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Tracking the long-distance travels of dust

Spring stirs pollen, and also dust — high-flying dust that's blown thousands of miles to reach North America in greater amounts than scientists have known before, with potential impact on the climate and air quality.
Mineral dust rises from dry expanses in Asia, Africa and the Middle East, rides upper atmospheric winds for days across the Pacific Ocean to the West Coast of the United States and beyond. More than two miles up, it can reach Maryland, where scientists at College Park, Greenbelt and Catonsville have been tracking its global travels with satellite-based instruments in a way they say is more accurate and covers a longer period of time than previous studies.
At the moment, Maryland scientists are at work on a three-year project funded by NASA to fathom the potential impact of dust on climate, following up a study published in the summer that quantified in a striking way the annual flow of mineral dust from abroad.
Publishing in the journal Science, researchers at the University of Maryland, College Park, the NASA Goddard Space Flight Center and the University of Maryland, Baltimore County showed that dust, not pollution from wildfires or burning fossil fuels, is the main ingredient in the stream of airborne particles reaching American shores. Dust crossing the Pacific, some 56 million tons of it a year, accounts for 88 percent of the total amount of airborne particles reaching the United States from overseas, the paper reported.
"Our estimated oversea contribution looks strikingly large," wrote Hongbin Yu, a research scientist at the University of Maryland and Goddard, in an email. "I think this could be one of the reasons Science published our results."
The study also found that the total of 64 million tons of dust and other airborne particles arriving in the United States from overseas every year is roughly comparable in weight, if not impact on air quality, to the 69 million annual tons of particles from domestic sources, including dust and material from burning fossil fuels.
In both its method and conclusion, the study represented a departure from previous research on the transcontinental movement of so-called "aerosols" —fine particles suspended in the atmosphere.
Earlier studies have relied chiefly on computer models and some satellite information, and focused on specific episodes of dust movement. This research used instruments mounted on two satellites to attempt to measure the size of the dust plume sweeping into North America over the course of a year.
This study also examined a larger expanse of air space, from the ground to between 6 and 12 miles above the Earth. The flow of dust across continents occurs mainly in high altitudes, with only about 5 percent of this mass crossing into North America in the air up to about 1.2 miles.
"Our study is the first measurement-based estimate of total contribution of oversea aerosols to the entire air shed over North America," said Yu, the lead author of the paper with six colleagues in Maryland.
Combining information from the two satellite-based instruments, the researchers distinguished dust from other types of particles and reckoned their varying heights in the atmosphere. Only the highest flying dust makes it all the way here from overseas, and researchers believe it has more of a potential effect on climate than on air pollution, which is composed of ozone, mercury and particles from wildfires, emissions from industrial plants and burning fossil fuels that are closer to the ground.
The dust and other particles stream to the United States from overseas all year long, but "spring is the most active season for trans-Pacific transport" due to cyclones and westerly winds at mid-latitudes, the study reported.
Two atmospheric scientists who are familiar with the Science report say it adds to a body of research on this subject by expanding the estimate of dust coming from overseas.
"It's an enormous number that their numbers are suggesting," said David J. Smith, a scientist at the NASA Kennedy Space Center in Florida.
Dan Jaffe of the University of Washington-Bothell, agreed that the estimated size is the study's chief contribution. He called it a "really excellent scientific paper," although he said the comparisons of relative amounts of overseas and domestic airborne particles could be misleading for suggesting an effect on air quality, as home-grown particles are the main contributors to air pollution near the ground.
While high-flying dust can affect the quality of the air people breathe when it occasionally falls to the ground, its chief impact is believed to be on climate. That impact is not well understood, not least because the effects pull in more than one direction.
Dust particles can reflect sunlight back into space, which has a cooling effect, but they can also absorb heat, which would warm the layer of atmosphere in which the dust is traveling, said Tianle Yuan and Lorraine A. Remer, research scientists at UMBC and co-authors of the study.
Dust that falls on pure snow would melt it more quickly, as it absorbs more heat than the snow because the dust is a darker color, said Remer. Faster snow melt, particularly on the West Coast, could potentially deplete summertime water supplies, Yu said.
Remer said a key question is the effect of dust on clouds.
"That's a very big unknown," she said. "We know very little about how dust acts in clouds."
Yu said recent studies in California have found evidence that dust transported from long distance triggers rainfall there. He said members of the research team are now in the second year of a three-year project funded by NASA to better understand the climate impact of this high-altitude dust.
This all becomes part of understanding the human impact on climate, because some dust is created by human activity such as deforestation, which ultimately can create deserts.
"To me a better understanding of dust effects on climate is essential," Yu said. "A better understanding of these effects is certainly of great benefit for climate change science and the society."

Source: By Arthur Hirsch, The Baltimore Sun

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