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AEROSOL
POLLUTION SLOWS DOWN WINDS AND
REDUCES RAINFALL The winds that blow
near the surface of the Earth
have two beneficial effects: They provide a renewable source of clean
energy
and they evaporate water, helping rain clouds to build up. But
aerosolized
particles created from vehicle exhaust and other contaminants can
accumulate in
the atmosphere and reduce the speed of winds closer to the Earth's
surface,
which results in less wind power available for wind-turbine electricity
and
also in reduced precipitation, according to a study by Stanford and
NASA
researchers. "These aerosol
particles are having an effect
worldwide on the wind speeds over land; there's a slowing down of the
wind,
feeding back to the rainfall too," says civil and environmental
engineering Associate Professor Mark Z. Jacobson, co-author of the
study with
the late Yoram J. Kaufman from NASA Goddard Space Flight Center, who
died in
May 2006. "We're finding a reduction of rain, and that can lead to
droughts and reduction of water supply." Jacobson and
Kaufman's study, based on NASA
satellite data of aerosol accumulation, measurements of wind speeds
over the
South Coast Basin in California and in China, and computer model
simulations
over California as a whole and the South Coast Basin, was published
online Dec.
27 in Geophysical Research Letters. The researchers
used both the model
and data to study the effects of aerosol particles on wind speed and
rainfall. Slower Winds, Less
Rain
Aerosol particles
floating in the atmosphere absorb
or scatter solar radiation, and prevent it from getting to the ground.
This
cools the Earth's surface and reduces daytime vertical convection that
mixes
the slower winds found near the ground with the faster winds at higher
altitudes. The overall effect is a reduction in the speed of
near-surface
winds, which Jacobson has calculated to be up to 8 percent slower in Clean and renewable,
wind power made up 1.5 percent
of the "The more pollution,
the greater the reduction
of wind speed," Jacobson says. Aerosol particles may be responsible for
the slowing down of winds worldwide. Wind supplies about 1 percent of
global
electric power, according to Jacobson. Slow winds may hinder
development of
wind power in Slower winds
evaporate less water from oceans,
rivers and lakes. Furthermore, the cooling of the ground provoked by
the
aerosol particles reduces the evaporation of soil water. What's more, the
accumulation of aerosol particles
in the atmosphere makes clouds last longer without releasing rain.
Here's why:
Atmospheric water forms deposits on naturally occurring particles, like
dust,
to form clouds. But if there is pollution in the atmosphere, the water
has to
deposit on more particles. Spread thin, the water forms smaller
droplets.
Smaller droplets in turn take longer to coalesce and form raindrops. In
fact,
rain may not ever happen, because if the clouds last longer they can
end up
moving to drier air zones and evaporating. Increasing "In California, [the
wind reduction] may imply
a 2 to 5 percent reduction in water supply, which translates into 0.5
to 1.25
million acre feet less a year" Jacobson says. (An acre foot is the
volume
of water needed to cover one acre of surface area to a depth of one
foot.) This
contributes to water scarcity in the state, which with its growing
population
will require an additional In fact, Gov. Arnold
Schwarzenegger recently called
for new dams in Northern California and near Fresno to store more water
from
Sierra snowmelt—an initiative that environmentalists have
vehemently
criticized. Jacobson says the elimination of aerosol pollution would
offset some
of the need for additional dams by increasing rainwater supply. "The aerosol
pollution in Jacobson advocates
replacing existing motor
vehicles with cleaner ones, such as renewable-energy powered
battery-electric
and hydrogen-fueled vehicles, and substituting contaminating power
plants with
networked wind farms. These actions would reduce particle emissions
practically
to zero, he says. The second-best option would be adding particle traps
to
existing vehicles and other sources of pollution. "If we want to solve
the global warming
problem, we have to replace most of the existing energetic
infrastructure with
wind and other renewable-based energy," says Jacobson, whose next step
will be to study the effect of greenhouse gases on winds. Jacobson's research
was supported by grants from
NASA and the California Energy Commission and involved the use of
NASA's
high-end computing center at
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