The composition of the atmosphere is about 80% of nitrogen and is 20% of oxygen by mass.
At a height of more than 20 kilometers above the surface of the earth this pressure decrease as the gases are attracted to gravity and thus decrease in air molecules at the height above or beyond this layer as the region of mixing ends.
Layers above the 30 kilometers are cool and temperature inversion occurs with a low density of air. If assuming the temperature to be fixed at the 25 degrees the pressure with atmosphere gaining height would still not change as rarefying of air particles and only the presence of earth's magnetic field.
As it will keep on getting thinner and thinner and merge into outer space.
