来自《耶鲁大学开放课程：环境政治与法律 第2集 第2集》

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  settle down when it's dry.
  在干燥条件下会沉降下来
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  The finer particles tend to aggregate.
  较小的颗粒则会凝聚
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  And then when a rain cloud intersects with a dust cloud as
  放射尘埃进入较低大气层后
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  they settle down into lower elements of the atmosphere,
  与积雨云相遇
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  that's when they rain down into the earth.
  那时它们便随雨水落至地面
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  And that creates a patchy pattern of contamination.
  由此形成片状分布的污染
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  It's a patchy pattern, but we can figure out what the
  这是片状分布 但今天我们可以算出
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  deposition rates were fifty years ago today,
  五十年前的沉积速率
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  just by taking soil samples.
  通过采集土样即可
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  It's very interesting.
  这项工作很有趣
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  So that you can back-calculate the half-life of
  这样就能倒推出铯137或
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  cesium-137 or strontium-90.
  锶90的半衰期