Thank you for the question. If you mean below
sea-level, but still on land (such as in Death
Valley), the answer is likely no, the
percentage of oxygen in air does not decrease when
you go below sea-level. The percentages of
different elements in the atmosphere are constant
up to about 60 miles above sea level, implying
that the percentages are also constant just below
sea level. However, I couldn't find any studies of
below see-level measurements.
The reason for unchanging composition of air up
to 60 miles is "turbulent mixing." There is
so much wind that all the air is well-mixed. I
would expect this to hold below sea-level as well,
where there is still wind.
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