How chemically homogeneous is air? By this, I mean if I buy compressed dry air from somebody like British Oxygen how much variation in composition might I expect between batches? For example, is Argon evenly distributed globally, by height, by geography etc? Is $\ce{CO2}$ removed? Are contaminants filtered out eg $\ce{NOx}$?
Oxygen/nitrogen ratios vary seasonally and geographically even at sea level on the order of 200 ppm.  Oxygen concentration has also been slowly decreasing in correlation with carbon dioxide increase.  

Try using the MSIS-E-90 Atmosphere Model which allow location, date, time and height input and provides composition output.