It means that the air near the Earth's surface is denser compared to the other parts.
Air Density
99.9% of the air particles are found in the lower part of the atmosphere because the air in this region is denser than the other regions.
Air density increases with increased pressure and as we move up in altitude, air pressure decreases.
Thus, as air pressure decreases with height, the density of air also decreases. This is why most of the air particles are found in the lower part of the atmosphere.
More on air density can be found here: https://brainly.com/question/23294784?referrer=searchResults
