1. Overview of Atmospheric Layers: Starting from the ground and moving upward, the atmosphere consists of several distinct regions:
• Troposphere: This is the lowest layer, ranging from 0 km to approximately 12 km. It contains nearly 80% of the atmosphere's total mass and is where all weather phenomena occur.
• Stratosphere: The second major layer, extending from roughly 12 km to 50 km. Unlike the troposphere, the temperature here increases with altitude due to energy absorption.
• Mesosphere: Located above the stratosphere (50 km to 85 km), this is the region where most meteors burn up upon entry.
• Thermosphere: The outermost major layer, starting above 85 km, which contains highly ionized gases and where auroras are formed.
2. Pinpointing the Ozone Layer: The ozone layer is a specific region of high ozone ($O_3$) concentration. Approximately 90% of the Earth's atmospheric ozone is found within the
Stratosphere. Specifically, this concentration is highest between 15 km and 35 km above the Earth's surface. This stratified region is what we formally call the "Ozone Layer."
3. The Critical Protective Function: The presence of ozone in the stratosphere is vital for terrestrial life because it acts as a planetary filter. It absorbs the vast majority (about 97-99%) of the Sun's high-frequency ultraviolet radiation, specifically UV-B rays.
Without this protective stratospheric shield, harmful UV-B radiation would reach the surface unimpeded, leading to catastrophic biological consequences. These include high rates of skin cancer and cataracts in humans, significant damage to terrestrial plant DNA, and the destruction of phytoplankton, which form the base of the marine food chain.