Core Concepts
In this article, you will learn the basic introductory concepts necessary to understand the role of ozone in atmospheric systems. You will learn the fundamental chemical reactions that create and destroy this atmospheric constituent. You will also learn the health and environmental impacts of ozone.
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Ozone
Ozone is a molecule made up of three oxygen atoms bonded in a bent geometry. There are two different kinds of ozone present in the atmosphere: stratospheric ozone and tropospheric ozone. These two types of ozone have different effects on the atmosphere and undergo different reactions.
Stratospheric Ozone
Stratospheric ozone is a thin layer of ozone gas, between 15 and 35 km above the Earth’s surface, that surrounds the Earth. Stratospheric ozone helps regulate the Earth’s climate by filtering UV light from the Sun.
UV filtering is done in the ozone layer through a photodissociation mechanism:
O3 + hv → O2 + O⦁
O⦁ + O2 → O3
In this mechanism, UV photons dissociate into an oxygen molecule (O2) and an oxygen radical (O⦁). When this reaction runs in reverse, an oxygen molecule combines with an oxygen radical to create a molecule of ozone, plus some heat. Thus, ozone converts UV photons into heat in the stratosphere. This extra heat causes an increase in temperature with an increase in altitude. This inversion in the stratosphere traps molecules in the troposphere.
Ozone can also be consumed in a different radical reaction, recombination:
O⦁ + O3 → 2O2
This reaction occurs slowly due to low temperatures and low molecule density in the stratosphere. However, there are trace chemical compounds (free radicals) that act as catalysts for this reaction, exponentially speeding the progression of the reaction. The constant cyclical nature of photodissociation and recombination results in the relative constant thickness of the ozone layer. This layer serves as protection to the earth from UV radiation. Depletion of the ozone layer most commonly occurs when chlorine or bromine radicals come into contact with ozone in the stratosphere. These chlorine and bromine atoms generally come from compounds that disperse due to UV exposure in the stratosphere. As these compounds disperse, the free chlorine and bromine atoms act as catalysts for the recombination reaction as seen below.
Cl⦁ + O3 → ClO + O2
ClO + O⦁ → Cl⦁ +O2
As can be seen above, this is a cyclical chain reaction; there is always a chlorine radical as a product. In this way, a single chlorine atom can destroy tens of thousands of ozone molecules.
Compounds with chlorine were commonly used in the 1960s and 70s until it was discovered that these compounds, including chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), release chlorine and bromine radicals into the atmosphere due to the effects of radiation. CFCs are highly stable molecules that are almost exclusively broken down by UV radiation; consequently, the chlorine radicals are produced higher in the atmosphere, allowing the reaction with ozone. The reactions involving CFCs and HCFCs in the atmosphere can be seen below.
The reaction below demonstrates the ability of a common compound used as a refrigerant (Freon-12: CCl2F2) to destroy ozone molecules.
CCl2F2 + hv → CClF2 + Cl⦁
Cl⦁ + O3 → ClO⦁ + O2
ClO⦁ + O⦁ → Cl⦁ + O2
Since discovering the damaging effects of CFCs and HCFCs, the Montreal Protocol was signed in 1987; effectively banning the use of CFCs and HCFCs and the ozone layer has begun naturally repairing itself. The image below demonstrates the changes in the ozone layer over the last 50 years and the predicted changes in the next 40 years. The hole in the ozone layer is caused by the meteorological conditions near Antarctica. The low temperatures near Antarctica are responsible for the formation of clouds that cause chemical changes, which promote the production of chemically active chlorine and bromine.
Tropospheric Ozone
Tropospheric ozone on the other hand is categorized as a greenhouse gas and powerful air pollutant. Tropospheric ozone is created when a reaction occurs with volatile organic compounds (VOCs) and nitrogen oxides (NOx). These organic compounds and nitrogen oxides are typically produced via combustion sources like large manufacturers and power plants. See the formulation of ozone from nitrogen oxide below.
NO2 + O2 + hv → NO + O3
Ozone Problems
Stratospheric ozone depletion occurs when more ozone particles are being destroyed than created. As ozone is destroyed, the layer of the atmosphere that filters UV radiation is thinned, allowing for more UV radiation to reach the Earth. When this layer of ozone is depleted, the Earth is exposed to a level of radiation that cannot be sustained by ecosystems on Earth. An increase in UV radiation leads to a greater risk of cataracts and skin cancer among many other complications. UV radiation can also negatively affect terrestrial and aquatic environments.
Tropospheric ozone is most likely to be at a high concentration in urban areas due to increased emissions of VOCs and NOx. It is additionally more likely in areas of high heat due to the catalyzing effect of higher temperatures on reactions. Tropospheric ozone can sometimes cause or worsen existing health concerns and damage crop growth. It can act as a greenhouse gas in the troposphere by absorbing heat from the Earth’s surface and containing it in the troposphere. Tropospheric ozone is damaging to crops as it enters plants through the stomata and oxidizes during respiration, effectively burning the plant. This form of ozone also plays a major role in the formation of photochemical smog (as seen in LA below).
Conclusion
Ozone is a naturally occurring compound that serves many different purposes in the atmosphere. Stratospheric ozone is beneficial and protects the Earth from UV radiation, reducing the risk of radiation symptoms to both humans and ecosystems. Tropospheric ozone is a polluting greenhouse gas that is formed when NOx and VOCs are exposed to sunlight and release compounds that react with existing molecules in the air. Chlorine and bromine radicals created through the dissociation of CFCs and HCFCs react with ozone in the stratosphere and destroy the molecules in a cyclical chain reaction, resulting in the depletion of the ozone layer.