Question

Explain why ozone destruction via the reaction of ${\mathrm{O}}_3$ with atomic oxygen does not occur to a significant effect in the lower stratosphere.

Short answer

The lower concentration of atomic oxygen and unfavorable conditions in the lower stratosphere limit the significance of ozone destruction via ${\mathrm{O}}_3$ and $\mathrm{O}$ reactions.

Step-by-step solution

Understand the Reaction Context

In the lower stratosphere, ozone (${\mathrm{O}}_3$) reacts with atomic oxygen ($\mathrm{O}$) in a process that could lead to ozone destruction. However, this reaction does not occur to a significant degree in this part of the atmosphere.

Examine the Concentration Factor

The concentration of atomic oxygen ($\mathrm{O}$) in the lower stratosphere is low compared to other layers of the atmosphere, particularly the mesosphere, where atomic oxygen is more abundant due to higher energy UV radiation breaking down ${\mathrm{O}}_2$. The scarcity of atomic oxygen in the lower stratosphere means there are fewer reactant molecules to participate in the destruction of ozone, lessening the impact of the reaction.

Consider Temperature and Pressure Conditions

The lower stratosphere has relatively lower temperatures and higher pressures compared to the upper layers of the atmosphere. These conditions are not as favorable for the reaction between ozone and atomic oxygen, further limiting any potential ozone destruction by this mechanism.

Explore Competing Reactions

Other reactions, such as those involving chlorofluorocarbons (CFCs) and nitrogen oxides, play a more significant role in ozone depletion in the lower stratosphere. These reactions are catalyzed by substances that are more prevalent and effective in this region, overshadowing the less effective reaction between ${\mathrm{O}}_3$ and $\mathrm{O}$.

Key concepts

Atomic Oxygen Concentration

In the lower stratosphere, the concentration of atomic oxygen ($\mathrm{O}$) is significantly lower compared to other atmospheric layers. This is primarily because the lower stratosphere is less exposed to high-energy UV radiation, which is responsible for breaking down molecular oxygen (${\mathrm{O}}_2$) into atomic oxygen.

• In the mesosphere, there is more UV radiation, leading to a higher presence of atomic oxygen.
• In contrast, the lower stratosphere lacks sufficient UV radiation to produce large amounts of atomic oxygen.
• This low concentration means fewer atomic oxygen molecules are available to react with ozone (${\mathrm{O}}_3$), resulting in minimal ozone destruction from this reaction.

Despite the possibility of ozone destruction through reaction with atomic oxygen, it is not significant in the lower stratosphere due to the lack of reactant molecules.

Temperature and Pressure Effects

Temperature and pressure are crucial factors influencing chemical reactions in the atmosphere. In the lower stratosphere, these conditions are not suitable for the reaction between ozone (${\mathrm{O}}_3$) and atomic oxygen ($\mathrm{O}$).

• Lower temperatures generally slow down chemical reactions, while higher temperatures can speed them up.
• The lower stratosphere is cooler compared to the upper layers, resulting in a less favorable environment for fast reactions.
• Moreover, higher atmospheric pressure here makes it difficult for reactants to collide effectively, further inhibiting the reaction.

This combination of lower temperature and higher pressure means that the kinetics of ozone destruction by atomic oxygen are not favorable in the lower stratosphere.

Competing Ozone Depletion Reactions

In the lower stratosphere, other chemical reactions are more prominent and effective in leading to ozone depletion. These competing reactions overshadow the relatively minor destruction that could be caused by atomic oxygen.

• Reactions involving chlorofluorocarbons (CFCs) release chlorine atoms, which are highly effective at breaking down ozone.
• Nitrogen oxides also contribute significantly to ozone depletion through a series of catalytic reactions.
• These substances and their ensuing reactions are more prevalent due to industrial and natural processes, making them the primary culprits in ozone layer depletion in the lower stratosphere.

Thus, even if ozone could potentially be destroyed by atomic oxygen, the effect is minor compared to these other powerful reactions.