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Spring 1999

MIRAGE uncovers the science behind pollution

Fifty years ago, New York City was the only metropolitan area in the world with a population of over 10 million. Now, it's just one of a crowd. By next year, 24 metropolitan areas worldwide will have a population of that magnitude, and 150-200 others will be over the 1 million mark, according to U.N. estimates. And like the great cities of the past, such as London with its deadly pea-soup fogs, each of today's multiplying megacities produces its own noxious blend of air pollution. With recent findings that pollution from China's cities travels as far as the U.S. West Coast, it's clear that megacity pollution is more than a local problem: it's a global issue.

Darrel Baumgardner.

"A lot of people are doing air quality monitoring to formulate mitigation strategies that hopefully will combat the local effects of pollution," says Darrel Baumgardner, a scientist in NCAR's Atmospheric Technology Division (ATD). But the regional and larger-scale effects remain largely unknown. "The work that's needed is to investigate the fundamental physical processes that underlie the formation of aerosols and secondary gases such as ozone. This has been done to some degree, such as extensive research in the [Los Angeles] basin, but studies on a broader scale and on megacities in developing nations just have not been done."

Guy Brasseur. (Photo by Carlye Calvin.)

Baumgardner and Guy Brasseur, director of NCAR's Atmospheric Chemistry Division (ACD), have launched a new initiative to begin filling that gap. Called Megacity Impact on Regional and Global Environments (MIRAGE), it will use models, laboratory studies, and field projects to learn what physical mechanisms produce and control urban pollutants, how the various pollutants interact, and especially how these chemicals are transported outside urban areas and affect the larger environment. At the same time, MIRAGE will build several exploratory models that can be used to analyze and predict regional impacts of urban pollution.

"What we're proposing to do is to play a coordinating role," says Brasseur. "We want to have a nucleus at NCAR, but we are inviting the community as a whole--other federal labs, federal agencies, and of course the universities--to try to build a program to look at these issues. Some work has already been done. We do not want to reinvent the wheel, but for example we would like to have a few campaigns to identify how pollutants are emitted in cities, get out of the boundary layer, and [in some cases] are transported outside the cities."

The first field project will take place in and around Mexico City in July 2000, if the NSF/NCAR C-130 aircraft is available. That site was chosen because Baumgardner has been working with Graciela Raga, an atmospheric science professor at the Universidad Autonoma de Mexico (UNAM) in Mexico City. In November 1997, Baumgardner, Raga, and Gregory Kok, now a visiting scientist in ACD, did a two-week experiment there, called Project Azteca, to measure aerosols. Having reaped "a lot of interesting measurements," the scientists saw a need for a more detailed study. As Baumgardner remembers, "During the June 1997 [NCAR] director's retreat, Guy had said that one of the key emphases in ACD was going to be air quality. The results of Project Azteca motivated me to discuss what NCAR could do in this area. Guy and I think NCAR can make a big impact because of our expertise in the areas of transport modeling, photochemistry, and aerosol measurements. We are putting together a program that will be focused on science rather than on policy making."

Brasseur hopes that MIRAGE will mark a new direction not just for ACD, but for NCAR. "I see this being an interdivisional activity. ACD will have a strong contribution, but [MIRAGE] also involves ATD. It should involve a strong meteorological group, so MMM [Mesoscale and Microscale Meteorology Division] has been invited to be a part." He adds, "We hope very much that as soon as there is a new director in ESIG [Environmental and Societal Impacts Group], this person will have some interest--that there will be a socioeconomic dimension as opposed to just pure chemistry or meteorology." Paraphrasing from Mark Hertsgaard's book Earth Odyssey, Brasseur says, "The big environmental problem of the future is less ozone depletion or global warming than it is poverty. What that means is that it's very difficult to separate the science from the economic and social factors, so we'll have to bridge these different aspects."

The Mexico City campaign

At the experimental site in Mexico City, an ecological park 400 meters above the basin, researchers can watch the rise of polluted air throughout the day. At 7:00 a.m. (top photo), despite lower-level smog, a hill some distance away (right background) is still visible, and the sky is clear in the top half of the photo. By afternoon, the pollution layer has reached the site, and nothing is visible through the brown cloud. (Photos by Darrel Baumgardner.)

