Sizing up black carbon in snow
Why black carbon particles could be larger in snow
The scientists identified several mechanisms that could explain how the black carbon particles end up larger when they are in the snow, including smaller particles sticking together in the air; larger particles being more likely to be deposited in snow; and larger particles being formed in fallen snow that undergoes temperature fluctuations. However, they hypothesize that more complex interactions of the black carbon and snowflakes could be also be involved, likely as the snow is forming in the atmosphere. The size of the black carbon in snow contains a “fingerprint” of these interactions, providing hope that it can help improve scientists’ understanding of how the particles are removed from the air.
Black carbon particles—often referred to as “soot particles”— in snow are larger than expected, according to a new study led by scientists at NOAA’s Cooperative Institute for Research in Environmental Sciences (CIRES). Black carbon in snow contributes to climate warming and this finding suggests that the warming produced by black carbon in snow could be currently overestimated by as much as 30 percent.
“For the first time, we looked at the size of these particles in snow, and found that they can be larger than in the air,” said lead author CIRES scientist Joshua Schwarz, who works at NOAA’s Earth System Research Laboratory. ”This is important for our understanding of how black carbon interacts with the atmosphere and how it can affect climate.”
Black carbon is a byproduct of combustion—both vehicle engines and cooking fires release these small dark particles into the air. Eventually the particles return to the ground in rain, snow, or by hitting the landscape. Whereas white snow reflects most of the sun’s rays, darker soot-tainted snow will absorb more of the incoming radiation, changing the energy absorbed by the landscape and the rate at which the snow melts and exposes darker ground.
It is not just the net amount of black carbon in the snow that impacts the absorption of the sun’s rays, however—theory suggests the size of the particles should also play a role. Larger particles will absorb less radiation than the same weight of smaller particles. Previous models have assumed that the light absorption of black particles in the snow is approximately the same size as those particles in the atmosphere: an assumption, which if incorrect, could lead to errors in the warming estimates made by these models.
To measure the size of soot particles in the snow—previously unexplored territory—a team of CIRES, NOAA, and Science and Technology Corp. researchers, used an instrument known as the Single Particle Soot Photometer (SP2). The SP2, when installed on aircraft for research flights, can determine the size of black carbon particles in the atmosphere. To measure black carbon’s size in snow, however, the scientists developed a method to apply the SP2 to measuring black carbon’s size in liquid—in this instance, melted snow.
The team found the black carbon particles to be larger than those typically observed in the air and published the unexpected findings in the journal Nature Scientific Reports. “It surprised us even more once we calculated the possible change in the amount of light the larger particles would absorb,” Schwarz said.
"These are important measurements and have the potential to alter how we represent black carbon-in-snow size distribution and optical properties, in global modeling efforts," Mark Flanner, of the Department of Atmospheric, Oceanic and Space Sciences at the University of Michigan. Flanner is one of 31 co-authors of a 4-year international study bounding the role of black carbon in climate. The study, published in January 2013 in the The Journal of Geophysical Research-Atmospheres concluded that black carbon exerts a substantially stronger warming effect on climate than quantified in the 2007 Intergovernmental Panel on Climate Change (IPCC) report.
The researchers based their findings on snow samples gathered in Colorado, but also saw indications of the large black carbon sizes in snow from remote Arctic regions. The tendency to form larger soot particles could be something that happens over large regions of the globe, Schwarz said.
While the scientists note that the climate warming due to black carbon in snow could be currently underestimated, they add that further work is needed to refine the estimate of 30 percent. “This piece of the puzzle opens the door to many questions,” says Schwarz. “A next step is to pin down the implications for climate and understanding of black carbon removal from the air by snow.”