Tiny dust particles from Asian deserts common over western United States

December 13, 2007

It has been a decade since University of Washington scientists first pinpointed specific instances of air pollution, including Gobi Desert dust, traversing the Pacific Ocean and adding to the mix of atmospheric pollution already present along the West Coast of North America.

Now a UW researcher is finding that dust from the Gobi and Taklimakan deserts in China and Mongolia is routinely present in the air over the western United States during spring months.

"We are interested in Asian dust that comes across the Pacific because particles can have an impact on health, as well as on visibility," said Emily Fischer, a UW doctoral student in atmospheric sciences.

"Most previous work has been very event specific, but this research looks at how the average background aerosol concentrations vary on a year-to-year basis."

Aerosols are tiny particles – such as dust, grains of sea salt, soot from fossil fuel combustion and smoke from forest fires – suspended in the air. Many of the aerosols are comparatively large, as much as 10 microns, which still is less than the width of a human hair.

Fischer found that in years with large Asian dust storms there was an increase in particles of 2.5 microns or less in the air over the western United States. Particles that small can be inhaled more deeply into the lungs and so are a greater health concern.

"Local pollution makes the biggest contribution to poor air quality in cities, but my study is looking at aerosols in remote regions like national parks," she said. "In these places dust can be a larger contributor to the total aerosol concentrations because there is little local pollution. While some of the dust pulses from Asia are small, some of them can be very large."

Fischer used two sets of data, gathered during March, April and May from 1998 through 2006, to correlate the dust kicked up in storms over Asian deserts and the appearance of dust in air over the western United States. She looked at dust levels in the air columns directly over the deserts, recorded by NASA satellites, and then paired that information with air quality data from ground stations in rural areas of the western United States for the same period.

The research is being presented at this year's annual meeting of the American Geophysical Union in San Francisco.

For the dust detected at ground stations in the United States, Fischer also looked for – and found – evidence of calcium, which is a tracer for desert dust.

"The calcium lends more confidence to our conclusion," she said.

While the results of the research are not unexpected, they provide supporting evidence that particles of 2.5 microns or smaller appear in higher concentrations in the western United States in years when there are high dust concentrations over Asian deserts.

"The transport of dust across the Pacific is not a new phenomenon," Fischer said. "But we are just beginning to understand it and quantify it on a year-to-year basis instead of on a case-by-case basis.

"We know that just having dust over Asia doesn't mean that it's going to come here. There is the transportation part of the puzzle, which I'm working on now. But we already know that some years are more favorable than others for dust to be transported across the Pacific."

Source: University of Washington