Where are most gasses in the atmosphere found?

Most gases in the atmosphere are found in the troposphere, which is the lowest layer of Earth's atmosphere, extending from the surface up to about 8 to 15 kilometers (5 to 9 miles) high. This layer contains roughly 75% of the atmosphere's mass and is where weather phenomena occur. Key gases such as nitrogen (78%) and oxygen (21%) are most concentrated in this region. Higher layers, like the stratosphere and mesosphere, have much lower densities of gases.