| Microscopic Pollutants Have Global Reach, Impact|
By Andrew Bridges
May 27, 2002
Atop Hawaii's Mauna Loa, thrust 13,677 feet into the sky, one would
expect nothing but the freshest air, save the occasional gaseous
burp from the volcano.
But environmental monitoring stations crowding the peak find
arsenic, copper and zinc that was kicked into the atmosphere five to
10 days earlier from smelting in China, thousands of miles distant.
When industrial pollution first showed up at Mauna Loa a few years
ago, scientists were startled. Now they know that the pollution that
dirties the world's largest cities affects the whole Earth.
"It turns out Hawaii is more like a suburb of Beijing," said Thomas
Cahill, a University of California at Davis atmospheric scientist.
Along the West Coast, a campaign to measure the pollutants as they
make landfall after bridging the Pacific ends this month.
Since April, scientists have used data gathered on the ground from
an airplane flying along the coast to measure aerosol pollutants
that waft eastward each spring, carried by the prevailing winds. For
the United States, China is a major source. For Europe, it's the
United States, and likewise down the line, complicating the blame
"It's kind of a natural human condition to point to someone else who
is causing your problems," said David Parrish, a research chemist
with the National Oceanic and Atmospheric Administration. "Each
state points to the state upwind and says, 'You're causing our
Scientists previously supposed only greenhouse gases like carbon
dioxide were so global in reach and effect. They now understand that
the microscopic, suspended particles of pollutants generically
called aerosols also wrap the globe, even if they persist for just
hours before settling to the ground. This class of pollutants
includes soot, salts, dust and other byproducts of the burning of
vegetation and fossil fuels.
"It happens on a small scale, but the implications are on a global
scale," said V. Ramanathan, an atmospheric scientist at the scripps
Institution of Oceanography.
During their time aloft, the particles affect everything from global
warming to human mortality to the rainfall that ultimately scrubs
them from the atmosphere. Given their tremendous variety in shape,
size, composition and distribution, their effects are unpredictable.
Scientists long though aerosols were day-trippers, settling close to
their point of origin. In fact, many are.
But beginning in the 1950s, scientists began noticing layers of haze
in such places as the Arctic, far from any significant source of
pollution. The haze suggested aerosols were capable of traveling
Now, armed with satellites, airplanes, balloons, and ship- and
land-based observatories, scientists track with accuracy the
pollutants and the winds that carry them.
"If there's anything we've learned over the years, it's there is a
lot of long-range transport up there that no one was ever aware of,"
said Ken Rahn, a professor of oceanography at the University of
Large storms can hoist a plume of particles high enough to hook up
with the jet stream. Once high enough, dust from the Sahara or smoke
from big fires "can easily travel half way around the globe," said
Yoram Kaufman, a senior scientist with the National aeronautics and
Once plugged into higher altitude winds, the sometimes vibrant
plumes can be charted.
The best example is the billowing clouds of dust licked up each
spring in Mongolia's Gobi Desert. That dust blows east, passing
through such cities as Beijing. There, the particles of dust are
frosted by various pollutants, many of them toxic. The noxious
confection continues to blow eastward, arriving in the United States
The problem isn't just China. Aerosols have been tracked from
Ontario to Rhode Island, and from Germany to Sweden. Within them
travel toxic metals, nutrients, viruses and fungi.
"We live in a small world. We breathe each other's air," Cahill said.
Nor is the problem new: Pollutants generated by smelting of ores by
the Greeks and Romans show up today, more than 2,000 years later, in
trace amounts in ice cores drilled from Greenland.
"These dust plumes don't go away right away. They can be carried
over great distances and are forcing people to take a global
perspective on pollution," said Barry Huebert, a professor of
oceanography at the University of Hawaii.
The effects of aerosols are obvious in cities such as Beijing, where
springtime mantle of dust and pollution cuts visibility to feet.
There, Cahill jokes, you can tell it's a sunny day if the sky is
bright brown. If it's rainy, it's still brown, just a darker shade.
"You're living in a sepia world," he said.
The tiny particles pose a serious health hazard, as they can lodge
deep in the lungs, contributing to increased mortality.
Aerosols may also harm agriculture by blocking portions of the
spectrum of light from the sun, effectively starving crops such as
wheat and rice of energy they need to grow.
The biggest worry, and the one least understood, is the effect
aerosols have on weather and climate.
Some aerosols can cool the planet by literally shading it from the
sun. Others can warm it by absorbing and trapping the sun's heat.
Aerosols may also have the peculiar ability to aid in the formation
of clouds, while retarding their rainfall, scientists reported in a
study in the journal of Science in December.
Water drops will coalesce in clouds, but they will not clump
together to form large drops that gravity pulls from the sky as
"We humans may be pushing precipitation away from populated
regions," said NASA's Kaufman.
The dust is not all bad news, though; the wafting plumes also carry
nutrients to regions that depends on them. In Hawaii, plant life
relies on Asian phosphorus and calcium to grow. Phytoplankton in the
waters off the Alaskan coast crave Asian iron, which blows eastward
by the millions of tons.
Still scientists believe it's important to trace the origin of the
pollutants and say they can do that by using the unique chemistry of
aerosols as a fingerprint.
Rahn, the oceanography professor, has developed a roster of about
150 compounds, each with its own distinct signature. With some work,
scientists can distinguish between soot from a power plant burning
low-sulphur coal in Colorado and a fire raging in the Brazilian rain
"Once the scientists say these particles are coming from here, here
and here, at that point it's finger-pointing," says Ramanathan, who
has proposed a national effort to study aerosols. "That's something
we have to leave to the politicians to figure out."