The correct option is 2 and 3 only.
Explanation
The atmosphere is a mixture of various gases, water vapour, and dust particles. While the major gases maintain a relatively stable ratio in the lower atmosphere, the composition varies significantly with altitude.
- Statement 1 is Incorrect: The proportion of gases does not remain constant up to the highest layers. The atmosphere is divided into two broad zones based on composition:
- The Homosphere (up to approximately 80 km), where the proportion of major gases is uniform.
- The Heterosphere (above 80 km), where gases separate into distinct layers based on their molecular weight and density.
- Statement 2 is Correct: Nitrogen is the most abundant gas in the atmosphere, accounting for approximately 78.08% of the total volume.
- Statement 3 is Correct: Nitrogen (78.08%) and Oxygen (20.95%) collectively constitute roughly 99% of the total volume of clean, dry air. The remaining 1% includes Argon (0.93%), Carbon Dioxide (0.036%), and trace gases such as Neon, Helium, and Ozone.
Key Takeaway: Nitrogen and Oxygen make up 99% of the atmosphere's volume. The uniformity of this gaseous mixture is maintained only in the lower atmosphere (homosphere), whereas the upper atmosphere (heterosphere) exhibits stratification.