NPR News: The Culprit In Rising Western U.S. Smog Levels: Asia

The Culprit In Rising Western U.S. Smog Levels: Asia
U.S. emissions of smog-forming pollutants have dropped, but smog levels in the western U.S. have increased each year. Now, researchers say, they've found out why — it's wafting from across the Pacific Ocean.

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