Question

What is meant by the expression photochemically dissociated as applied to stratospheric \(\mathrm{O}_{2}\) ?

Short answer

Photochemical dissociation of stratospheric \(\mathrm{O}_{2}\) involves breaking \(\mathrm{O}_{2}\) molecules into individual atoms using UV light.

Step-by-step solution

Understand the Context

Stratospheric \(\mathrm{O}_{2}\) refers to the molecular oxygen found in the stratosphere, a layer of the Earth's atmosphere. In this context, 'photochemically dissociated' is a term from atmospheric chemistry.

Define Photochemical Dissociation

Photochemical dissociation is a process where molecules are broken down into smaller parts due to the absorption of light. This occurs when high-energy photons, typically in the ultraviolet (UV) spectrum, provide enough energy to break chemical bonds.

Apply Photochemical Dissociation to \(\mathrm{O}_{2}\) in the Stratosphere

In the stratosphere, \(\mathrm{O}_{2}\) can absorb UV light from the sun, causing the oxygen molecule to split into two individual oxygen atoms. This process is crucial for forming ozone \(\mathrm{O}_3\) as these free oxygen atoms react with other \(\mathrm{O}_{2}\) molecules.

Key concepts

Stratospheric Oxygen

Stratospheric oxygen, denoted as \( \mathrm{O}_{2} \), refers to the oxygen molecules found within the stratosphere, which is the second layer of Earth's atmosphere. The stratosphere lies above the troposphere and extends up to about 50 kilometers above the Earth's surface.
A notable aspect of the stratosphere is its stability and lack of weather-related turbulence, stemming from the fact that the layer's temperature increases with altitude due to the absorption of ultraviolet (UV) radiation by ozone and other gases.
Understanding stratospheric oxygen is crucial because it is directly involved in key atmospheric processes, particularly those involving sunlight. Oxygen in the stratosphere is subject to interactions with solar energy, which plays a pivotal role in atmospheric chemistry.

• Stratospheric oxygen is continuously involved in complex reactions influenced by solar radiation.
• The condition of the stratospheric oxygen has significant implications for ozone production and atmospheric balance.

Ozone Formation

Ozone formation is a critical process occurring in the stratosphere, where molecular oxygen \( \mathrm{O}_{2} \) undergoes photochemical reactions to form ozone \( \mathrm{O}_{3} \).
The process is initiated when high-energy UV light from the sun hits \( \mathrm{O}_{2} \) molecules, splitting them into two separate oxygen atoms. This dissociation is made possible by the absorption of UV photons, which contain sufficient energy to break the bonds in an \( \mathrm{O}_{2} \) molecule.
Once the oxygen atoms are free, they can react with other \( \mathrm{O}_{2} \) molecules to form ozone. This ozone formation process is part of the ozone-oxygen cycle.

• Free oxygen atoms combine with \( \mathrm{O}_{2} \) molecules, forming \( \mathrm{O}_{3} \).
• Ozone acts as a critical shield, protecting life on Earth by absorbing harmful UV radiation.

This cycle continues, ensuring that the concentration of ozone in the stratosphere remains relatively stable, maintaining an essential balance in protecting the biosphere.

Atmospheric Chemistry

Atmospheric chemistry is the study of the chemical composition and transformations within Earth's atmosphere. It encompasses a range of processes that occur due to the interaction of gases with solar radiation and with each other. These processes include photochemical reactions like those leading to ozone formation.
Atmospheric chemistry in the stratosphere is particularly important because it deals with the formation and decomposition of ozone, an essential gas for life on Earth. Ozone absorbs a significant amount of ultraviolet (UV) radiation from the sun, which prevents these harmful rays from reaching the surface.
The balance and dynamics of atmospheric chemistry involve:

• The transformation of gases due to sunlight exposure, also known as photochemical reactions.
• Interplay between different atmospheric layers, influencing weather and climate.

Atmospheric chemistry ensures Earth's climate stability and supports the protection of living organisms by minimizing the exposure to dangerous UV radiation through the maintenance of the ozone layer.