Another occurrence came in April 2001, when windstorms kicked up dust from the Gobi Desert in Mongolia and pushed it to more than 10,000 feet in altitude, where it could be captured by strong winds and pushed directly across the Pacific. In that event, reported last November in a paper published in the journal EOS Transactions, half the particulate matter was smaller than 2.5 microns and half was 2.5 to 10 microns.
Measurements were taken at 110 sites in the Interagency Monitoring Program for improved Visual Environments network, spread throughout the United States in relatively pristine environments, such as national parks and wildlife refuges. Readings on April 10, 2001, averaged across the country gave a value of nine micrograms per cubic meter for particulates 10 microns or smaller. By April 16, the average reached 16 micrograms, even though measuring stations tend to be far from industrial pollution sources.
"Clearly by April 16, that dust from Asia was all over the United States, and it didn't dissipate immediately," Jaffe said. "It didn't go back down to nine until May, and there were several pushes of it coming across."
Before 2001, particulate measurements from the 110 interagency network sites dating back to the mid-1980s show just one comparable event, in 1998.
Asian pollution comes across on winds pulled by a sort of high-speed conveyor belt, with a low-pressure system over the Aleutian Islands and a high-pressure cell near Hawaii acting like twin gears that propel the air mass. Various forces over the Pacific can affect the altitude at which the pollution enters North America, but eventually it drops low enough, between ground level and 3,500 feet, to have an impact on air quality.
In the 2001 event, the effects were more dramatic in some urban settings that already had much-higher background levels of particulate matter in the air, since they were substantially closer to industrial pollution so
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Contact: Vince Stricherz
vinces@u.washington.edu
206-543-2580
University of Washington
13-Feb-2004