Atop Hawaii's Mauna Loa, thrust 13,680 feet into the sky, one would expect nothing but the freshest of air, save the occasional gaseous burp from the volcano.
But environmental monitoring stations crowding the peak record arsenic, copper and zinc kicked into the atmosphere five to 10 days earlier from smelting in China, thousands of miles away.
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 UC 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.' "
The effects can be 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. The probable source: China.
"It's nothing that is going to get anyone sick. It's just that it shouldn't have been there," Cahill said.
Scientists previously supposed that only greenhouse gases such as carbon dioxide were so global in reach and effect. They now understand that the microscopic, suspended particles of pollutants--generically called aerosols by scientists--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 the byproducts of the burning of fossil fuels and vegetation.
"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. Scientists long thought aerosols were day-trippers, since they were assumed to settle from the atmosphere close to their point of origin. In fact, many are. The cloud of smoke, dust and other pollutants from the attack on the World Trade Center didn't travel far, researchers say.
But beginning in the 1950s, scientists began noticing layers of haze in places such as the Arctic, far from any significant source of pollution. The haze suggested that 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 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 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.
When plugged into higher-altitude winds, the sometimes vibrant plumes can be charted.
The best examples are the billowing clouds of dust kicked up each spring in Mongolia's Gobi Desert. That dust blows east, passing over cities such as Beijing. There, the particles are frosted by various pollutants, many of them toxic. The noxious confection continues to blow eastward, arriving in the United States within days.
The problem isn't just China. Aerosols have been tracked from the Sahara to the Caribbean, from Ontario to Rhode Island, 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, 2,000 or more 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 the springtime mantle of dust and pollution cuts visibility to feet.