It is known that the atmosphere is divided into some layers. In which one among the following layers, is the percentage composition of Helium gas maximum?

The Earth's atmosphere is divided into the homosphere and the heterosphere. In the heterosphere, which begins around 80 km and includes the upper thermosphere and exosphere, gases are distributed in distinct layers based on their atomic weight due to gravity [1]. Heavier gases like nitrogen and oxygen dominate the lower layers, while the lightest elements, hydrogen and helium, migrate to the outermost margins [3]. The exosphere, starting above 480 km, is the uppermost layer where the air is extremely rarefied [1]. It is primarily composed of individual atoms of hydrogen and helium that are weakly bonded by gravity [2]. While helium is a trace gas in the lower atmosphere (approximately 5 ppm), its relative percentage composition is maximum in the exosphere because heavier gases are virtually absent at these extreme altitudes [1].

Sources

1. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > Dirunion of Atmosphere > p. 6
2. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Permanent Gases of The Atmosphere > p. 271
3. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Atmospheric Escape > p. 280