The Earth’s atmosphere contains a natural shield against the sun’s intense energy, known as the ozone layer. This layer is a region of elevated ozone concentration, located primarily in the stratosphere between 15 and 35 kilometers above the planet’s surface. Ozone, a molecule composed of three oxygen atoms (O3), performs the function of absorbing most of the harmful ultraviolet (UV) radiation constantly streaming from the sun. Without this protective atmospheric layer, the high-energy radiation would sterilize the Earth’s surface, making life as we know it impossible.
Categorizing Ultraviolet Radiation
Ultraviolet radiation is a segment of the electromagnetic spectrum categorized by wavelength, ranging from 100 to 400 nanometers (nm). This spectrum is subdivided into three distinct bands based on energy level and biological impact: UVA, UVB, and UVC.
The shortest and most energetic band is UVC, spanning wavelengths from 100 to 280 nm. UVC radiation is extremely damaging to life, but the ozone layer and ordinary oxygen molecules block it completely, meaning none of it reaches the Earth’s surface.
The next band, UVB, covers the range of 280 to 315 nm and is the primary cause of sunburn and DNA damage. The ozone layer absorbs most of this radiation, screening out 97% to 99% of the average frequency. Even with this high level of absorption, the small amount of UVB that does penetrate is enough to cause biological harm.
The least energetic band, UVA, has the longest wavelengths, from 315 to 400 nm. Ozone only weakly absorbs UVA, so the majority of this radiation passes through the atmosphere and accounts for approximately 95% of the total UV light reaching the ground. While less intense than UVB, UVA is a concern for long-term skin health, as it penetrates deeper into the skin layers.
The Chemical Mechanism of UV Absorption
The ozone layer’s protective action is governed by a set of natural chemical reactions known as the Chapman Cycle, or the ozone-oxygen cycle. This cycle details the constant creation and destruction of ozone molecules in the stratosphere, a process that effectively converts harmful UV energy into harmless heat.
The cycle begins when high-energy UVC photons, with wavelengths less than 242 nm, strike an ordinary oxygen molecule (O2). This intense energy causes the O2 molecule to split into two separate, highly reactive oxygen atoms (O).
Each of these free oxygen atoms then quickly collides with another O2 molecule to form a molecule of ozone (O3). This formation reaction releases energy as heat into the stratosphere, which contributes to the atmosphere’s temperature profile. Ozone molecules are then ready to absorb the less energetic, but still dangerous, UVB radiation.
When an ozone molecule absorbs a UVB photon, it undergoes photolysis, breaking apart into an O2 molecule and a single free oxygen atom (O). The energy from the UV photon is consumed in this chemical bond-breaking process.
The resulting oxygen atom can then immediately react with another O3 molecule, or combine with an O2 molecule to re-form ozone, continuing the cycle. The net result of these reactions is that penetrating UV radiation is intercepted, and its energy is dissipated as thermal energy in the upper atmosphere, preventing it from reaching the Earth’s surface.
Consequences of Reduced Ozone Protection
A reduction in the stratospheric ozone layer allows a greater amount of biologically damaging UVB radiation to reach the Earth’s surface. The primary concern for human health is a significant increase in the incidence of skin cancers, including the more common non-melanoma types and the highly lethal malignant melanoma. Increased UV exposure is also strongly linked to the development of cataracts, a clouding of the eye’s lens that impairs vision.
The immune system is also suppressed by elevated UVB levels, which can reduce the body’s ability to fight off infectious diseases. The increased radiation also affects marine ecosystems. Phytoplankton, the microscopic organisms that form the base of the aquatic food web, are particularly vulnerable to UV damage, with increased exposure impairing their growth and motility.
Terrestrial plant life is also negatively impacted, as UVB radiation can reduce the growth rates and yields of many crop species, including rice and soybeans. The radiation alters plant chemistry, which can reduce the nutritional value of food crops and disrupt the balance of plant species.