Thre are no "layers" in the atmospher, it is a gradient from the surface on up. The lowest area, the troposhere, is the densest. However the bottoms of mine shafts would have even denser air and the most air molecules per unit volume.
Exoshere
Most of the gas molecules in the atmosphere are found in the lower part of the atmosphere, called the troposphere. This is the layer of the atmosphere closest to the Earth's surface, where most weather events occur and where most living organisms exist.
The troposphere is the layer of the atmosphere that contains most of the atmosphere's total mass. It is the layer where weather occurs and where most of Earth's air is found.
Exosphere
The exosphere is the outermost layer of the Earth's atmosphere and holds the fewest gas molecules and air particles. This layer extends from about 500 km above the Earth's surface to the edge of space.
The mesosphere is the layer of the atmosphere that protects the Earth's surface from most meteoroids. Meteoroids burn up in this layer due to the high temperature caused by the friction with the air molecules.
Exosphere
Exosphere
The ozone layer is the layer that is found at the highest level of the atmosphere. is the layer found in the stratosphere protecting us against harmful solar radiations like Ultraviolet Radiations.
fluorocarbons affect the air quality by destroying the air molecules in the ozone layer
Air molecules spread out and heat up primarily in the troposphere, which is the lowest layer of the Earth's atmosphere. This layer extends from the surface up to about 8 to 15 kilometers (5 to 9 miles) high, depending on geographic location and weather conditions. The troposphere is where most weather phenomena occur, and it is heated by the Earth's surface, causing air to rise and cool as it expands.
it is called temperater inversion