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Figure 1 further shows that the French thermonuclear tests in
mid-1968 abruptly ended the influence of the 6th Chinese test debris
in the Southern Hemisphere,
plunging the ratio to below 1%.
The
Chinese 8th test in December of 1968 apparently generated a ratio of
Pu-239/Sr-90 of about 2% since the pattern in the Northern Hemisphere
was not markedly affected in early 1969.
The Pu-239/Sr-90 in the lower stratosphere,
pause,
just above the tropo-
is illustrated in Figure 2, and exhibited a similar pattern with
one striking difference.
At this altitude the high ratios generated
by small French tests in the Southern Hemisphere in mid-1967 are clearly
seen.
Most important to note, however,
is that in the Northern Hemis-
phere the highest ratios observed in the entire period shown were
greater than 2.5% but less than 3%, and in early 1969 virtually all of
the data were below 2.5%.
This observation holds true not only for the
lowest stratosphere but also for altitudes as high as 19.2 km
(Fig.
1}.
Surface Air Data
The Pu-239/Sr-90 ratios in surface air for the period 1966 through
mid~1969 are shown in Figure 3.
This figure was derived from the data
Obtained in the Health and Safety Laboratory (HASL) Surface Air Sampling Program (9).
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