This blog post examines the dual nature of ozone: how it shields life by blocking ultraviolet rays in the upper atmosphere, yet harms plants and humans at ground level.
Ozone (O₃) is formed when oxygen atoms (O) combine with oxygen molecules (O₂). It is a highly toxic substance, potent enough to be used as a disinfectant when diluted. In the surface atmosphere, it is known to damage plant chloroplasts and human lung tissue. Conversely, ozone has the property of absorbing ultraviolet radiation harmful to life, playing a protective role for Earth’s life forms by blocking UV rays in the upper atmosphere.
At ground level, ozone is produced when nitrogen oxides undergo chemical reactions upon exposure to strong sunlight. Nitrogen oxides are emitted during fuel combustion, primarily in the form of nitric oxide (NO) and nitrogen dioxide (NO₂). Like ozone, nitrogen oxides are chemically highly unstable and convert to the more stable nitrogen dioxide by combining with oxygen atoms. When exposed to sunlight, nitrogen dioxide breaks down again into nitrogen oxides and oxygen atoms. These oxygen atoms then combine with oxygen molecules to form ozone. Hydrocarbons act as catalysts in this ozone formation process.
Ozone in the upper atmosphere is primarily formed in the lower stratosphere at low latitudes. Oxygen molecules absorb ultraviolet radiation and break down into oxygen atoms. These broken-down oxygen atoms then bond with other oxygen molecules to form ozone. Nitrogen molecules or oxygen molecules also act as catalysts in this process. The stratosphere is the atmospheric layer extending from the lowermost troposphere up to approximately 50 km in altitude. Unlike the troposphere, where vertical air circulation is active, the stratosphere experiences increasing temperatures with altitude, preventing convection. The stratospheric temperature is determined proportionally by the amount of ultraviolet radiation absorbed by ozone. Most ozone is concentrated in the lowest layer of the stratosphere, known as the ozone layer.
Ozone layer depletion is influenced by nitrogen oxides emitted during aircraft operations and nuclear testing, but it is primarily caused by freon gases (CF₂Cl₂ or CFCl₃), which are classified as major greenhouse gases alongside carbon dioxide. Developed and used since the late 1920s, CFCs are highly stable and do not decompose when exposed to sunlight in the troposphere. This allows them to spread throughout the Earth’s atmosphere over long periods via atmospheric circulation. Freon gases only decompose when exposed to ultraviolet radiation in the stratosphere, releasing chlorine atoms (Cl). These chlorine atoms chemically react with ozone to form chlorine monoxide (ClO), which then reacts with oxygen atoms to revert back to chlorine atoms. This cycle repeats, leading to ozone depletion.
During winter, when sunlight is very weak, a massive circular vortex forms in the lower stratosphere above Antarctica, driven by strong rotating winds. Air containing freon gases and water vapor is drawn into this vortex from lower latitudes via the global atmospheric circulation. The water vapor within this incoming air transforms into ice crystals, trapping the freon gases within them. This process repeats, causing ice crystals containing freon gas to accumulate continuously within the vortex throughout winter. When sunlight reaches this region in spring, the vortex weakens and dissipates. As the ice crystals melt, chlorine atoms rapidly released from the trapped freon gas intensively destroy ozone. The reason Antarctic ozone depletion did not appear for half a century after CFC development was that it took an extremely long time for CFCs to be transported to and accumulate above Antarctica.
Meanwhile, the Arctic vortex is not as strong as the Antarctic one, resulting in a more convoluted shape. There is significant mixing between the air inside the vortex and the surrounding air, and it does not persist as long. Consequently, ozone depletion is less severe in the Arctic than in the Antarctic. However, as global warming progresses, stratospheric temperatures are projected to decrease, potentially strengthening and enlarging both the Antarctic and Arctic vortices. This is because while increased greenhouse gas concentrations in the atmosphere cause temperature rises in the troposphere, their unique thermal structure in the stratosphere actually tends to cool it. Therefore, if changes in the intensity of polar vortices do occur alongside global warming, the pattern of ozone layer depletion will likely differ significantly from what we see today.
