Why is air more dense at sea level?

The atmospheric pressure and density decrease with increase in altitude. At sea level, the atmospheric pressure is maximum due to high air pressure. Hence air is denser at sea level.

Explanation: -

It can be explained using Boyle's Law.

According to Boyle's law, the pressure is inversely proportional to volume, and, as we know that as height increases, pressure decreases. At the sea level, the pressure is more than that at high altitude, so, obviously, the volume of air will be less at the sea level.

Now, In the formula for density, if we keep the mass constant, we find that Density varies inversely with Volume. And, as already I've already explained above, volume at the sea level is less than that at higher altitudes. So, consequently, the density of air is more at the sea level.

Use the formulae for density and the relation given by Boyle's law.