The formation and dissociation of stratospheric ozone (O₃) are crucial processes in the Earth’s atmosphere that protect life on Earth by absorbing harmful ultraviolet (UV) radiation from the Sun.
The stratospheric ozone layer is located approximately 10 to 50 kilometers above the Earth’s surface and plays a vital role in maintaining the planet’s climate and protecting living organisms from excessive UV radiation.
Formation of Stratospheric Ozone
The formation of stratospheric ozone occurs through a series of photochemical reactions involving oxygen molecules (O₂) and UV light from the Sun. The process can be divided into three main steps:
1. Photodissociation of Oxygen Molecules
- Reaction:
[ \text{O}_2 + \text{UV-C} \rightarrow 2\text{O} ] Ultraviolet light with wavelengths less than 242 nm (UV-C) strikes an oxygen molecule (O₂), causing it to dissociate into two individual oxygen atoms (O). - Explanation: The high-energy UV-C radiation breaks the O₂ molecule into two oxygen atoms, which are highly reactive.
2. Formation of Ozone Molecules
- Reaction:
[ \text{O} + \text{O}_2 \rightarrow \text{O}_3 ] An individual oxygen atom (O) reacts with an oxygen molecule (O₂) to form an ozone molecule (O₃). - Explanation: The oxygen atom generated in the first step reacts with an oxygen molecule to create ozone. This reaction is facilitated by the presence of a third molecule, often referred to as a “collision partner,” which absorbs the excess energy released during the reaction.
3. Ozone Formation in the Stratosphere
- Reaction:
[ \text{O} + \text{O}_2 \rightarrow \text{O}_3 ] The ozone molecules formed are distributed throughout the stratosphere, contributing to the formation of the ozone layer.
Dissociation of Stratospheric Ozone
The dissociation of stratospheric ozone involves the breakdown of ozone molecules into oxygen molecules and atoms. This process is essential for regulating the concentration of ozone in the stratosphere and is influenced by UV radiation.
1. Photodissociation of Ozone
- Reaction:
[ \text{O}_3 + \text{UV-C} \rightarrow \text{O}_2 + \text{O} ] Ultraviolet light with wavelengths less than 330 nm (UV-C) strikes an ozone molecule (O₃), causing it to dissociate into an oxygen molecule (O₂) and an oxygen atom (O). - Explanation: The high-energy UV-C radiation breaks down the ozone molecule, leading to the formation of oxygen atoms and molecules. This process is part of the natural ozone-oxygen cycle, which maintains the balance of ozone in the stratosphere.
2. Ozone-oxygen Cycle
- Cycle:
- Formation: [ \text{O}_2 + \text{UV-C} \rightarrow 2\text{O} ]
- Ozone Formation: [ \text{O} + \text{O}_2 \rightarrow \text{O}_3 ]
- Ozone Dissociation: [ \text{O}_3 + \text{UV-C} \rightarrow \text{O}_2 + \text{O} ]
- Ozone Destruction: [ \text{O} + \text{O}_2 \rightarrow \text{O}_3 ]
- Explanation: The ozone-oxygen cycle describes the continuous formation and destruction of ozone in the stratosphere. The balance between these processes determines the concentration of ozone in the ozone layer.
Summary
- Formation: Stratospheric ozone is formed when UV-C radiation dissociates O₂ molecules into individual oxygen atoms, which then react with other O₂ molecules to form ozone (O₃).
- Dissociation: Ozone molecules are broken down by UV-C radiation into oxygen molecules and atoms, which then participate in the ozone-oxygen cycle to maintain a balance of ozone in the stratosphere.
These processes are essential for protecting the Earth from excessive UV radiation and maintaining a stable climate.