Redox and acid-base reactions in planetary atmospheres
We most certainly can call atmospheres acidic or basic.  Put some quicklime, or even magnesia, in our air and you see that it undergoes an acid-base reaction with acidic materials (water vapor, carbon dioxide) in our atmosphere.  Over time, our weakly acidic atmosphere gradually dissolves in water and then reacts with alkali and alkaline earth metal silicates too, turning rock into clay.

Our atmosphere became acidic as it picked up oxygen, which is indeed an "acid former" through the volatile (thus atmospheric) compounds it forms with several nonmetals.  In contrast, the gas (or at least, clouds therein) on the giant planets contain some ammonia and would appear basic if we brought typical Earth chemicals there.