Clouds: Lighter than air
but laden with lead
Monday, April 20,
2009
By sampling clouds -- and making their own -- researchers have
shown for the first time a direct relation between lead in the sky
and the formation of ice crystals that foster clouds. The results
suggest that lead generated by human activities causes clouds to
form at warmer temperatures and with less water. This could alter
the pattern of both rain and snow in a warmer world.
The lead-laden clouds come with a silver lining, however. Under
some conditions, these clouds let more of the earth's heat waft
back into space, cooling the world slightly. Atmospheric lead
primarily comes from human sources such as coal.
The international team of researchers reported their results in the
May issue of Nature Geoscience. The collaboration included
researchers from institutions in the United States, Switzerland and
Germany.
"We know that the vast majority of lead in the atmosphere comes
from man-made sources," said atmospheric chemist Dan Cziczo of the
Department of Energy's Pacific Northwest National Laboratory and
study author. "And now we show that the lead is changing the
properties of clouds and therefore the balance of the sun's energy
that affects our atmosphere."
Globe Trotting for Lead
Scientists first attempted to goad rain from the sky with silver
and lead iodide in the 1940s. Since then, researchers have known
that lead can pump up the ice crystals in clouds. But daily human
activities also add lead to the atmosphere. The top sources include
coal burning, small airplanes flying at the altitude where clouds
form, and construction or wind freeing lead from the ground. Cziczo
and colleagues wanted to know how lead from these sources affects
clouds.
To find out, the researchers collected air from high atop a
mountain peak on the Colorado-Wyoming border. In their high
altitude lab, they created artificial clouds from the air in a
cloud chamber about the size of a small refrigerator. Half of the
ice crystals they plucked from the synthetic clouds, they found,
contained lead.
The team then collected a dollop of real cloud atop a mountain in
Switzerland. About half of those ice crystals also contained lead.
But finding lead in an incriminating position doesn't mean it
causes ice crystals.
To determine whether lead causes ice crystals and clouds to form,
the team turned to a lab in Germany that houses a cloud chamber
three stories tall, as well as a smaller chamber in Switzerland.
They created dust particles that were either lead-free or contained
one percent lead by weight, which is about what scientists find in
the atmosphere. They put these dust particles into the chambers and
measured the temperature and humidity at which point ice nucleated
around the dust.
They found that lead changed the conditions under which clouds
appeared. The air didn't have to be as cold or as heavy with water
vapor if lead was present.
"Most of what nucleates clouds are dust particles," said Cziczo.
"Half of the ones we looked at had lead supercharging them."
Leaden Clouds, Cooler Climes
To investigate what this might mean for the earth's climate, the
researchers simulated the global climate with either lead-free dust
particles floating around, or with either 10 percent or all of them
containing lead.
The computer simulation showed that the clouds they looked at --
typically high, thin clouds -- formed at lower altitudes and
different locations in the northern hemisphere when lead was
present in dust particles. This will probably affect precipitation,
said Cziczo.
"In our atmosphere, lead affects the distribution and density of
the kinds of clouds we looked at," said Cziczo, "which might then
affect where and when rain and snow fall."
Clouds at lower altitudes let more of the earth's heat, or
so-called longwave radiation, escape out to space. So
lead-triggered clouds could partly offset global warming due to
greenhouse gases.
But that doesn't mean lead in the atmosphere will simply cool the
planet, said Cziczo, since they looked at only one type of cloud.
Cloudy skies are far more complicated than their wispy image lets
on.
"This work highlights how complex these interactions between lead
and water vapor and temperature are," said Cziczo. "They're not as
simple as greenhouse gases."
Future work will look at the type of lead and how much is needed to
affect clouds and precipitation, as well as the atmospheric
distribution of the metal dust.
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DOE/Pacific Northwest National Laboratory
Thanks to DOE/Pacific Northwest
National Laboratory for this article.