Even as local efforts to improve air quality have been making headway over western North America, increasing flows of unhealthy and agriculturally harmful emissions of ozone have been blowing in from Asia.

While scientists have recognized this phenomenon for some time, they are just now beginning to put some hard numbers on it.  An exhaustive international study of databases developed since the 1980s found an increase of 29 percent in "background ozone" entering the lower atmosphere over western North America during springtime since 1984.

This NASA satellite image visually captures the windborne movement of Asian air pollution during an especially severe dust storm in April 2001.

A team headed by Owen R. Cooper of the University of Colorado at Boulder reported their new results in the current issue of the journal Nature.  It includes researchers from Norway, France and Canada, measurements taken by cooperating European airlines, computer and satellite research from NASA and NOAA facilities, the National Center for Atmospheric Research, Caltech and the University of Washington.

"In springtime, pollution from across the hemisphere, not nearby sources, contributes to the ozone increases above western North America," Cooper said in a release issued by NOAA.  "When air is transported from a broad region of south and east Asia, the trend is largest."

Exactly how much of the springtime ozone increase comes solely from Asia, the researchers can't be sure, Cooper said, "But we can say that the background ozone entering North America increased over the past 14 years and probably over the past 25 years."

While ozone flow from distant sources is easiest to detect in spring, researchers are interested in quantifying the Asian ozone contribution especially during summer, when sunlight most seriously damages ground-level air quality across western North America.