risks associated with its deterioration.
Since the discovery of the ozone layer and scientific advances in its environmental chemistry, social and political concern has been increasing., promoting international treaties and changes in consumption and production habits. Below, we present a detailed and comprehensive guide, in user-friendly and completely updated language, on everything you need to know about the chemistry of the ozone layer, its composition, the mechanisms of its formation and destruction, and the current and future challenges it faces.
What is ozone and where is it found?
Ozone (O3) is an allotropic form of oxygen, consisting of three atoms of this elementIt is a colorless or slightly bluish gas in high concentrations and is notable for its strong, characteristic odor, perceptible even in very small quantities after a thunderstorm or under certain environmental conditions. Ozone plays very different roles depending on where it is located in the atmosphere, making it necessary to distinguish between two major locations: the stratosphere and the troposphere.
In the stratosphere, between 15 and 50 km altitude, there is about 90% of the ozone present in the entire atmosphere.This area is commonly called the ozone layer, vital for life on Earth, as it filters out harmful ultraviolet radiationIf all stratospheric ozone were compressed to sea-level pressure, its thickness would be only 3 mm, but this thin layer is essential to protect us from problems such as skin cancer and cataracts.
In the troposphere, i.e. from the surface to approximately 15-18 km altitude, Ozone is considered a secondary pollutantHere, far from protecting us, it can cause irritation, respiratory problems, and contribute to photochemical smog, one of the main air pollution problems in large cities and industrial areas.
Chemical and physical properties of ozone
Ozone is one of the most powerful oxidants present in nature.It is an unstable molecule, since its three oxygen atoms tend to separate easily, returning to the diatomic form (O2). Its density is 2,14 kg/m³ and it is highly soluble in water. —although much less stable than in air, with a half-life of approximately 20 minutes compared to the 12 hours it can last as ambient gas.
Its melting point is -192 ºC and its boiling point is -112 ºC, turning blue in high concentrations. Being a very strong oxidant, ozone reacts rapidly with other molecules and compounds, especially those containing nitrogen, volatile organic compounds or halogens such as chlorine and bromine..
The ozone cycle in the stratosphere: natural formation and destruction
Knowledge about the mechanisms of formation and destruction of stratospheric ozone was consolidated by physicist Sydney Chapman in 1930., through a series of photochemical reactions known as the Chapman cycle. This cycle explains how, under natural conditions, the amount of ozone remains relatively constant thanks to the balance between its formation and destruction.
Formation of stratospheric ozone: It all starts when high-energy ultraviolet radiation (wavelength less than 240 nm, UV-C category) hits oxygen molecules (O2). This sufficiently energetic radiation breaks (dissociates) the O molecules2 into individual oxygen atoms (O).
- O2 + UV radiation → O + O
- O + o2 + M → O3 + M (where M is any neutral molecule, usually N2 at O2, which absorbs excess energy and stabilizes the ozone molecule).
Therefore, the area with the greatest ozone production is the equatorial stratosphere, since that is where ultraviolet radiation hits most intensely.However, stratospheric winds distribute ozone towards polar latitudes.
Once formed, Ozone absorbs UV-B radiation, leading to its decomposition into O2 and an oxygen atom, in a reverse reaction:
- O3 + UV radiation → O2 + O
Under natural conditions, Atomic oxygen can also react with ozone to form two diatomic oxygen molecules:
- O3 + O → 2 O2
This set of reactions maintains the ozone concentration balanced as long as no external factors come into play that alter said balance.However, this delicate balance is easily altered by the action of certain molecules and radicals introduced by human activity.
You can learn more about how the ozone layer is formed in this article..
Ecological importance of the ozone layer
The ozone layer is essential for life as we know it.It acts as a shield that filters most of the sun's ultraviolet-B and ultraviolet-C radiation, preventing it from reaching the Earth's surface. Without this natural filter, UV radiation would be lethal to most living beings and would affect both terrestrial and aquatic ecosystems.
