Wildfire Particles Travel a Long Way from Home

Fast, J. D., Pacific Northwest National Laboratory

Aerosol Properties

Aerosol Processes

Thomas J, C Polashenski, A Soja, L Marelle, K Casey, H Choi, J Raut, C Wiedinmyer, L Emmons, J Fast, J Pelon, K Law, M Flanner, and J Dibb. 2017. "Quantifying black carbon deposition over the Greenland ice sheet from forest fires in Canada." Geophysical Research Letters, 44(15), 10.1002/2017GL073701.

Science

Impurities in surface snow reduce the snow’s surface reflectivity (albedo), which leads to heating of the snow. The magnitude of this effect depends on the concentration and season of deposition. Scientists at the U.S. Department of Energy (DOE) participated in a study that examined the processes contributing to the transport of air impurities to the Greenland ice sheet. Through the use of satellite measurements and regional-scale modeling, researchers identified wildland forest fires in western Canada as the source of impurities in snow samples from the northwestern Greenland ice sheet. While the model captured the timing of the particle deposition event, it under-predicted the impurity concentration at all sites by a factor of 2 to 100—similar to or better than other models.

Impact

Atmospheric process predictions critically depend on knowledge of impurity amount, concentration, and location in the troposphere, as well as the amount and location of deposition on snow and ice. This study showed the need for better model descriptions of wildfire smoke estimates and aerosol removal from the atmosphere by rain or snow, referred to as precipitation scavenging. Improved model descriptions are needed to correctly predict smoke particle deposition on snow cover, which is critical for determining how aerosols from fires affect Earth systems.

Summary

Researchers presented a new data set that describes deposition of impurities on snow across the northwestern Greenland ice sheet from July 2013 to April 2014. They found that about 57 percent of the deposition occurred during two snow storms in a one-week period (July 27-August 2, 2013). Scientists identified likely transport pathways by using measurements from two Earth-observing satellites. Then, they ran a regional-scale chemical transport model that used known aerosol sources over North America. Previously, scientists supported by DOE’s Atmospheric System Research program developed process modules for the model that describe the aerosol life cycle and cloud-aerosol interactions.

Researchers combined the model results and satellite images to identify large wildland fires in western Canada as the source of impurities deposited on snow in Greenland during the one-week period. While the model accurately simulated the timing of smoke transport to Greenland, the simulated impurity concentrations in snow were a factor of 2 to 100 too low, depending on the sampling site. The low simulated concentrations of impurities in snow could ultimately lead to underestimates of snowmelt. The new data set will be a valuable resource for evaluating how models simulate wildfire smoke and precipitation scavenging.