Ozone (\(\text{O}_3\)) is a molecule consisting of three oxygen atoms bonded together, a chemical structure that makes it highly reactive. Depending on its location in the Earth’s atmosphere, ozone can be either a protective shield or a harmful pollutant. The vast majority of ozone resides high above the surface in the stratosphere, where it performs an indispensable function for life on Earth. However, when this molecule forms near the ground, it becomes a toxic component of air pollution.
Stratospheric Ozone: Earth’s Natural Sunscreen
Stratospheric ozone is often called “good ozone” because of the protective role it plays for all biological systems. This ozone layer exists primarily within the stratosphere, roughly 10 to 30 miles (15 to 50 kilometers) above the Earth’s surface, containing about 90% of all atmospheric ozone.
The formation of this naturally occurring ozone is a continuous cycle driven by solar radiation. High-energy ultraviolet (UV) radiation from the sun, specifically UV-C, strikes ordinary oxygen molecules (\(\text{O}_2\)) and splits them into two separate, single oxygen atoms (\(\text{O}\)). These highly reactive single oxygen atoms then quickly combine with intact \(\text{O}_2\) molecules to form the three-atom ozone molecule (\(\text{O}_3\)).
The primary function of this stratospheric ozone is to act as an atmospheric filter against incoming solar radiation. It absorbs the sun’s most damaging ultraviolet wavelengths, particularly UV-B and all UV-C radiation, preventing them from reaching the planet’s surface. By absorbing UV-B, the ozone molecule itself breaks apart, but it quickly reforms, maintaining a steady concentration that shields the planet. This absorption process also generates heat, which helps maintain the temperature structure of the atmosphere.
Ground-Level Ozone: A Secondary Pollutant
In contrast to the naturally formed stratospheric layer, ground-level ozone is considered “bad ozone” because it is a harmful air pollutant that forms near the Earth’s surface in the troposphere. Unlike other pollutants, this ozone is not emitted directly from any source, which classifies it as a secondary pollutant. Instead, it is created through a complex series of chemical reactions involving other precursor pollutants.
The two main ingredients necessary for the formation of ground-level ozone are nitrogen oxides (\(\text{NO}_{\text{x}}\)) and volatile organic compounds (VOCs). \(\text{NO}_{\text{x}}\) are produced from the combustion of fossil fuels in motor vehicles, power plants, and industrial boilers. VOCs come from sources like gasoline vapors, chemical plants, solvents, and natural biogenic sources.
These precursor chemicals react together through a photochemical process that requires intense sunlight and warmer temperatures to accelerate the reaction. The chemical reaction is most active during the summer months and in urban areas where precursor emissions are highest, leading to the formation of ozone that contributes significantly to smog. Because the chemical process takes time, the ozone can form downwind from the initial source of the \(\text{NO}_{\text{x}}\) and VOCs, sometimes affecting rural communities far from urban centers.
Differing Effects on Human and Environmental Health
The presence of stratospheric ozone is directly linked to the reduced risk of several health issues in humans. It lowers the global incidence of skin cancers, cataracts, and immune system suppression.
This protective shield also extends its benefits to the environment, preventing damage to terrestrial and aquatic ecosystems. Without the stratospheric layer, the intense UV radiation would significantly impair the growth and productivity of various crops and marine organisms. The continued repair of this layer is an ongoing effort that protects global health and food security.
Conversely, ground-level ozone is a highly reactive gas that irritates and damages living tissue upon inhalation. As a strong oxidant, it can penetrate deep into the lungs due to its low solubility, leading to inflammation and a reduction in lung function. Exposure to unhealthy concentrations can trigger asthma attacks, aggravate bronchitis, and increase the risk of premature mortality, especially in people with pre-existing respiratory and heart conditions.
Beyond human health, this harmful ozone also negatively impacts the broader environment as it reacts with the plant surfaces. Elevated concentrations can damage the leaves of sensitive vegetation, leading to reduced growth, lower crop yields, and compromised forest health. This makes ground-level ozone a dual threat, affecting both public health and the economic productivity of agriculture.