are 3000 feet above sea level (most of the valley is a little lower than that, around 2500 feet) At 3000 ft. about 10% of the air is below, 90% is still overhead.
It is the weight of the 90 % that is still above that determines the atmospheric pressure in Tucson. If 100% of the atmosphere produces a pressure of 1000 mb, then 90% will produce a pressure of 900 mb,