The ozone layer is one of the most fascinating and relevant topics in current environmental science. Although at first glance it may seem like a matter reserved for scientists and meteorologists, its thickness, its variations and the importance of its conservation have direct implications on the daily lives of all of usFrom protection against ultraviolet radiation to its impact on human health and ecosystems, understanding the ozone layer is critical to assessing the risks to the planet and the solutions we can implement.
In the following lines, you'll immerse yourself in a complete tour that covers the physical nature of the ozone layer, how it is measured and monitored, the main threats to its integrity, the historical evolution of its state, and the achievements—and remaining challenges—in its protection. In addition to reviewing the scientific foundations, you'll discover how variations occur over time and space, what instruments are used to measure it, and, above all, why this gaseous layer is so important to preserve. is crucial for the continuity of life on Earth.
What is the ozone layer and why is it vital?
The ozone layer is an area of the Earth's atmosphere, located mainly in the stratosphere, where most atmospheric ozone is concentrated. This gas, whose chemical formula is O3, is composed of three oxygen atoms and has unique properties that differentiate it from common oxygen (O2).
It extends approximately between 15 and 40 kilometers above the Earth's surface, reaching its highest concentration around 25 kilometers. However, if all the ozone in the stratosphere were compressed at ambient pressure, we would form a very thin layer between 2 and 3 millimeters thick, a surprising fact considering its significant protective role.
The main function of the ozone layer is to filter and absorb most of the ultraviolet radiation (UV-B and UV-C) coming from the Sun. Without this natural barrier, harmful radiation would reach the Earth's surface unhindered, causing devastating effects: an increase in diseases such as skin cancer and cataracts, crop damage, harm to marine life, and disruptions to terrestrial and aquatic ecosystems.
The very existence of life on Earth, as we know it, depends on this delicate gas shield. Therefore, any relevant alteration in its thickness or composition has a direct impact on environmental and human health.
Formation and destruction of stratospheric ozone
The formation and destruction of ozone in the stratosphere is a dynamic process, the result of complex chemical and physical balances driven primarily by solar ultraviolet radiation.
Ozone is generated when UV radiation with a wavelength shorter than 240 nm hits oxygen molecules (O2). This energy "breaks" the molecules, separating the atoms, which then join with other oxygen molecules to form ozone (O3). This mechanism was described by Sydney Chapman in 1930 and is known as the Chapman Cycle.
The essential reaction can be summarized as follows: sunlight breaks down molecular oxygen into individual atoms, and these atoms subsequently recombine with O2 to generate ozone (O3). Ozone, in turn, can be destroyed by UV radiation of lower specificity, releasing molecular oxygen and oxygen atoms. This back-and-forth reaction maintains the natural balance of the ozone layer, provided there are no external disturbances.
Other factors, such as the presence of halogenated compounds (e.g., chlorofluorocarbons, CFCs, and halons) or increased nitrogen oxide (NOx) content, can drive catalytic reactions that accelerate ozone destruction.
In polar regions, especially during the Antarctic spring, what we know as the “ozone hole” occurs. In these areas, factors such as low temperatures, the formation of polar stratospheric clouds, and the accumulation of halogenated compounds contribute, triggering massive, seasonal destruction of the layer.
Ecological and health importance
The role of the ozone layer in preserving life is essential and irreplaceable. By absorbing more than 97% of UV-B radiation and almost all of UV-C, prevents lethal doses of solar radiation from reaching the Earth's surfaceIn this way, the layer protects living beings from:
- Skin cancer: Exposure to unfiltered UV radiation increases the risk of melanomas and other skin tumors.
- Cataracts and eye damage: UV radiation can cause serious eye conditions, even leading to blindness.
- Immunosuppression: There is evidence that increased exposure to UV-B decreases the effectiveness of the immune system in humans and animals.
- Alterations to ecosystems: The reduction of the layer can affect photosynthesis and alter food chains in seas, lakes, rivers and forests.
- Agricultural impact: Increased radiation negatively affects crop productivity and quality.
The ozone layer also plays a relevant role in climate dynamics, since by absorbing UV radiation, contributes to the warming of the stratosphere and regulates global atmospheric temperature.
How is the thickness and concentration of the ozone layer measured?
The “thickness” of the ozone layer is not expressed as a direct physical thickness, but as a measure of the amount of ozone present along a vertical column of the atmosphere. The standard form is the Dobson Unit (DU), which represents the amount of ozone that, compressed under normal conditions of pressure and temperature, would form a layer 0,01 mm thick.
The global average value of ozone in the atmosphere is considered to be around 300 DU, although there are variations depending on the geographical location and the season of the year.. For example, at the poles (especially during the Antarctic spring) values can drop below 150-220 DU during ozone hole episodes.