The consequences of an increase in UV-B radiation due to the deterioration of the ozone layer include:
- Increase in cases of skin cancer and cataracts in humans.
- Alteration of the immune system, leading to an increase in diseases.
- Reduction in agricultural and forestry productivity due to damage to crops and forests.
- Impact on aquatic ecosystems, especially of radiation-sensitive planktonic organisms.
- Disturbances in the food chain and photosynthesis in plant organisms.
Furthermore, Stratospheric ozone is responsible for the increase in temperature in the stratosphere, by absorbing UV radiation and transforming it into heat, which determines the thermal structure of the Earth's atmosphere and climatic stability.
Tropospheric ozone: the forgotten pollutant
In contrast to stratospheric ozone, ozone present in the troposphere is a secondary pollutant created by photochemical reactions. between nitrogen oxides (NOx), volatile organic compounds (VOCs), and the action of sunlight. These precursors come primarily from road traffic, industrial processes, and biogenic emissions.
Tropospheric ozone:
- Contributes to the formation of photochemical smog, especially in summer and in anticyclonic zones.
- It is toxic to human health, causing eye and throat irritation, respiratory problems and aggravating diseases such as asthma.
- Causes damage to vegetation and reduces crop yields.
- Contributes to global warming as a greenhouse gas.
Its levels increase during the central hours of the day, especially in rural areas and on the outskirts of large cities., since that is where there is less traffic and, therefore, less consumption of the ozone generated.
Destruction of the ozone layer: causes and consequences
For much of the 20th century, the ozone cycle was thought to be unchanging. However, the introduction of new chemicals, especially chlorofluorocarbons (CFCs), halons, and bromides, radically altered this balance.
CFCs – compounds containing chlorine and fluorine – widely used in refrigeration, air conditioning, aerosols and foams, have proven to be extremely stable and able to reach the stratosphere without degrading.Once there, ultraviolet radiation breaks them down, releasing extremely reactive chlorine and bromine atoms.
A single chlorine atom can destroy up to 100.000 ozone molecules before being eliminated by atmospheric processes.These reactions occur in catalytic cycles, where the catalyst (the halogen) is released intact and can continue to destroy more ozone:
- Cl + O3 → ClO + O2
- ClO + O → Cl + O2
The cycle begins again, generating multiplied damage over time.
You can find out what the destruction of the ozone layer consists of..
The hole in the ozone layer
Beginning in the 80s, satellites and measuring stations in Antarctica detected a worrying decrease in the thickness of the ozone layer during the southern spring.Ozone concentrations over the South Pole decreased by up to 70% during September and October.
The term "ozone hole" is used to describe areas where the total ozone content falls below 220 Dobson Units. (UD). Satellite images show how, every spring, much of Antarctica is covered by this "vacuum zone," even affecting populated regions in the southern hemisphere.
The ozone hole has reached surfaces exceeding 25 million km for several weeks.2, almost twice the size of the Antarctic continent. In September 2006, the lowest value ever was recorded, with just 85 DU over eastern Antarctica.
More details on the evolution of the ozone hole.
Impacts on health and ecosystems
The depletion of stratospheric ozone has serious consequences for public health and the environment.Unfiltered ultraviolet-B radiation can penetrate to the surface, increasing the incidence of:
- Skin cancer (melanoma and non-melanoma)
- Cataracts and eye damage
- Suppression of the immune system
- Reduction in the yield of sensitive crops and alterations in the cycles of aquatic ecosystems
- Problems in marine life, especially in larval stages of phytoplankton and fish
In the troposphere, the presence of ozone is associated with respiratory and cardiovascular problems, especially in vulnerable groups such as the elderly, children, pregnant women, and people with chronic illnesses.
The European Union and the World Health Organization have established limits for exposure to ambient ozone, recommending not exceeding 100 µg/m3 as a daily average, since higher concentrations can cause coughing and irritation, as well as reduced lung function and increased mortality in sensitive individuals.
