Epic survey finds regional patterns of soot and dirt on North American snow

C. Dang in EurekAlert via the University of Washington: Snow is not as white as it looks. Mixed in with the reflective flakes are tiny, dark particles of pollution. University of Washington scientists recently published the first large-scale survey of impurities in North American snow to see whether they might absorb enough sunlight to speed melt rates and influence climate.

The results, published in the Journal of Geophysical Research, 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 Sarah Doherty, a research scientist at the UW's Joint Institute for the Study of the Atmosphere and Ocean.

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.

....Their main focus was black carbon, 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....

A snowy landscape in Theodore Roosevelt National Park in North Dakota. National Park Service photo