Something's in the air in ACD: aerosols

UCAR > Communications > Staff Notes > April 1999

April 1999

Something's in the air in ACD: aerosols

Since NCAR's founding in 1960, world population has doubled from 3 to 6 billion. This map shows the 16 urban areas that qualified as megacities (populations above 10 million) in 1996. The number of megacities is expected to reach 24 by next year. (Data courtesy United Nations Statistics Division; illustration by Mike Shibao.)

Most of the Atmospheric Chemistry Division's major field projects over the past decade have taken place in sparsely populated areas: the Arctic and Antarctic, the middle of the Pacific. In these relatively pristine places, ACD has examined the farthest reaches of human influence on the atmosphere, with other variables kept to a minimum.

Now ACD is taking its field expertise to the world's second biggest urban complex, Mexico City, as part of a multiyear initiative to study the life cycle of aerosols and air pollution. The project involves a new ACD group, a shift in divisional focus, and potential collaborations throughout NCAR.

Aerosols (airborne particles of dust, soot, and the like, often mixed with liquids) are a confounding factor in global climate models because of their cooling influence on the earth and their secondary effects on cloud cover. They also have a more immediate impact in the urban areas where ever-greater numbers of people live and breathe (or try to). The Mexico City project will examine a rich stew of ingredients--pollutants, dust, organic emissions from vegetation, and volcanic products--all interacting amid the intense oxidation produced by a low-latitude, high-altitude sun. (The city lies near 19.5°N and at about 2,250 meters, or 7,400 feet.)

MIRAGE becomes a reality

The Megacity Impact on Regional and Global Environments project was born in an NCAR workshop last October. MIRAGE will use models, laboratory studies, and field projects to learn more about the physical mechanisms that produce and control urban pollutants, how the various pollutants interact, and especially how these chemicals are transported outside the urban areas to affect the larger environment.

Darrel Baumgardner. (Photo by Carlye Calvin.)

The leaders of MIRAGE are ACD director Guy Brasseur and Darrel Baumgardner (Atmospheric Technology Division). An external advisory panel includes Nobel laureate Mario Molina (Massachusetts Institute of Technology). Molina will also be a principal investigator in a major field phase planned for Mexico City in the summer of 2000.

"Guy and I think NCAR could make a big impact because of our expertise in the areas of transport modeling, photochemistry, and aerosol measurements," says Darrel. MIRAGE has attracted the interest of scientists from almost every NCAR division, including a number of postdoctoral fellows from the Advanced Study Program (ASP). In fact, Guy hopes that MIRAGE will mark a new direction not just for ACD, but for NCAR. Paraphrasing from Mark Hertsgaard's book Earth Odyssey, Guy says, "The big environmental problem of the future is not ozone depletion or global warming, 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."

Darrel has a head start on MIRAGE. He has been in Mexico City since October as a visiting professor at the Universidad Nacional Autonoma de Mexico, working with Graciela Raga, who is also a member of the MIRAGE advisory panel. In November 1997 the two scientists collaborated with Greg Kok (formerly in ATD, now a visiting scientist with ACD) on Project Azteca. This two-week study of Mexico City's atmosphere was based at an ecological park about 400 meters (1,300 feet) above the city.

Project Azteca resulted in "a lot of interesting measurements" and whetted the participants' appetites for a more in-depth study, says Darrel. Ozone levels were as high as 300 parts per billion, more than twice the U.S. and Mexican health standards. The daily increase in carbon monoxide (CO) and ozone was tied to abrupt wind shifts that brought pollutants up the hillside to the research site. The rapid increase "was like [watching someone] turning on a switch," says Darrel, as the CO levels rose by almost an order of magnitude within minutes.

The preliminary results suggest that motor vehicles are responsible for the aerosols that most affect visibility and public health. However, significant amounts of sulfur dioxide and sulfate aerosols stem from local volcanic activity under some weather conditions. "This complicates the interpretation of the data," says Darrel. "The pilot project produced a wealth of information but clearly shows the need for much more extensive observational and theoretical studies."

Next year promises to be one of ACD's biggest for field work in some time. As currently planned, the Mexico City project will immediately follow another ACD experiment called Tropospheric Ozone Production about the Spring Equinox (TOPSE), in which the NCAR/NSF C-130 will be used to study photochemistry as sunlight returns to the far north next spring. This timing will save money, since the C-130 will already be equipped and ready to roll. In fact, the aircraft may be able to simply extend a southward trek from Canada to Mexico City, collecting data all the way; such a long swath of measurements over a continent is rare.

