Question

What is the two-step mechanism by which the hydroxyl free radical is produced in clean air?

Short answer

Hydroxyl radicals are produced through the photodissociation of ozone followed by a reaction with water vapor.

Step-by-step solution

Photodissociation of Ozone

In the first step, ozone (O_3) in the atmosphere absorbs ultraviolet (UV) light from the sun. This energy causes the ozone molecule to split into an oxygen molecule (O_2) and an oxygen atom (O). The reaction can be represented as: \[ \text{O}_3 + \text{UV light} \rightarrow \text{O}_2 + \text{O} \] This process is known as photodissociation and is crucial for the generation of hydroxyl radicals.

Reaction with Water Vapor

In the second step, the free oxygen atom (O) produced in the first step reacts with water vapor (H_2O) present in the air. This reaction forms hydroxyl radicals (OH) and molecular oxygen (O_2). The reaction can be written as: \[ \text{O} + \text{H}_2\text{O} \rightarrow 2\text{OH} \] Hydroxyl radicals are highly reactive and play an essential role in atmospheric chemistry by decomposing pollutants.

Key concepts

Photodissociation

Photodissociation is a fascinating process that occurs when ultraviolet (UV) light from the sun is absorbed by molecules, leading to the breaking of chemical bonds. In our atmosphere, this often involves ozone (\(\text{O}_3\)), which absorbs UV light and undergoes a transformation. As a result, the strong energy from the UV light causes the ozone molecule to split into an oxygen molecule (\(\text{O}_2\)) and a single oxygen atom (\(\text{O}\)). This reaction can be represented as:

• \[\text{O}_3 + \text{UV light} \rightarrow \text{O}_2 + \text{O}\]

This process is not just interesting from a scientific perspective but is also essential in creating conditions necessary for additional chemical reactions in the atmosphere.

Ozone Decomposition

Ozone decomposition is a critical component of atmospheric chemistry, particularly in the context of forming hydroxyl radicals. After the photodissociation of ozone, the single oxygen atom produced becomes highly reactive. It continues its journey by reacting with water vapor in the atmosphere. In this reaction, the free oxygen atom (\(\text{O}\)) combines with water (\(\text{H}_2\text{O}\)) to produce hydroxyl radicals (\(\text{OH}\)). The equation for this transformation is:

• \[\text{O} + \text{H}_2\text{O} \rightarrow 2\text{OH}\]

Hydroxyl radicals are vital to breaking down pollutants because they are highly reactive, leading to the decomposition of harmful substances in the air. Thus, ozone decomposition through these steps is crucial in preserving the delicate balance of atmospheric chemistry.

Atmospheric Chemistry

Atmospheric chemistry involves the study of chemical processes that take place in the atmosphere. It is through these processes that substances such as hydroxyl radicals are continuously produced and transformed. The formation of hydroxyl radicals, for instance, shows how different elements in the atmosphere interact under the influence of sunlight.
These radicals act as natural cleaners, breaking down pollutants and toxins. This considerable responsibility makes them allies in maintaining air quality. They help in the decomposition of contaminants like volatile organic compounds (VOCs) and nitrogen oxides, minimizing pollution levels. Understanding such mechanisms is fundamental to atmospheric chemistry, allowing us to gauge the impact of these reactions on air quality and climate. These studies are essential to predict how human activities may alter atmospheric balance.

Ultraviolet Light Absorption

Ultraviolet light absorption is a key factor in many atmospheric processes, serving as the driving force behind chemical reactions like photodissociation. UV light from the sun is particularly potent, with enough energy to break chemical bonds.
When molecules like ozone absorb UV light, they receive this energetic push that leads to dissociation, forming reactive species that initiate further reactions.
The ability of UV light to kickstart these reactions is crucial in understanding atmospheric dynamics. While UV light is responsible for harmful effects such as skin damage in humans, in the atmosphere, its role is indispensable as it aids in forming necessary reactive compounds. These compounds contribute to balancing natural processes and breaking down pollutants, maintaining atmospheric health.