13. Which one of the following gases is present in thestratosphere at a concentration higher than itsconcentration in troposphere?
    (1) Nitrogen (2) Oxygen
    (3) Ozone (4) Carbon dioxide

Earth’s atmosphere is a complex, layered system where different gases are distributed in unique ways. While most gases, such as nitrogen, oxygen, and carbon dioxide, maintain relatively similar concentrations across the lower atmospheric layers, one gas stands out for its dramatic difference between the troposphere and stratosphere: ozone (O₃). This article explores which gas is present at a higher concentration in the stratosphere compared to the troposphere, why this is the case, and the implications for atmospheric science and life on Earth.

The Structure of Earth’s Atmosphere

The atmosphere is divided into several main layers, each with distinct characteristics:

• Troposphere: The lowest layer, extending from the surface up to about 8–15 km (5–9 miles), where weather occurs and most of the atmosphere’s mass is found.

• Stratosphere: The layer above the troposphere, extending from about 10–50 km (6–31 miles) above the surface. It contains the ozone layer.

• Mesosphere, Thermosphere, and Exosphere: Higher layers with increasingly thin air and different roles in atmospheric processes.

Gas Concentrations in the Troposphere and Stratosphere

Most gases in the atmosphere, including nitrogen, oxygen, and carbon dioxide, are well-mixed throughout the troposphere and stratosphere. Their concentrations are nearly identical in both layers:

• Nitrogen (N₂): About 78% by volume in both the troposphere and stratosphere.

• Oxygen (O₂): About 21% by volume in both layers.

• Carbon Dioxide (CO₂): Present at about 0.04% (400 parts per million) in both layers.

The main differences in gas concentrations between these layers involve ozone and water vapor:

• Ozone (O₃): Present in much higher concentrations in the stratosphere than in the troposphere. The stratosphere contains the ozone layer, where ozone concentrations can reach 2–8 parts per million, compared to much lower levels in the troposphere.

• Water Vapor (H₂O): Present in much higher concentrations in the troposphere, where it plays a key role in weather and climate, but is almost absent in the stratosphere.

Why Ozone Is Unique

Ozone’s distribution is unique because it is formed and destroyed by specific chemical reactions that are most active in the stratosphere:

• Ozone Formation: In the upper stratosphere, ultraviolet (UV) radiation splits oxygen molecules (O₂) into oxygen atoms, which then combine with other oxygen molecules to form ozone (O₃).

• Ozone Destruction: Ozone is also broken down by UV radiation and by chemical reactions involving human-made substances like chlorofluorocarbons (CFCs).

• Ozone Layer: The balance between formation and destruction results in a stable layer of ozone in the stratosphere, known as the ozone layer.

In the troposphere, ozone is present in much lower concentrations and is considered a pollutant, contributing to smog and respiratory problems.

Comparing the Options

Let’s review each option in the context of gas concentrations in the troposphere and stratosphere:

• Nitrogen: Concentration is nearly the same in both layers.

• Oxygen: Concentration is nearly the same in both layers.

• Ozone: Concentration is much higher in the stratosphere due to the presence of the ozone layer.

• Carbon Dioxide: Concentration is nearly the same in both layers.

Therefore, ozone is the only gas among the options that is present at a higher concentration in the stratosphere than in the troposphere.

The Importance of the Ozone Layer

The high concentration of ozone in the stratosphere is crucial for life on Earth:

• UV Protection: The ozone layer absorbs most of the sun’s harmful ultraviolet-B (UVB) radiation, preventing it from reaching the surface and protecting living organisms from DNA damage, skin cancer, and other health effects.

• Atmospheric Stability: The absorption of UV radiation by ozone also warms the stratosphere, creating a temperature inversion that stabilizes the atmospheric layers below.

Why Other Gases Do Not Show This Pattern

Nitrogen, oxygen, and carbon dioxide are well-mixed throughout the lower atmosphere because they are relatively inert and do not participate in strong, localized chemical reactions like ozone does. Water vapor, while important, is concentrated in the troposphere due to its sources and sinks (evaporation from the surface and condensation/precipitation).

Common Misconceptions

• All Gases Are Higher in the Stratosphere: Incorrect. Only ozone shows a significant increase in concentration in the stratosphere compared to the troposphere.

• Ozone Is a Pollutant Everywhere: Ozone is a pollutant in the troposphere but is essential and beneficial in the stratosphere.

• Carbon Dioxide or Oxygen Are Higher in the Stratosphere: Their concentrations are similar in both layers.

Summary Table

Key Takeaways

• Ozone is present at a much higher concentration in the stratosphere than in the troposphere, due to the presence of the ozone layer.

• Nitrogen, oxygen, and carbon dioxide have similar concentrations in both the troposphere and stratosphere.

• The ozone layer is essential for absorbing harmful UV radiation and protecting life on Earth.

• Water vapor is much more abundant in the troposphere than in the stratosphere.

Conclusion

Among the gases listed—nitrogen, oxygen, ozone, and carbon dioxide—ozone is the only one present in the stratosphere at a higher concentration than in the troposphere. This unique distribution is the result of specific chemical processes that create and sustain the ozone layer, which plays a vital role in protecting life on Earth from the sun’s harmful ultraviolet radiation.

Understanding the distribution and importance of ozone in the stratosphere is essential for atmospheric science, environmental protection, and public health. Protecting the ozone layer remains a global priority, as its integrity is crucial for maintaining the balance of life on our planet.