Dirty snow speeds runoff in mountains

SEATTLE – Soot on snow causes winter snowpacks in the Cascades and other Western mountain ranges to melt faster, leading to runoffs earlier in the spring, scientists say.

Researchers at the Pacific Northwest National Laboratory in Richland, who studied the effect of pollution on snowpack, found soot can melt as much as a couple of inches of snow in some areas of the West.

“These changes can affect the water supply, as well as aggravate winter flooding and summer droughts,” said Yun Qian, an atmospheric scientist with the laboratory and author of the computer modeling study. The lab is part of the federal Department of Energy.

Soot from diesel trucks and smokestacks lands on snow and darkens it. The dirty snow reflects less sunlight and soaks up more heat than new snow, melting snowpacks as much as a month early in the spring.

Results from computer modeling show that the soot warms the snow and the air above it by as much as 1.2 degrees Fahrenheit.

Phil Mote, Washington state’s climatologist, questioned how big an effect soot could have in the Northwest. Because the role of soot isn’t well known, its impact “cannot be quantitatively determined,” said Mote, who is also a research scientist at the Climate Impacts Group at the University of Washington.