Ozone layer

The ozone layer is a vital atmospheric region characterised by elevated concentrations of ozone relative to the surrounding air. Although it contains only trace amounts of the gas, it plays a critical role in absorbing biologically harmful ultraviolet radiation from the Sun, thereby supporting the survival of life on Earth. Found predominantly within the lower stratosphere, the layer displays considerable spatial and seasonal variations linked to global circulation and solar influences.

Discovery and Early Scientific Understanding

The existence of the ozone layer was first established in 1913 by the French physicists Charles Fabry and Henri Buisson. They discovered that sunlight reaching the Earth’s surface lacked wavelengths shorter than approximately 310 nanometres, a discrepancy that could only be explained by atmospheric absorption. Subsequent comparison of the missing spectral region identified ozone as the responsible absorbing species.
Further advancements in understanding came from the British meteorologist G. M. B. Dobson, who developed ground-based spectrophotometry capable of measuring stratospheric ozone. Between 1928 and 1958 he established a global observation network that still provides important atmospheric data. His research laid the foundation for modern ozone monitoring, and the Dobson Unit (DU), which quantifies the total ozone in a vertical column of air, is named in his honour.

Formation and Photochemical Processes

The ozone layer owes its existence to photochemical reactions driven by solar ultraviolet radiation. These mechanisms, first described comprehensively by the British physicist Sydney Chapman in 1930, involve splitting dioxygen molecules into individual oxygen atoms. The free atoms subsequently react with unbroken dioxygen to form ozone. The process can be summarised as:

• Ultraviolet radiation with wavelengths below about 240 nanometres photolyses dioxygen into atomic oxygen.
• Atomic oxygen reacts with dioxygen to form ozone.
• Ozone itself is broken down when it absorbs further ultraviolet light, yielding dioxygen and atomic oxygen.

These reactions constitute the ongoing ozone–oxygen cycle. Although ozone is unstable, the balance between formation and destruction under stratospheric conditions allows it to persist for long periods. Approximately 90 per cent of the atmosphere’s ozone is concentrated in the stratosphere.

Structure, Location and Distribution

The ozone layer occupies a broad band within the lower stratosphere, typically situated between about 15 and 35 kilometres above the Earth’s surface. Concentrations within this region peak at around 8 to 15 parts per million by volume, a value significantly higher than the global atmospheric average of approximately 0.3 parts per million. Despite its importance, the layer would be only a few millimetres thick if compressed to sea-level pressure.
The distribution of ozone is neither uniform nor static. It differs with latitude and season due to atmospheric circulation patterns and changes in solar radiation. The majority of ozone is generated over the tropics where sunlight is most intense. Stratospheric wind systems, particularly the Brewer–Dobson circulation, then transport this ozone-rich air towards higher latitudes. As a result, ozone concentrations tend to be lower near the equator and higher near the poles, peaking in spring and reaching minimum levels in autumn in the northern hemisphere. These patterns also reflect the influence of land–sea contrasts and mountain ranges, which affect the strength of atmospheric circulation. Prior to significant anthropogenic influences, northern polar springtime ozone values could exceed 600 DU, whereas the Antarctic region typically displayed maximum values around 400 DU.

Absorption of Ultraviolet Radiation

The ozone layer is essential because it absorbs the vast majority of the Sun’s medium-frequency ultraviolet radiation, specifically in the range from 200 to 315 nanometres. Around 97 to 99 per cent of this potentially harmful radiation is filtered out, preventing its arrival at the Earth’s surface. Ultraviolet radiation is classified according to wavelength:

• UVA (315–400 nm): Weakly absorbed by ozone and nitrogen, allowing most of it to reach the ground. It contributes to skin ageing and long-term tissue damage.
• UVB (280–315 nm): Partially absorbed by ozone. Excessive exposure can cause sunburn, cataracts, genetic damage and increased risk of skin cancer. Small amounts reaching the surface are necessary for vitamin D production in mammals.
• UVC (100–280 nm): Extremely harmful but completely absorbed by a combination of dioxygen and ozone, preventing it from reaching the surface.

The efficiency of ozone absorption is at its maximum around a wavelength of 250 nanometres. At 290 nanometres, the intensity of ultraviolet light at the top of the atmosphere is hundreds of millions of times greater than at ground level, demonstrating the effectiveness of the ozone shield.

Historical Depletion and Global Response

In 1985 scientists reported a dramatic decrease in stratospheric ozone levels, particularly over Antarctica, a phenomenon widely known as the ozone hole. Research identified industrially produced organohalogen compounds—especially chlorofluorocarbons and bromofluorocarbons—as the primary cause. These substances, although heavier than air, are efficiently mixed through the homosphere and eventually reach the stratosphere. There they are broken down by ultraviolet light, releasing chlorine and bromine radicals capable of catalysing the destruction of tens of thousands of ozone molecules each.
The consequences of ozone depletion included increased ultraviolet exposure at the surface, with potential impacts on ecosystems, human health and climatic balance. Recognising the severity of the issue, international agreements such as the Montreal Protocol led to restrictions and eventual bans on many ozone-depleting substances. By the early twenty-first century evidence indicated that ozone depletion had stabilised and that recovery had begun, although complete restoration will take several decades. Since 2009 nitrous oxide has been identified as a leading anthropogenic contributor to ozone loss.
The United Nations General Assembly has designated 16 September as the International Day for the Preservation of the Ozone Layer to commemorate global efforts to protect this atmospheric resource.

Variations and Extraterrestrial Ozone

Ozone distribution is influenced by natural atmospheric processes as well as anthropogenic factors. Seasonal variations, circulation dynamics and polar meteorological conditions create regions of both enhanced and diminished ozone abundance. The Arctic commonly exhibits high ozone values during March and April, whereas Antarctic ozone levels are at their lowest during September and October due to strong polar vortex conditions and enhanced catalytic destruction.
Beyond Earth, ozone has also been detected elsewhere in the Solar System. Venus, for example, possesses a thin ozone layer located roughly 100 kilometres above its surface, though its concentration is far lower than that of the Earth.