Scientists have long known that pollution and dust from China travels over the Pacific to the western United States. What they haven't been able to figure out is how much. Until now.

In a paperpublished in the latest issue of the scholarly journal Environmental Science and Technology and picked up by Chemical & Engineering News, a team of geochemists announced that they have developed a method for tracing fine airborne particulate pollution (also known as PM2.5 because the particles are less than 2.5 microns wide) with...