Lower atmosphere mass distribution Because gas density decreases with altitude, about 70% of the total atmospheric mass is found in the lowest ~5 km. This lower region is known as the:

Introduction / Context:
Atmospheric mass is concentrated near Earth’s surface because pressure and density decrease rapidly with height. Understanding naming and properties of layers is essential for dispersion modelling and environmental assessments.

Given Data / Assumptions:

• Approximate statement: a significant majority of atmospheric mass is in the lowest several kilometres.
• We need the correct name for this layer.

Concept / Approach:
The lowest layer of the atmosphere is the troposphere. It extends from the surface to the tropopause (height varies with latitude and season). Most mass and almost all weather reside here due to gravitational compression and strong mixing.

Step-by-Step Solution:

Verification / Alternative check:
Standard atmospheric models attribute the majority of mass to the troposphere; pressure halves approximately every 5.5 km, demonstrating rapid mass decline with altitude.

Why Other Options Are Wrong:

• Ionosphere/Exosphere: Far above the bulk mass region.
• Stratosphere: Begins above the troposphere; contains much less mass than the troposphere.
• None of these: Incorrect because “troposphere” is correct.

Common Pitfalls:
Mixing up layer names or assuming uniform density with height; the atmosphere is highly stratified.

Final Answer:
Troposphere