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](http://bluemoon.ucsd.edu/publications/ralph/3_Seasonal.pdf) 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](http://ccmc.gsfc.nasa.gov/modelweb/models/msis_vitmo.php) which allow location, date, time and height input and provides composition output.


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