The results, published in the , show that North American snow away from cities is similar to Arctic snow in many places, with more pollution in the U.S. Great Plains. They also show that agricultural practices, not just smokestacks and tailpipes, may have a big impact on snow purity.

During their almost 10,000-mile trek across North American snowfields, the researchers were particularly interested in the Bakken oil fields of northwest North Dakota.

“With all this oil exploration, diesel trucks and new oil wells, people wondered: Is there a huge amount of air pollution making the snowpack darker?” said lead author , a research scientist at the UW’s .

What they found was that these activities do appear to be adding extra soot to the snow, but perhaps just as important is the dirt. Disturbance from clearing oil pads, new housing sites and all the extra truck traffic on unpaved roads means dirtier snow. But even away from the oil fields, soil is disturbed by agriculture.

“Our work suggests that land use and farming practices might matter as much as diesel emissions in many parts of the Great Plains,” Doherty said.

Doherty was part of a team of 91̽»¨atmospheric scientists who spent the winter of 2013 driving across northwestern U.S. states and some Canadian provinces to get a firsthand look at the continent’s snow.

The project involved collecting hundreds of snow samples from 67 sites away from any cities or major roads. The trip took the researchers from Seattle to North Dakota to Churchill, Manitoba. Every few days they melted and filtered the snow in their motel rooms, then back at their 91̽»¨lab they shone light through a filter to see how much light was blocked, and did chemical analyses to determine what particles were responsible.

Their main focus was , a very light-absorbing particle emitted by burning diesel, coal or wood. Many countries have regulated black carbon because of its effects on air quality and human health, but more recently climate scientists also have become interested because the tiny particles darken the snow and hasten melting.

The cleanest samples they collected were from northern Canada, with overall levels of black carbon, or soot, similar to that of Arctic snowpack. The Pacific Northwest and Rocky Mountain states had levels slightly higher. The Great Plains readings were more variable and sometimes two to three or more times higher than in other parts of the country, typically 15 to 70 nanograms of soot per gram of snow.

Doherty previously worked with co-author , a 91̽»¨emeritus professor of atmospheric sciences, on a 2006-2010 he led of snow in the Arctic. Warren and Doherty also worked with Chinese collaborators in 2010 s of snow in northern China, all using the same techniques so the combined results can provide a first-ever global map of snow cleanliness.

Results from China showed rates of pollution tens to hundreds of times greater than in North America, with the highest rate in northeast China of 1,220 nanograms of soot per gram of snow, likely because of industrial activity and other emissions in the Beijing area. But dirt and desert dust also were prevalent in central North China snow.

“For a lot of the central U.S. and north China Great Plains the snow is not very deep. In the U.S., almost the whole area is agricultural fields and in China there is a lot of animal grazing,” Doherty said. “When the wind blows the dirt gets lofted, maybe just 10 feet off the ground, and gets mixed in with the snow.” North Dakota locals refer to the mixture as “snirt.”

The new paper documents how much light is blocked, and at which wavelengths, by filtered snow samples. Other co-authors and snow collectors were research professor and graduate students and , all in 91̽»¨atmospheric sciences.

A companion by Dang and Hegg involved a chemical analysis of the North American samples to pinpoint exactly which compounds are contained in the snow.

“A lot of the focus in climate models has been on black carbon, because it’s a pollutant and it’s very dark,” Doherty said. “But the snow is darkened by other things as well, like organics, and also by dust and soil that can get in the snowpack.”

In fact, they found that in the Great Plains states up to half of light absorption is due to organic matter, or “brown carbon” from burning fossil fuels and from soil that mixes in with falling snow.

The deposits affect both global and local climates. Pollution on the Himalayan glaciers, for instance, is raising concerns that it will speed melt rates and harm water supplies. For U.S. farmers, changes in the snow’s reflectivity could affect when the spring melt will occur and when meltwater will drain out.

Whether the pollution the researchers found in North Dakota is enough to change snow melt timing will have to be answered by region-specific climate models, Doherty said.

“But first the models have to do a more accurate job of representing the amount of dirt that’s in the snowpack,” she added.

The work was funded by the Environmental Protection Agency and the China Scholarship Fund.

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For more information, contact Doherty at 206-543-6674 or sdoherty@uw.edu and Dang at chengd1@uw.edu. High-res version of the images are available .

The new study concludes that , the soot particles in smoke and smog, contributes about twice as much to global warming as previously estimated, even by the 2007 Intergovernmental Panel on Climate Change.

“We were surprised at its potential contribution to climate,” said , a 91̽»¨ atmospheric scientist and one of four coordinating lead authors.

The silver lining may be that controlling these emissions can deliver more immediate climate benefits than trying to control carbon dioxide, she said.

The paper was made freely available online today (Jan. 15) in the .

Some previous research had hinted that models were underestimating black-carbon emissions, Doherty said, from such things as open burning of forests, crops and grasslands, and from energy-related emissions in Southeast Asia and East Asia.

Black carbon’s role in climate is complex. Dark particles in the air work to shade the Earth’s surface while warming the atmosphere. Black carbon that settles on the surface of snow and ice darkens the surface to absorb more sunlight and increase melting. Finally, soot particles influence cloud formation in ways that can have either a cooling or warming impact.

The report surveyed past studies and included new research to quantify the sources of black carbon and better understand its overall effect on the climate.

Doherty was executive director of the in 2009 when policy groups were seeking better information on the benefits of reducing black-carbon emissions. The research team undertook a comprehensive assessment, funded by IGAC and the U.S. National Oceanic and Atmospheric Administration.

“Because of a lack of action to reduce carbon dioxide emissions, the policy community is asking what else we can do, particularly to help places like the Arctic that are melting much more quickly than we had anticipated,” Doherty said. “We hope reducing black-carbon emissions buys us some time. But it doesn’t replace cutting back on CO2 emissions.”

While carbon dioxide has a half-life of 100 years, black carbon stays in the atmosphere for only a few days.

The authors investigated various sources of black carbon to see which reductions might have the most short-term cooling impact. Regulating emissions from diesel engines followed by replacing some wood- and coal-burning household stoves, authors find, would have the greatest immediate cooling impact.

“If you’re just thinking about impact on climate, you would want to be strategic about which sources you cut back on,” Doherty said. “We looked at the overall impact because some of these sources also emit associated particles that can have counteracting effects.”

Black carbon contributes to climate change in the mid to high latitudes, including the northern United States, Canada, northern Europe and northern Asia, as well as affecting rainfall patterns of the .

The report incorporates data that Doherty, a member of the UW’s , and co-author , a 91̽»¨professor of atmospheric sciences, gathered between 2007 and 2009 to measure soot on Arctic snow. Calculating black carbon deposits in the Arctic is difficult, so data are essential for testing and correcting models.

First author , now at the University of Illinois, earned a doctoral degree at the 91̽»¨in 2000 that combined engineering, chemistry and atmospheric science to measure emissions from burning that have atmospheric importance.

“Mitigating black carbon is good for curbing short-term climate change, but to really solve the long-term climate problem, carbon dioxide emissions must also be reduced,” Bond said in a .

In related research, Doherty, Warren and 91̽»¨graduate student Cheng Dang will travel next month to Colorado, Wyoming, the Dakotas, Saskatchewan, Manitoba and elsewhere to collect snow samples and investigate black carbon’s effects on North America’s Great Plains.

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For more information, contact Doherty at 206-543-6674 or sarahd@atmos.washington.edu.