Key chemical reactions in ozone destruction
The accelerated destruction of ozone in the stratosphere is mainly due to catalytic cycles involving reactive chemical species.These reactions are essential to understanding how ozone depletion occurs and what factors accelerate it.
- Halogenated radicals (Cl, Br, ClO, BrO)
- Nitrogen radicals (NO NO2)
- Hydroxyl radicals (OH) and peroxyl (HO2)
Those that have the greatest impact on ozone destruction are the reactions associated with ClO and BrOCatalytic cycles allow a single chlorine or bromine molecule to destroy thousands or even up to 100.000 ozone molecules before being removed or neutralized.
You can consult about the layers of the atmosphere and their influence on ozone..
Measurement and monitoring of the ozone layer
The measurement of ozone in the atmosphere is carried out primarily using the parameter “Dobson Unit” (DU), which expresses the thickness that total ozone would occupy if compressed under normal pressure and temperature conditions. One DU is equivalent to 2,69 × 1020 ozone molecules per square meter.
Vertical ozone profiles are obtained using ozonesondes and satellites equipped with spectrophotometers, such as the GOMOS instrument installed on Envisat. Typical values range from 200 to 500 DU, with a global average of around 300 DU.
International Actions: The Montreal Protocol and the Kigali Amendment
The seriousness of the problem of ozone layer depletion prompted unprecedented international action.In 1985 the Vienna Convention for the Protection of the Ozone Layer was signed, paving the way for the adoption of the Montreal Protocol in 1987Almost all countries in the world have ratified agreements that prohibit or strictly regulate the production and consumption of ozone-depleting substances (ODS).
The success of the Montreal Protocol has been resounding.The phasing out of CFCs, halons, and other compounds has halted the decline and begun the recovery of the ozone layer since the beginning of the 21st century. However, substitutes such as HCFCs and HFCs continue to require additional regulation, especially due to their potential to contribute to global warming.
International cooperation has been key to protecting the ozone layer.
Ozone layer recovery and future prospects
The latest measurements show a positive trend towards the recovery of the ozone layer., although this process will be slow due to the long lifetime of the emitting compounds in the atmosphere. It is estimated that, if current policies continue, a full recovery to pre-1980 levels could be achieved around 2075.
Climate change also influences recovery, since the increase in greenhouse gases could alter stratospheric circulation and temperature, affecting ozone distribution. International cooperation and strict environmental policies are essential to maintain and accelerate this trend.
What we can do as citizens to protect the ozone layer
We can all contribute to the care of the ozone layer through small daily actions and the adoption of responsible habits:
- Choose products labeled “CFC-free” or “ozone-friendly.”
- Prefer fire extinguishers and cooling systems that do not contain halons, CFCs, or HCFCs.
- Avoid using aerosols with harmful propellants; they come in cream, stick, or mechanical spray forms.
- Keep your refrigeration and air conditioning equipment in good condition and use certified technicians for maintenance.
- Do not use methyl bromide for domestic or agricultural fumigation.
- Reduce car use, use public transport, walk or cycle.
- Share the importance of the topic in your family, educational, and work circles.
- Participate in campaigns and activities to raise awareness about environmental protection.
The role of education and social awareness
Environmental education is a key element in achieving the protection of the ozone layer.Informing and educating new generations about the importance of this natural shield, the risks associated with its deterioration, and the actions needed to prevent it are essential to avoid repeating the mistakes of the past.
Educational institutions, the media, and social organizations play a fundamental role in disseminating information and generating collective awareness.
Every informed person contributes to the defense of our planet.
The chemistry of the ozone layer is an example of the complexity and fragility of the vast environmental systems that sustain life. Although the challenges have been enormous, humanity has demonstrated that international cooperation and civic engagement can reverse dangerous trends. However, success is not guaranteed: it will depend on continued vigilance, innovation, and shared responsibility in every decision that affects our environment.