Courtesy Daniel J. Jacob, Harvard University
In March, Mother Nature offered a surprising twist on spring's colorful bounty: Yellow snow blanketed the busy streets of Seoul, and creamy red snowflakes caused panicked residents of eastern Russia to flood their nearest police stations. The culprit was neither an exploding factory or an act of sabotage, or anything else in the immediate area. The strange weather patterns resulted from a mixing of local snowstorms with dust and sand from China's Gobi Desert, which had been thrust into the air by cyclones before traveling east.
Once thought to be a local problem, recent research indicates air pollution is going global. Dust and industrial emissions hitchhike on high-speed, high-altitude winds, crossing oceans and affecting countries and continents far from their point of origin.
A Harvard study recently submitted to the journal Atmospheric Environment found that in 2001, 41% of the days with the worst dust visibility in the US were due to Asian sources. That same year, a plume of North American pollution traveled across the Atlantic, elevating ozone levels in the Alps by 33%. Qinbin Li, a researcher at Jet Propulsion Laboratory, published a study in the Journal of Geophysical Research, which reported that transatlantic emissions were responsible for 20% of Europe's ozone air quality violations.
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Experts and policy makers' growing understanding of mobile air pollution seems to have ushered in an era of finger pointing. Europe is watching North America, North America is eyeing Asia and Asia wants to know how much of its air quality problems are caused by European, and even North American, emissions.
"Everyone is worrying about everybody else," said Li.
Domestically, our chief worry is East Asia. While the region's effects currently make up only a small part of total US pollution levels, scientists warn that its influence will increase as China continues to rapidly industrialize.
Dust and pollution travel great distances via systems called "warm conveyor belts," says Tracey Holloway, an atmospheric pollution expert at the University of Wisconsin. Conveyor belts occur when cold air contacts warm air, driving the warmer air—and any pollution it's carrying—upwards. Pollution and dust can also travel with passing storms, circling the globe on the jet stream 15 km up, until gravity or countervailing weather systems force them to descend. In 2001, satellites tracked a monster cloud of Gobi dust, which produced hazy skies across China and dumped 52,500 tons of particulate matter—the equivalent weight of 290 Boeing 747s—throughout the US.
Dan Jaffe, head of the University of Washington's air monitoring program, said Asian emissions are episodic in nature and generally form only a background effect on local air quality.
"The pollutants are transported 7,000 to 8,000 kms and there's quite a bit of dilution," he said. "It's not like we're getting smog directly from Beijing."
This spring, to quantify the Asian influence off the Washington coast, Jaffe's team used a twin propeller Beechcraft Duchess airplane to collect air samples that could be analyzed for levels of manmade ozone, particulate matter and mercury. In Washington, the National Center for Atmospheric Research launched a C-130 plane to do similar sampling. And in Hawaii, NASA joined the effort, using a modified DC-8 airplane.
Industrial emissions form ozone, which produces smog, causing respiratory and heart disease and even death. Particulates are composed of both natural mineral particles and the soot and ash caused by burning fossil fuels. Of greatest concern is PM 2.5 (particles 2.5 microns or smaller), which can lodge deep in the lungs of people and animals. Mercury isn't a health threat when in the atmosphere, but if it gets into lakes and oceans, it can accumulate in the fish and other seafood we eat.
In 2003, a group at Atmospheric and Environmental Research, Inc., in San Ramon, CA, computed that between 5% and 36% of all airborne mercury in the US comes from Asia.