As currently planned, the MIRAGE Mexico City project will immediately follow another ACD experiment called Tropospheric Ozone Production about the Spring Equinox, in which the NSF/NCAR C-130 aircraft will be used to study photochemistry in the far north. This timing will save money. "When the plane comes back from TOPSE, it will be fully equipped with atmospheric chemistry instruments," says Brasseur. Baumgardner and his colleagues can use this instrumentation for their measurements, which encompass both a large number of chemical species--ozone, carbon monoxide and dioxide, oxides of nitrogen, and others--and a wide variety of parameters--aerosol properties, particle sizes, radiative fluxes, photolysis rates, and the state of the atmosphere. (Brasseur hopes the C-130 can simply keep flying south from Canada to Mexico City, collecting data all the way; this kind of long swath of measurements over a continent is very rare.)

If the plan is approved, the C-130 will make two deployments to Mexico, each four weeks long. One of the deployments would be in July, during Mexico's wet season, and the other in November at the beginning of the dry season. Six ten-hour flights are scheduled during each of these time periods. These long flights will allow the researchers to sample the pollution layer as it develops and then follow the polluted air as it drains through the mountain passes around the Mexico City basin. Some flights will also sample upwind of the pollution for comparison.

Nobel laureate Mario Molina of the Massachusetts Institute of Technology, a member of MIRAGE's advisory panel (see sidebar), is a principal investigator for the Mexico City program. Besides Raga, three other UNAM professors and a half-dozen students will be involved. Two students from UCAR's Signficant Opportunities in Atmospheric Research and Science (SOARS) program will also collaborate: Stephanie Rivale (University of Colorado) and Monica Rivera (University of Rochester).

Model modules

Current pollution models were developed for use in the industrialized nations, and often for specific cities, such as Los Angeles. These models don't transfer well to megacities in the developing countries. For one thing, L.A.'s smog has a different chemical composition than the acrid murk over Bombay or Shanghai, where more coal and wood may be burned and cars have fewer pollution controls. Also, Brasseur explains, the models are mostly statistically based, using records of temperature, wind, levels of pollutants, etc., to simulate the probable resulting conditions. "We would like to build models more on first-order principles, to solve the basic physical and chemical equations. We would like to team up with people who are starting to develop these [physically based] models and try to lead their effort and our effort into a community tool that can be made available at many places in the world." He envisions that MIRAGE will eventually provide a pollution-modeling toolkit offering a number of generic "modules."

Susan Durlak, a postdoctoral fellow in NCAR's Advanced Study Program, is developing the aerosol module. She has begun modifying a three-dimensional model of urban atmospheres developed by Anthony Wexler (University of Delaware) to incorporate "all the basic physics and chemistry of aerosols," she says. The completed module could be used for a wide range of modeling activities. For example, NCAR's global sulfate-aerosol modelers have already met with Durlak to discuss applications for their work.

Once the aerosol module is done, Durlak will add the data from Project Azteca. "I hope to plug it into a regional model [encompassing Mexico City] so we can see where the pollution goes and plan where to fly the airplane to get good data" during the field project. It might be possible to adjust the resulting model for use in other megacities.

Another module will address the photochemistry that takes place when hydrocarbons are oxidized and then interact with nitrogen oxides. Since these photochemical reactions sometimes result in long-lived compounds that may be transported long distances, they are important to understanding how urban pollution affects the regional environment. And finally, a chemical transport model is expected to be an important part of the study.

To get involved

Brasseur and Baumgardner are planning a community workshop in the fall, which they hope will be widely attended. Those interested are invited to contact Brasseur at 303-497-1456 or brasseur@ucar.edu. For more information about the Mexico City experiment, contact Baumgardner in Mexico City at 011-525-622-4053 or darrel@ucar.edu and look at the Web site for this project.

MIRAGE Advisory Panel

Gregory Carmichael, University of Iowa
Robin Dennis, NOAA/Environmental Protection Agency
Sylvia Edgerton, U.S. Department of Energy
Jake Hales, U.S. Department of Energy
Mario Molina, Massachusetts Institute of Technology
Anne Thompson, NASA Goddard Space Flight Center
Anne-Marie Schmoltner, NSF
Min-Xing Wang, Chinese Academy of Sciences, Beijing
Anthony Wexler, University of Delaware


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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Tue Apr 4 15:10:30 MDT 2000