Compare the air at 18,000 feet (atmospheric pressure 280 mm hg) to the air at sea level (760 mm hg). what specific changes in the primary atmospheric gases (nitrogen, oxygen, carbon dioxide) might occur

There are no changes in primary atmospheric gases even if height is 18,000 ft.

It feels like oxygen is low at such heights but it is not. At height of 18000 ft oxygen will be same in air (21%) but due to decrease in pressure it will be difficult to breathe.

Explanation:

As we move to higher altitudes, the pressure reduces if we compare it with sea level but the important thing to note is composition of air remains the same which is 78% Nitrogen, 21% oxygen and 1% other gases (including Carbon dioxide). There are no changes in primary atmospheric gases even if height is 18,000 ft but the problem which can be faced is breathing because pressure is low at such height and it will be difficult for lungs to take oxygen in.

It feels like oxygen is low at such height but it is not. At height of 18000 oxygen will be same in air(21%) but due to decrease in pressure it will be difficult to breathe.