Local air pollution could be from Mongolian desert
New research shows that pollutants are hitching rides on dust plumes from Asia and coming over to America.
SAN FRANCISCO — Thomas Cahill has a warning for Northern Hemisphere residents. Cleaning up polluters in your own part of the world won't guarantee you have clean air, he says. Nasty stuff can drift in from sources half way around the world - for example, the highest levels of arsenic in Nevada come from Mongolia.
Dr. Cahill, an atmospheric scientist at the University of California at Davis, is part of an international team that has finally had a good look at what's in the Asian dust plumes that cross the Pacific, spread over North America, and drift out over the Atlantic. They're finding that toxic metals and other man-made pollutants are hitching rides on the desert dust, making the plumes an effective long-distance pollution transport system.
In recent years, scientists have become increasingly concerned about the large-scale environmental effects of aerosols - particles suspended in air.
Some aerosols can cause cooling by reflecting incoming sunlight back into space. Others absorb sunlight and warm up the air. This can offset or enhance the regional effects of carbon dioxide-driven global warming.
But, the climate effect may be more subtle than just cooling or warming. In the Dec. 7 issue of the journal Science, Paul Crutzen and V. Ramanathan at the University of California in San Diego explain that one of the biggest effects could be to reduce the planet's fresh water supply.
They took part in an international study of the effects of Asian aerosols on the Indian Ocean. Dr. Ramanathan explains they found that aerosols are cutting down the sunlight going into the ocean. That sunlight evaporates seawater that later falls out of the air as rain. "So as aerosols cut down sunlight by large amounts, they may be spinning down the hydrological cycle of the planet," Ramanathan says.
Scientists need to know more about the nature and extent of atmospheric aerosols before they can fully assess any proposed environmental effects. Additionally, an understanding of the effect of aerosols is crucial to the accuracy of computer simulations of climate change.
That could prove difficult.
The study of the Asian dust plumes showed an unexpected complex structure that will be hard for computers to simulate, says Barry Huebert from the University of Hawaii at Honolulu.
For example, the aerosol was sometimes distributed in as many as 14 distinct layers stacked up vertically through two kilometers of atmosphere. "Imagine trying to model 14 different layers ... [it's] a tremendous challenge." he says.
Huebert was lead scientist for the dust-plume study, which was called ACE (Aerosol Characterization Experiment). Carried out last spring, it included the great dust plume that brought gloom to parts of western North America.
Presenting early results of their research at a meeting of the American Geophysical Union here, the ACE scientists noted that the Asian dust plumes are partly a natural phenomenon. Winds pick up quartz dust and other particles mainly from deserts and become part of the a system that sustains life in distant regions. In the North Pacific, for example, lack of iron limits the growth of microscopic algae that form the base of the food chain. Dust plumes bring extra iron to fertilize the algae.
That, in turn, supports more fish. Likewise, in Hawaii, plants not fed by humans have a hard time getting enough calcium and phosphate. Asian dust brings these needed nutrients. In such cases, the long- distance pollution "is a good thing," Huebert says.
Humans have superimposed their particles on this natural long-distance aerosol transport system. In some cases, this is direct pollution from farming, factories, and mining operations. In other cases, humans have diverted water that once filled lake beds. The dried beds now add their dust to that of the natural deserts.
Asian dust is not the only aerosol mix science needs to take into account. Deserts and human activity on other continents also produce aerosols that need extensive study.
However, Asia may be the most important Northern Hemisphere source. Thomas Cahill notes: "Between the big spikes [like last April's storm], there's [less visible] Asian dust almost all the time. So we live in a small world. We breathe each other's air."