The density of air in the atmosphere is the maximum where the largest number of air particles are found and also they are squeezed by the pressure of air above it into a compressed volumetric space. The maximum squeezing pressure will be at the lowest layer of atmosphere i.e troposphere.
The air density (i.e the number of air molecules per unit volume of atmosphere) is the highest near the earth’s surface/sea surface. Thus, Troposphere being the lowest layer has the maximum air density.
