Dear Lars and Paul,

The oxygen content of dry air is usually quoted as 20.946 (+/- 0.002 per
cent) by volume.

Whilst there are variations in the oxyegn content of air the magnitude of
variation, is small. On a seasonal basis the oxygen to nitrogen ratio varies
by about 100 per meg, the equivalent of a variation  between 210,000 ppmV
and 209,980 ppmV. On a longer time scale there is a gradual reduction
in the oxygen content of the atmosphere by about 20 per meg per annum. These
variations are driven by the burning of fossil fuels and terrestrial and
oceanic productivity and respiration. Not surprisingly there are also inter
hemispheric
varaiations in the oxyegn levels of a similar magnitude to above, given that
productivity cycles are out of phase between the hemispheres.

I suspect these variations are far smaller than you may be thinking of.

As a starting point here are some references that may be of interest and
use.

Bender et al., 1996, Variability of the O2/N2 ratio of southern hemispheric
air........, Global Biogeochemical cycles, 10, 9-21

Keeling et al., 1993, What atmospheric oxygen measurements can tell us about
the carbon cycle, Global Biogeochemical Cycles, 7, 37-67

Keeling et al., 1996, Global and henispheric CO2 sinks deduced from
measurements of the atmospheric oxygen/nitrogen ratio, Nature, 381, 218-221

and references there in.

Good luck,

Paul