Scientists size up black carbon in snow

Contact: Monica Allen, monica.allen@noaa.gov, 301-734-1123 or Jane Palmer, 303-492-6289

Next-generation Hurricane Weather and Research Forecasting model (HWRF)

This is a real-time version of the next-generation, triply-nested HWRF in August 2010, with Hurricanes Danielle and Earl in the Atlantic and Hurricane Frank in the Pacific. In 2007, HWRF started as a double-nested model with 27 km grid and an inner moving nest at 9 km over the storm region. By 2015, HWRF had become a triply-nested model with an 18 km grid, then a moving nest of 6 km, and a much finer resolution of 2 km over the storm region. Credit: NOAA/AOML

Using an innovative approach, scientists have measured – for the first time – the size of black carbon particles in snow, and discovered that they can be larger than expected. The finding can help researchers better account for how black carbon changes the climate-related properties of a snowy landscape.

It is well known that white snow reflects sunlight and that dark material absorbs it. When black carbon particles in the atmosphere are deposited in and on snow and ice, they change the balance of reflection and absorption. Theory says that the size of the black carbon particles matters. A given weight of larger particles tends to absorb less sunlight than the same weight of smaller particles, because the interior of a larger particle is "shaded" by its own surfaces. Thus, larger particles would have a smaller warming effect on snow. But until now, researchers haven't measured the size of those dark particles in snow, and models assumed that they were similar to their size in the atmosphere.

Researchers from NOAA, the NOAA Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado, and the Science and Technology Corp., using a NOAA-developed technique with unique capabilities for analyzing black carbon in liquid samples such as snow melt, found that the size of the black carbon particles in snow is larger than the size typically observed in the air. This unexpected finding was published online March 1 in the journal Nature Scientific Reports.

"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. This surprised us even more once we calculated the possible change in the amount of light they would absorb," said the study's lead author Joshua Schwarz, a CIRES research scientist working 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, from sources such as vehicle engines and cook fires. The small particles are released into the atmosphere and return to the Earth’s surface in rain, snow or by colliding with the ground.

The scientists identified several mechanisms that could explain how the particles in snow are larger, 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 cycling. However, they hypothesize that more complex interactions of black carbon and snowflakes could be also be involved as the snow is forming in the atmosphere. The size of the black carbon in snow contains a "fingerprint" of these interactions, providing clues about how airborne fine particles are removed from the air.

The scientists used an instrument known as the Single Particle Soot Photometer (SP2) for this work. The SP2 is an innovative instrument that has flown aboard research aircraft and can determine the size of black carbon particles in the atmosphere. To measure black carbon particle size in snow, however, the scientists developed a method to apply the SP2 to measuring size in liquid samples (melted snow, for instance).

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. Therefore, the researchers reason that formation of larger soot particles could happen over large regions of the globe.

The authors suggest that the climate warming due to black carbon in snow could be currently overestimated by about 30 percent, but that further work is needed to refine this estimate.

"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 by snow."

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