Description:
<jats:title>Significance</jats:title>
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The specific cause of the long-term decrease in stable nitrogen isotope ratio (
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N/
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N) of ice core nitrate beginning ∼1850 is a subject of debate, hindering the efforts to understand changes in the global nitrogen cycle. Our high-resolution record of ice core
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N/
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N combined with model calculations suggests that the decrease is mainly caused by equilibrium shift in gas−particle partitioning of atmospheric nitrate due to increasing atmospheric acidity resulting from anthropogenic emissions of nitrogen and sulfur oxides. Our high-resolution record also reveals a leveling off of
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N/
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N ∼1970, synchronous with changes in acidity and sulfate and nitrate concentrations. This leveling off suggests a measurable reduction in air pollution following the implementation of the US Clean Air Act of 1970.
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