Ozone (\(\text{O}_3\)) is a gas molecule consisting of three oxygen atoms bonded together. The molecule is chemically identical regardless of where it exists in the atmosphere. The difference between “good” and “bad” ozone is entirely a matter of altitude, determined by the atmospheric layer in which it resides. The same compound that is necessary for life high above the Earth becomes a harmful pollutant when it forms at ground level.
Stratospheric Ozone: The Earth’s Natural Sunscreen
The “good” ozone is concentrated in the stratosphere, extending from roughly 10 to 50 kilometers above the surface. This region contains the ozone layer, where about 90% of atmospheric ozone naturally resides. Ozone is continuously created and destroyed here in a natural cycle driven by solar radiation and oxygen molecules.
Ozone formation begins when high-energy ultraviolet-C (UV-C) radiation strikes an oxygen molecule (\(\text{O}_2\)). This energy causes the molecule to split apart, creating two separate, highly reactive oxygen atoms (\(\text{O}\)). Each free oxygen atom then collides with an intact \(\text{O}_2\) molecule, bonding to form the ozone molecule (\(\text{O}_3\)).
The primary function of stratospheric ozone is to act as a planetary sunscreen. It effectively absorbs almost all UV-C radiation and a significant portion of harmful ultraviolet-B (UV-B) radiation. This absorption protects all life on Earth from radiation that would otherwise cause severe genetic damage. When an \(\text{O}_3\) molecule absorbs a UV-B photon, it breaks down into \(\text{O}_2\) and \(\text{O}\), converting the harmful radiation into harmless heat.
Tropospheric Ozone: A Harmful Secondary Pollutant
In contrast, the ozone found in the troposphere, the lowest layer of the atmosphere up to about 10 kilometers, is considered “bad.” This ground-level ozone is a major component of photochemical smog and a significant air quality concern. Unlike stratospheric ozone, this ozone is primarily a pollutant created indirectly by human activities.
Ground-level ozone is not directly emitted from pollution sources; rather, it is a secondary pollutant. It forms through complex photochemical reactions involving precursor chemicals: nitrogen oxides (\(\text{NO}_{\text{x}}\)) and volatile organic compounds (VOCs). These precursors are released by sources such as industrial facilities, power plants, and motor vehicle exhaust.
The chemical reaction requires intense sunlight, which provides the energy needed to drive the process. This explains why ground-level ozone pollution peaks on hot, sunny days, especially in urban areas. As a powerful oxidant, tropospheric ozone is toxic to living tissues. In humans, it irritates and damages the lining of the lungs, exacerbates conditions like asthma, and reduces overall lung function. This ozone also damages plant life, reducing agricultural crop yields and harming forest ecosystems.
The Consequences of Imbalance: Depletion Versus Concentration
The two distinct ozone problems center on a reversal of concentration: too little ozone in the stratosphere and too much in the troposphere. The stratospheric imbalance is characterized by depletion, famously manifested as the “ozone hole.” This severe thinning occurs seasonally over the Antarctic, facilitated by unique meteorological conditions that accelerate ozone destruction.
The depletion is caused by human-made chemicals, notably chlorofluorocarbons (CFCs), once common in refrigerants and aerosol propellants. When these stable chemicals reach the stratosphere, UV radiation breaks them down, releasing chlorine and bromine atoms. These atoms catalytically destroy ozone molecules faster than they can naturally reform. The consequence of this thinning is that increased levels of harmful UV-B radiation reach the surface, raising the risk of skin cancer, cataracts, and immune system suppression. International action, like the 1987 Montreal Protocol, has successfully phased out these substances, setting the ozone layer on a path toward gradual recovery.
Conversely, the tropospheric imbalance is marked by a harmful concentration of ozone at ground level. This excess ozone forms the toxic smog that plagues many populated regions. Managing this pollution is challenging because the precursor chemicals can be transported long distances by wind before they react to form ozone.
The excess ground-level ozone also acts as a greenhouse gas, absorbing outgoing infrared radiation and contributing to the warming of the Earth’s surface. This links the local air quality issue directly to global climate change, creating a double environmental burden. The problem is complicated because weather conditions intensified by climate change, specifically hotter temperatures, accelerate the photochemical reactions that produce more ground-level ozone.