Question

If an average \(O_{3}\) molecule "lives" only 100-200 seconds in the stratosphere before undergoing dissociation, how can \(\mathrm{O}_{3}\) offer any protection from ultraviolet radiation?

Short answer

Although ozone molecules in the stratosphere have a short lifespan of 100-200 seconds, they offer protection against ultraviolet radiation through a continuous cycle of formation and dissociation. Ozone (O3) forms by reacting with oxygen (O2) molecules and UV radiation, and when it dissociates, it absorbs harmful UV photons in the process. This constant formation and dissociation of ozone ensure that it continuously filters out and prevents harmful UV radiation from reaching Earth's surface, providing essential protection to life forms.

Step-by-step solution

Understanding Ozone Formation

Ozone (O3) is formed in the stratosphere primarily through the interaction of ultraviolet (UV) radiation with diatomic oxygen (O2) molecules. UV radiation splits O2 into two separate oxygen atoms (O), which can then react with other O2 molecules to form O3. This process of ozone formation can be represented by the following reactions: 1. Photodissociation of O2: \[O_{2} + \text{UV radiation} \longrightarrow 2 O\] 2. Formation of O3: \[O + O_{2} \longrightarrow O_{3}\] These reactions ensure that as long as there are sufficient O2 molecules and UV radiation present, new ozone molecules will be continually formed.

Understanding Ozone Dissociation

Ozone molecules in the stratosphere have relatively short lifespans, lasting only around 100-200 seconds before dissociating. The dissociation of ozone occurs through the absorption of UV radiation and can be represented by the following reactions: 1. Photodissociation of O3: \[O_{3} + \text{UV radiation} \longrightarrow O_{2} + O\] 2. Recombination of O: \[O + O_{2} \longrightarrow O_{3}\] The dissociation of ozone molecules also has the effect of "consuming" harmful UV radiation to protect Earth's surface from its damaging effects.

Understanding the Absorption of Ultraviolet Radiation by Ozone

Ozone in the stratosphere absorbs significant amounts of incoming ultraviolet radiation, preventing it from penetrating deeper into the Earth's atmosphere and causing harm to life forms. This absorptive capacity of ozone is thanks to its chemical structure, which allows it to absorb UV photons with a wavelength between 240-290 nm. When an ozone molecule absorbs a photon, it undergoes photodissociation, breaking apart into an O2 molecule and a single oxygen atom, as described in Step 2.

Conclusion

Despite the relatively short lifespan of ozone molecules in the stratosphere, they offer critical protection against ultraviolet radiation. Ozone is continually formed through the interaction of UV radiation with O2, and its dissociation acts as a natural barrier that "consumes" harmful UV rays. This cyclical process ensures that ozone constantly absorbs and filters out harmful UV radiation, offering lasting protection to the Earth's surface and life forms.

Key concepts

Ozone Formation

Ozone formation is a key atmospheric process essential for protecting life on Earth from the harmful effects of ultraviolet (UV) radiation. It occurs high up in the stratosphere, the second layer of the atmosphere that extends from about 10 to 50 kilometers above the Earth's surface.

Ozone, or \( O_{3} \), is created when oxygen molecules, or \( O_{2} \), interact with UV radiation. The energy from this radiation causes the oxygen molecules to split into two separate oxygen atoms, a process known as photodissociation. These highly reactive oxygen atoms (\text{O})) then collide and combine with other oxygen molecules \( O_{2} \) to form ozone \( O_{3} \).

Despite the transient nature of individual ozone molecules, the ongoing interaction between UV radiation and \( O_{2} \) ensures a steady regeneration of the ozone layer, creating a dynamic balance that sustains the protective shield.

Photodissociation

Photodissociation is the process by which a chemical bond is broken by the absorption of a photon, meaning a particle of light. This phenomenon is crucial in the formation and breakdown of ozone in the atmosphere.

In the stratosphere, photodissociation begins when UV radiation hits an \( O_{2} \) molecule. The energy absorbed from UV light splits \( O_{2} \) into two separate oxygen atoms:\[O_{2} + \text{UV radiation} \longrightarrow 2 O\]

These free oxygen atoms are highly reactive and can quickly react with another \( O_{2} \) molecule to create ozone, thus continuing the ozone formation cycle. When ozone absorbs UV radiation, it also undergoes photodissociation, splitting into an \( O_{2} \) molecule and a free oxygen atom, and effectively absorbing harmful UV rays in the process.

Ultraviolet Radiation Absorption

Ultraviolet (UV) radiation is a type of electromagnetic radiation emitted by the Sun, and it's known for its potential to damage living organisms. The Earth’s atmosphere, specifically the ozone layer, plays a vital role in absorbing this radiation and protecting life on the planet.

Ozone is exceptionally good at absorbing UV rays, especially those in the UV-B range, which have wavelengths between 240 and 290 nanometers. When an ozone molecule absorbs UV-B radiation, it undergoes photodissociation and momentarily splits into an \( O_{2} \) molecule and an individual oxygen atom. This absorption process shields the Earth’s surface from a significant portion of the UV radiation that would otherwise be harmful to DNA and other biological molecules.

The ozone layer's ability to continually absorb UV radiation is due to the constantly occurring process of photodissociation and reformation of ozone, making it an effective UV filter.

Ozone Layer Protection

The ozone layer's role in safeguarding life on Earth cannot be overstated. It acts as a natural sunscreen, absorbing and blocking the majority of the sun's harmful ultraviolet radiation.

Various human activities, particularly the release of chlorofluorocarbons (CFCs) and other ozone-depleting substances, have led to the thinning of the ozone layer, a phenomenon famously known as the 'ozone hole.' Recognizing the danger, the global community has taken steps to reduce and phase out the use of these harmful chemicals through international agreements like the Montreal Protocol.

Protecting and restoring the ozone layer is not only about reducing ozone-depleting substances but also involves understanding and mitigating the impacts of climate change, as stratospheric temperature changes can affect ozone chemistry. Continuous scientific monitoring and research are crucial to ensure that the ozone layer remains intact, safeguarding the planet for future generations.