This is called a "baseline." There are records dating back long before 2002.

Journal of Atmospheric and Terrestrial Physics (click here)
Volume 1, Issue 1, 1950, Pages 49-55

Chemical analysis of stratosphere samples from 50 and 70 km height
K.F.Chackett, F.A.Paneth, E.J.Wilson
Londonderry Laboratory for Radiochemistry, University of Durham UK

Received 17 May 1950, Available online 1 April 2003.

By means of V2 rockets samples of air from heights ranging from 50 km to 70 km have been obtained in the U.S.A. and sent to us for chemical analysis. These have been tested for oxygen, argon, neon and helium. Except in the case of oxygen, the results show that the composition of the air at these heights is not appreciably different from that at ground level. A large deficit of oxygen was found but this is attributable to the method of sampling.