UV radiation is a form of invisible energy released by the sun. While some UV energy is necessary, such as for photosynthesis, excessive exposure can damage living systems. The atmosphere functions as a protective shield, intercepting the vast majority of this incoming solar radiation. This defense prevents the most energetic and harmful UV wavelengths from penetrating to the surface, maintaining conditions necessary for life.
Location of Massive UV Absorption
The layer responsible for absorbing over 95% of incoming UV radiation is the stratosphere. It extends from approximately 6 to 31 miles (10 to 50 kilometers) above the Earth’s surface. Within the stratosphere lies the ozone layer, where the concentration of ozone molecules (\(O_3\)) is significantly higher. The ozone layer’s primary function is to serve as the planet’s natural UV filter, absorbing these high-energy rays.
The highest density of ozone is found between 15 and 30 kilometers in altitude. Even though ozone makes up only a small fraction of the gases, its unique molecular structure allows it to effectively capture and neutralize solar radiation. This efficient absorption process is the main reason why the temperature in the stratosphere increases with altitude.
The Chemical Process of Ozone Shielding
The mechanism that shields the Earth involves a continuous creation and destruction process known as the ozone-oxygen cycle, or Chapman cycle. This cycle begins when high-energy UV radiation breaks apart a diatomic oxygen molecule (\(O_2\)) into two separate, reactive oxygen atoms (\(O\)). Each free oxygen atom then collides with an intact \(O_2\) molecule to form ozone (\(O_3\)).
Another UV photon strikes the newly formed ozone molecule, breaking it apart into an \(O_2\) molecule and a single oxygen atom. This destruction process absorbs the incoming UV energy, converting it into heat. The cycle constantly regenerates the ozone shield while simultaneously absorbing damaging UV radiation before it can reach the lower atmosphere.
Categorizing the Remaining UV Radiation
The radiation that successfully passes through the stratospheric shield is categorized into three types based on wavelength.
Ultraviolet C (UVC)
UVC has the shortest wavelengths (100–280 nanometers) and is the most energetic and harmful. It is almost completely absorbed by the atmosphere and ozone layer, meaning virtually none reaches the surface.
Ultraviolet B (UVB)
UVB has medium wavelengths (280–315 nanometers) and is mostly absorbed by the ozone layer, though a small percentage still reaches the ground. UVB is the primary cause of sunburn and is capable of damaging DNA in skin cells.
Ultraviolet A (UVA)
UVA has the longest wavelengths (315–400 nanometers) and is the least energetic. It is not significantly filtered by the ozone layer and accounts for approximately 95% of the UV radiation that reaches the Earth’s surface. Although less intense than UVB, UVA penetrates deeper into the skin, contributing to premature aging and skin cancer development.
Effects on Living Organisms
The residual UVA and UVB radiation that penetrates the atmosphere affects living organisms both beneficially and negatively. Exposure to UVB radiation is necessary for triggering the synthesis of Vitamin D in the skin, which is important for bone health and immune function. This beneficial effect requires only brief periods of sun exposure.
Negative impacts are associated with overexposure to both UVA and UVB. Both types of radiation can cause direct and indirect damage to DNA within cells. UVB is linked to mutations, while UVA contributes to damage through the generation of reactive oxygen species. Chronic exposure can also suppress the immune system, cause premature skin aging, and increase the risk of eye conditions like cataracts.