|Tiny Pollutants Have Global Reach and Effect |
By Andrew Bridges
May 25, 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
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, after intense study, 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, 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 and 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 game.
"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 problems."'
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 by atmospheric scientists
- also wrap the globe, even if they persist for just hours before settling
tations 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 thought aerosols were day-trippers, settling close to their
point of origin. In fact, many are. The big cloud of smoke, dust and pollutants
from the attack on the World Trade Center didn't travel very far, researchers
Beginning in the 1950s, scientists began noticing layers of haze in places like
the Arctic, far from any significant source of pollution. The haze suggested
aerosols were capable of traveling jet-setter distances.
Now, armed with satellites, airplanes, balloons, ship- and land-based
observatories, scientists track with accuracy the pollutants and the winds that
"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 Rhode Island.
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 halfway across the globe," said Yoram Kaufman, a senior scientist
with the National Aeronautics and Space Administration.
Once plugged into higher altitude winds, the sometimes vibrant plumes can be
The best example is the billowing clouds of dust kicked up each spring in
Mongolia's Gobi Desert. That dust blows east, passing through cities like
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 within days.
There, its effects are dramatic: In May 1998, Cahill and others measured the
highest atmospheric concentrations of arsenic ever seen in the western United
States in tiny Jarbidge, Nev., population 12.
"It's nothing that is going to get anyone sick. It's just that it shouldn't have
been there," Cahill said.
The problem isn't just China. Aerosols have been tracked from the Sahara to the
Caribbean, 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 the 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 like Beijing, where the 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 make for spectacular sunsets, but they also pose a serious
health hazard, as they can lodge deep in the lungs, contributing to increased
Aerosols may also harm agriculture by blocking portions of the spectrum of light
from the sun, effectively starving crops like wheat and rice of the 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) are clearly right at the center of some important climatic issues,"
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
Science in December.
Water drops will coalesce around aerosols in clouds, but not clump together to
form the larger drops that gravity pulls from the sky as rain.
"We humans may be pushing precipitation away from populated regions," said
The dust is not all bad news, though; the wafting plumes also carry nutrients to
regions that depend on them. In Hawaii, plant life relies on Asian phosphorus
and calcium to grow. Phytoplankton - those bottom feeders of the food chain - 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
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-sulfur coal in Colorado and a fire
raging in the Brazilian rain forest.
"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."