The measurement is carried out through specific instruments:
- Dobson and Brewer Spectrophotometers: These are optical devices that measure ultraviolet radiation from the Sun before and after it passes through the atmosphere. This allows us to calculate the total concentration of ozone in the column.
- Ozone probes: They are weather balloons equipped with sensors that, as they ascend, record data on ozone concentrations depending on altitude.
- Weather satellites: Equipped with advanced sensors, they enable global mapping and historical analysis of the distribution and evolution of the ozone layer.
Meteorological and research centers such as the State Meteorological Agency (AEMET) in Spain, or the Izaña Observatory in the Canary Islands, are international references in atmospheric ozone monitoring.. These institutions work in a network, sharing data globally and facilitating real-time assessment of the layer's condition.
Thickness variations: natural and anthropogenic causes
The thickness and concentration of the ozone layer vary naturally throughout the year, between different regions, and also due to human-caused causes.
Natural causes include:
- Latitude and season: Polar regions typically record lower values in spring due to specific photochemical processes. Equatorial regions, receiving greater UV radiation, experience greater ozone production.
- Solar activity: Changes in solar radiation, solar cycles, and eruptions temporarily affect the production and destruction of ozone.
- Meteorological processes: Planetary waves, polar vortices and other atmospheric circulation phenomena influence the distribution and transport of stratospheric ozone.
- Volcanic eruptions: The ejection of particles and gases can punctually reduce ozone by several chemical pathways.
The main threat to the balance of the ozone layer comes from human activities.The continued use and emission of halogenated chemicals, especially CFCs and halons, since the mid-20th century, They are responsible for the accelerated loss of ozone in large regions of the planet.
These substances, once emitted into the atmosphere, can take years to reach the stratosphere, where UV radiation breaks them down, releasing extremely reactive chlorine and bromine atoms. These atoms They destroy ozone through catalytic reactions in which a single molecule can eliminate up to 100.000 O molecules.3 before being neutralized.
The process of ozone destruction by halogenated compounds
The catalytic destruction of ozone by chlorinated and brominated compounds is the most significant route of ozone depletion in recent decades. The molecules responsible are mainly chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halons, carbon tetrachloride, and methyl chloroform, among others.
The main mechanism is that, after reaching the stratosphere, these substances undergo photolysis due to UV radiation, releasing chlorine or bromine atoms. Subsequently, they participate in cyclic reactions with ozone:
- The chlorine atom reacts with an ozone molecule, forming chlorine monoxide (ClO) and molecular oxygen.
- Chlorine monoxide reacts with an oxygen atom, releasing chlorine again and closing the cycle.
Similarly, brominated compounds, such as halons and methyl bromide, follow similar pathways and are, in fact, even more effective at destroying ozone. A single atom of bromine can be up to 45 times more effective than one of chlorine.
The reactions are intensified in the polar regions during winter and spring, due to the presence of polar stratospheric clouds. These clouds provide surfaces for normally inactive compounds to transform into highly active species, ready to destroy ozone when solar radiation returns at the end of winter.
The ozone hole phenomenon
The “ozone hole” refers to a region – mainly over Antarctica – where the total ozone content falls below 220 DU during the austral spring (August to November).
This phenomenon was initially detected in the 70s and 80s through field and satellite observations. Its appearance and evolution are related to:
- Atmospheric isolation from the polar vortex: During the southern hemisphere winter, a jet stream separates Antarctic air from the rest of the planet, allowing low temperatures to accumulate and polar stratospheric clouds to form.
- The presence of halogenated compounds: These transform, on the surfaces of polar clouds, into highly reactive forms that initiate intensive destruction as soon as sunlight appears.
The area of the ozone hole has reached more than 25-29 million square kilometers in some years, more than twice the surface area of the Antarctic continent. Although this phenomenon is most intense over Antarctica, less pronounced episodes have also been observed in the Arctic.
The impact of this phenomenon has been particularly worrying in southern regions such as Argentina and Chile, where increased ultraviolet radiation has caused health alerts, crop damage, and harm to wildlife.
Historical evolution, surveillance and recovery
Since the first signs of accelerated destruction in the 70s, the international scientific community, government agencies, and multilateral organizations have intensified their monitoring and study of the state of the ozone layer.
Monitoring is done through:
- Networks of spectrophotometers and ozone probes: Distributed around the world, they collect data in real time and are part of international consortia such as the World Ozone and UV Data Centre (WOUDC).
- Weather satellites: They allow for global and detailed monitoring of the layer, identifying trends, seasonal anomalies, and the evolution of ozone holes.
- Regional research centers: Like the Izaña Observatory (Spain), which leads calibration campaigns and cutting-edge technology in ozone measurement.
Spain stands out in Europe for its network of instruments and initiatives, such as co-leading the EUBREWNET network, dedicated to providing consistent, high-quality data on ozone and UV radiation. In addition, it has more than twenty-five measuring stations and an ultraviolet index prediction system for all the country's municipalities.