Observations of atmospheric oxygen in clean air between 50 degrees N and 60 degrees S, mainly over the oceans, yield an almost constant value of 20.946 percent by volume in dry air. Since 1910 changes with time over the globe appear to be either zero or smaller than the uncertainty in the measurements.

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The figures consist of five figures that are plots of the concentration of oxides of nitrogen measured at different locations. These begin on page 15. .

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