ESS 6.4.1 The UV and Ozone

Learning Objectives

Summarise solar radiation and outline the UV radiations that can pass through and absorbed by the stratospheric ozone
Describe the impacts of UV radiation
Describe the ozone layer and ozone equilibrium

Solar Radiation

The Sun emits electromagnetic (EM) radiation across a wide spectrum of wavelengths, ranging from low-frequency radio waves to high-frequency gamma rays. This electromagnetic spectrum encompasses infrared (IR) radiation, visible light, and ultraviolet (UV) radiation, each playing distinct roles within the Earth’s biosphere. Nearly all energy available at the Earth’s surface originates from solar radiation.

Electromagnetic waves are defined by their wavelength and frequency, which are inversely related—shorter wavelengths correspond to higher frequencies, while longer wavelengths have lower frequencies. Frequency, measured in hertz (Hz), represents the number of wave cycles passing a fixed point per second.

Radio waves possess long wavelengths and low frequencies. They are primarily used in communication technologies, including television, radio, and satellite transmission. Radio waves travel efficiently through the atmosphere and pose no known harm to human health.

Infrared (IR) radiation occupies the portion of the EM spectrum with wavelengths longer than those of visible light, ranging from approximately 700 nanometres (nm) to 1 millimetre (mm).

Visible light encompasses wavelengths between 400 and 700 nm and is perceptible to the human eye. This segment of the spectrum is essential for photosynthesis and therefore fundamental to life on Earth.

Ultraviolet (UV) radiation has wavelengths shorter than visible light, spanning approximately 10 to 400 nm. UVA (315–400 nm) contributes to vitamin D synthesis but can also cause sunburn and cataracts. UVB (280–315 nm) is capable of inducing DNA damage, while UVC (100–280 nm) is largely absorbed by atmospheric ozone before reaching the Earth’s surface.

Gamma radiation represents the shortest wavelength and highest energy region of the EM spectrum, originating primarily from radioactive decay of atomic nuclei.

UV Radiation

Ultraviolet (UV) radiation possesses shorter wavelengths, higher frequencies, and greater energy than most other forms of electromagnetic radiation, resulting in increased potential harm to living organisms. All three forms of UV radiation—UVA, UVB, and UVC—can cause biological damage.

UVA radiation is both immunosuppressive and mutagenic in humans, and carcinogenic in animals. Research indicates that exposure to radiation between 360–380 nm can suppress immune function in humans, whereas radiation between 320–350 nm does not. UVA exposure has also been linked to the development of skin cancer.

Responses to UVB radiation vary among individuals within the same species. The extent of stress imposed on organisms and ecosystems by elevated UVB exposure can be influenced by additional environmental factors, such as water or nutrient scarcity. The stratospheric ozone layer protects the Earth by absorbing all incident UVC radiation (the most energetic and shortest wavelength) and most UVB rays. However, ozone depletion over polar regions, particularly Antarctica and the Arctic, allows a greater proportion of UV radiation to penetrate the atmosphere.

Impacts of UV

Ultraviolet radiation adversely affects both terrestrial and aquatic ecosystems as well as human health. It reduces photosynthetic efficiency in phytoplankton and can damage DNA, leading to mutations and carcinogenesis. In humans, excessive exposure to UV radiation results in sunburn, premature skin ageing, and ocular conditions such as cataracts.

Exposure to UVB radiation has been linked to increased incidence of skin cancer, as well as cataracts and snow blindness. UVB exposure also suppresses photosynthesis and reduces crop productivity in major agricultural species such as maize, rice, soybeans, wheat, and cotton. Furthermore, it decreases plankton productivity, notably in Antarctic waters affected by ozone depletion, where reductions of 6–12% have been observed.

Declines in stratospheric ozone have led to measurable increases in UVB radiation reaching the surface—estimated at 6–14% above pre-1980 levels. Between 2010 and 2020, springtime UVB levels increased by approximately 14% in parts of the Northern Hemisphere and up to 40% in the Southern Hemisphere. Over the same period, stratospheric ozone concentrations decreased by 3% and 6% in the Northern and Southern Hemispheres, respectively. A 30% increase in UVB intensity could severely impact global crop yields. Experimental studies (e.g., Schneider et al., 2022) have shown reduced growth in red algae (Gracilaria cornea) under elevated UVB exposure.

Globally, over 1.5 million new cases of skin cancer were diagnosed in 2020, resulting in approximately 120,000 deaths. UV exposure also contributes to premature ageing and ocular damage; around 15 million people worldwide are blind due to cataracts, up to 10% of which are attributable to UV exposure. Childhood sun exposure is a significant risk factor for skin cancer later in life, and UV radiation can suppress immune function. Children and adolescents are particularly vulnerable due to their thinner skin and developing ocular structures.

UVC radiation, though largely absorbed by the ozone layer, can damage DNA at the molecular level. Studies of microorganisms in desert and high-altitude environments have documented high levels of resistance to UVC radiation, suggesting adaptive mechanisms to extreme UV exposure.

Stratospheric Ozone

Ozone (O₃) is a highly reactive triatomic form of oxygen produced through photochemical reactions involving solar ultraviolet radiation. Stratospheric ozone absorbs a significant portion of incoming solar UV radiation, thereby reducing the amount that reaches the Earth’s surface and protecting living organisms from its harmful effects.

Ozone formation occurs when molecular oxygen (O₂) is photodissociated by solar UV radiation into two atomic oxygen (O) atoms. Each O atom subsequently combines with an O₂ molecule to form ozone (O₃). This continuous cycle of formation and destruction maintains a dynamic equilibrium that has persisted for millions of years. Consequently, ozone layer thickness varies temporally and spatially.

Although ozone constitutes a small fraction of the atmosphere, its ecological and climatic significance is profound. Most stratospheric ozone is formed over equatorial and tropical regions, where solar radiation is most intense. Atmospheric circulation then transports ozone toward higher latitudes, where it tends to accumulate. The highest concentrations occur between 16 and 35 kilometres above the Earth’s surface—especially between 16 and 25 km—forming a protective shield against harmful radiation.

Ozone absorbs UV radiation within the wavelength range of 100–400 nm and some outgoing terrestrial infrared radiation (10,000–12,000 nm), making it both a UV filter and a greenhouse gas. Natural balance in ozone levels is maintained through simultaneous photochemical production and destruction, including breakdown by nitrous oxide (N₂O).

Ozone Equilibrium

The concentration of ozone (O₃) in the atmosphere remains relatively stable over long timescales due to a dynamic equilibrium between its formation and destruction processes. Nonetheless, both spatial and seasonal variations occur globally. This equilibrium is critical for maintaining the protective capacity of the ozone layer and, by extension, the stability of Earth’s biosphere.