A new study from scientists at the Lawrence Berkeley National Laboratory (Berkeley Lab), published in Nature Climate Change, has quantitatively demonstrated that black carbon-also known as soot, a pollutant emitted from power plants, diesel engines and residential cooking and heating, as well as forest fires-reduces the reflectance of snow and ice, an effect that increases the rate of global climate change.

Soot can travel great distances and settle back to earth in remote areas far from the emission source. If it deposits on snow-covered areas such as the poles or glaciers, it darkens the snow and ice, with the result that less solar radiation is reflected back into space. More heat is retained near the earth's surface, speeding up global warming.

Although computer models of global climate have estimated this effect, the impact of soot on snow and ice albedo had not been thoroughly measured until now.

Odelle Hadley and Thomas Kirchstetter of Berkeley Lab's Environmental Energy Technologies Division developed new techniques to generate snow in the laboratory, and to mix it in varying concentrations with soot, which normally does not mix well in water.

Using these methods, they measured the reflectance of snow with concentrations of soot varying from none to 1,700 parts per billion (ppb), which spans the range of concentrations measured in snow worldwide.

"We were able to demonstrate clearly that soot in snow reduces its albedo [reflectance]," says Kirchstetter. "We also showed that as you increase the concentration of soot in the snow, you further decrease its reflectance."

Adds Hadley: "Another goal of our study was to validate the snow radiation modules used in general circulation models that predict anthropogenic climate change."

The researchers also demonstrated that the greater the grain size of snow, the larger the decrease in its reflectance associated with a fixed amount of soot. Larger-grained snow allows sunlight to travel deeper into the snowpack than smaller-grained snow. Grain size is a proxy for the snow's age because larger-grained snow is older than smaller-grained snow.

Black carbon depositing on snow may cause it to melt and refreeze into larger grains more quickly than would normally occur. The same amount of black carbon causes a bigger decrease in reflectance of large-grained snow than smaller-grained snow.

The researchers were able to work out the quantitative relationship between increasing black carbon deposition and snow reflectance reduction with increasing snow grain size-a relationship that had been estimated in computer models, but not verified until now.

These results are significant because they provide an experimental check on the methods used to calculate the impact of black carbon on global climate in computer models.

Hadley and Kirchstetter's research show that there is good agreement between their lab measurements and the Snow Ice and Aerosol Radiation (SNICAR) Model, which is being used by the Intergovernmental Panel on Climate Change in its next climate assessment report.

How soot accelerates climate warming
Emissions of carbon dioxide are the largest contributor to global climate change. Black carbon, a particle emitted during fossil fuel and biomass combustion, adds further warming.

"Theoretical calculations suggest that small amounts of soot, 10 to 100 ppb by mass, can decrease the reflectance of snow 1 to 5 percent," says Hadley.

"This reduction contributes to climate change because it allow less of the sun's radiation to reflect back into space. Snow is the most reflective natural surface on earth."

As snow falls it washes black carbon out of the air onto the snow pack. Typical field concentrations of black carbon are measured at 10 to 20 ppb, but in places scientists have measured concentrations as high as 500 ppb.

In snow covered regions, including the Arctic and the Himalayas, the local radiative forcing due to soot deposition is comparable to that exerted by carbon dioxide added to the atmosphere since preindustrial times. (Radiative forcing is a measure of how pollutants alter earth's radiation balance with space, and scientists use it to compare the relative impacts of various pollutants on climate.)

Snow-making in the lab
"We needed to pioneer new techniques to do this study, including developing a way to make snow in the laboratory, and to get soot into water," says Hadley. The researchers solved the first problem with a stack of Styrofoam coolers, liquid nitrogen, and a pressurized spray vessel.

They sprayed the water into the top of the cooler stack with liquid nitrogen at the bottom. As the water droplets met the cold air (-100 degrees C to -130 degrees C) below, it turned to snow. They learned to control the size of the snow grains by changing the nozzle size and water pressure through the nozzle.

They developed a method of generating soot with no other contaminants (such as oil) with the help of a type of non-premixed methane-air flame created by another Berkeley Lab scientist, Don Lucas.

And they captured the soot they created using a filter, and exposed it to ozone, which is known to render soot particles chemically more prone to distribute themselves evenly in water. They developed, as well, a new method for measuring the amount of soot in water.

With these methods in place, the team now had a way of creating water with any desired soot concentration, and then turning it into snow, whose reflectance they could measure. They developed ways of using an integrating sphere-equipped spectrometer to measure the reflectance of snow.

In addition to the experimental work, they estimated the effect of black carbon on snow using the SNICAR model as a step toward verifying the impacts predicted by climate models. SNICAR was developed by former Berkeley Lab researcher Mark Flanner, now at the University of Michigan.

Next steps
Hadley's and Kirchstetter's research provides strong experimental evidence that the climate models are correctly estimating the effect on climate of less solar radiation reflected back into space because of the decrease in snow and ice's reflectance.

In future work, they aim to investigate if the black carbon is causing the earth's snow and ice to melt faster, an effect that scientists suspect may be happening, but has not yet been demonstrated. Previous research by former Berkeley Lab scientist Surabi Menon suggests that black carbon contributes significantly to the melting of glaciers in the Himalayas.

They are also working with the University of California's Central Sierra Snow Lab to begin studying how black carbon travels through snow as the snow pack melts. This research was supported by the California Energy Commission's Public Interest Energy Research program, the U.S. Department of Energy Office of Science, and a Lawrence Berkeley National Laboratory E.O. Lawrence Fellowship for Hadley. The article "Black Carbon Reduction of Snow Albedo" can be found online here.