Though such events happen infrequently, when large pockets of Asian pollution do make it across, the effects on air quality this side of the Pacific can be dramatic, said Daniel Jaffe, an environmental science professor at the University of Washington, Bothell.
The most pronounced effects can push the levels of atmospheric ozone and small particulates beyond limits set by the U.S. Environmental Protection Agency, which raises health issues for those with respiratory problems, Jaffe said Friday during a presentation at the American Association for the Advancement of Science meeting in Seattle.
When particulates get as small as 10 microns, about one-tenth the width of a human hair, they are small enough to pass through the nose and into the respiratory system. At 2.5 microns or smaller they can begin to have noticeable health effects, particularly for people with conditions such as asthma.
"This clearly has health implications, but it's a rare event," Jaffe said. "How rare we don't know for sure, but from the record it looks like it's something on the order of three occurrences every five years."
One such event happened last summer, when pollution from Siberian forest fires crossed the Pacific and boosted levels of atmospheric ozone, an important summertime pollutant, at stations in Washington state and British Columbia. From May 27 through June 9, surface ozone levels exceeded the long-term average for May and June. On June 6, Enumclaw, Wash., about 35 miles southeast of Seattle, recorded an eight-hour average of 96 parts per billion, well beyond the EPA limit of 80 parts per billion. The data, which has not yet been published, indicates that the plume from the Siberian fires contributed about 15 parts per billion to that reading.
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 sources. For instance, Tucson, Ariz., measured particulates of 10-microns or smaller at 85 micrograms per cubic meter on April 17 and 18, while Savannah, Ga., registered an 85 on April 20. Salt Lake City, Winston-Salem, N.C., and Aspen, Colo., all had measurements in the 70s at some point during the event, and Atlanta registered a 67 on April 20.
About half the particulates measured in those areas came from Asian dust and half came from local pollution sources, Jaffe said.
"If you were driving in this, you'd wonder what was going on. It's a noticeable aerosol loading," he said.
He noted that particulates of 2.5 microns or smaller can begin having health effects at concentrations of 40 micrograms per cubic meter. Since roughly half the particulates arriving from Asia were that small, that would mean cities where particulates of 10 microns or smaller registered 80 or above would have had enough small particulates for residents with respiratory problems to feel the effects.
Jaffe said the 2001 and 2003 events have given greater gravity to a phenomenon he first reported in 1998.
"It's gone from a geophysical curiosity to a point where we can now say that, 'Yes, this occurs at large-enough levels occasionally that it can affect our air quality,'" he said.
For more information, contact Jaffe at 425-352-5357 or firstname.lastname@example.org