Snow manufactured in the laboratory, magnified 500x. Image: Lawrence Berkeley National Laboratory
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 ppb, which spans the range of concentrations measured in snow
“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
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 laboratory 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
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%,” says Hadley. “This reduction contributes to climate change because it
allows 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.
in the laboratory
“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 C to -130 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.
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 Laboratory to begin studying how black carbon travels
through snow as the snow pack melts.