Does air pressure increase or decrease with an increase in altitude?

decrease.

Explanation:

The air pressure is given by

P = h x d x g

where, h is the height of air column above the surface and g be the acceleration due to gravity and d be the density of air.

As we go up, the height of air column decreases, density of air decreases and acceleration due to gravity also decreases, so the value of pressure decreases at altitudes.

increase

Explanation:

think of climbing a mountain. the higher you go the harder it is to breathe. its because air pressure is increasing