If use of the C-130 is approved for MIRAGE, the aircraft will make six ten-hour flights during three weeks in the summertime rainy season. Another six flights in January 2001 would study the urban plume during the Mexican dry season. 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. A mobile aerosol laboratory is now being assembled in Mexico to take ground-level measurements of particle evolution. It will be used next summer by Stephanie Rivale and Monica Rivera, protégés in the Significant Opportunities in Atmospheric Research and Science (SOARS) program operated by UCAR.

A heterogeneous approach

One of the big unknowns in the atmosphere of Mexico City and other megacities is how gaseous, liquid, and solid components evolve over time. The study of such interplay is called heterogeneous chemistry. Knowledge of this field grew in the late 1980s as scientists began to understand the role of heterogeneous chemistry in the depletion of stratospheric ozone.

Fred Eisele and Dave Hanson. (Photo by Carlye Calvin.)

ACD has beefed up its expertise in this area with the arrival of scientist Dave Hanson. Dave joined NCAR in January after nine years at NOAA's Aeronomy Laboratory. He will lead a new ACD group with a focus on atmospheric heterogeneous chemistry. Some of the first few experiments will be chosen based on their relevance to MIRAGE. Dave is working closely with ACD scientist Fred Eisele, who has been steering the division's aerosol research for several years.

Dave is continuing work begun by Steve Ball, a recently departed ASP postdoc, carrying out "a few simple experiments" to see how particles are formed from sulfuric acid and water. "There are various models that predict a particle nucleation rate, but there are very few reliable lab measurements with which to test them," says Dave.

When ammonia is added to the mix, particle formation rates increase dramatically. "We need to work harder to better quantify this effect," says Dave. Ammonia is commonly found in the atmosphere and has both agricultural and industrial sources. In recent field campaigns, Fred and colleagues found that the rate of ultrafine particle formation was from 10,000 to 1 million times greater in the boundary layer than predictions from models that do not take ammonia into account. "We're finding a lot of discrepancies with the existing models and we're trying to find out why," says Fred.

The answer may lie with the smallest aerosols, as tiny as one to two nanometers. Most current analysis techniques in this range are geared only to counting and sizing particles. Fred is now building a mass spectrometer intended to provide particle composition as well as total mass for clusters as small as one to five molecules. That will allow study of the roles of other species, including organic molecules, in particle formation and growth.

"The interaction of organic molecules with particles, and the composition and atmospheric impact of organic particles, are hot topics," says Dave. He will be collaborating with ACD's Laboratory Kinetics group (including John Orlando and Geoff Tyndall) and ASP's Hans Friedli and Barb Noziere on the behavior of hydrocarbons and plant-emitted organic components, such as isoprene, in producing ozone and interacting with aerosols.

Once the sun gets to work, things get even more complex. As hydrocarbons are oxidized, they become more soluble and less volatile. This makes them more likely to produce ozone but also increases their affinity toward aerosols. It's a tug-of-war whose outcome remains unknown in the urban setting, says Geoff. "As [MIRAGE] gets closer, I'm sure we'll be focusing more closely on these partially oxidized compounds."

Modeling a megacity's air

As its title indicates, MIRAGE is concerned with more than Mexico City alone. Its focus is megacities: urban clusters of more than 10 million people. Fifty years ago, New York was the world's only megacity by this definition. By next year, there will be 24 megacities across the world, with 150 to 200 other cities topping the 1 million mark, according to the United Nations. 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.

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 developing areas. 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, current models are mostly statistically based, according to Guy. "We would like to build models more on first-order principles. . . . We'd 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."

Sue Durlak, a postdoctoral fellow in ASP, is developing the aerosol module, modifying a three-dimensional model of urban atmospheres developed by Tony Wexler (University of Delaware). Eventually her module will include "all the basic physics and chemistry of aerosols," she says. It could be used for a wide range of modeling activities. Sue is keeping in touch with other NCAR scientists who are modeling sulfates, such as Mary Barth of ACD/MMM. (See UCAR Highlights, for more on Mary's sulfate modeling.)

Once the aerosol module is done, Sue will compare its results to those from Project Azteca. "We hope to use that comparison to guide some preliminary field work leading up to the [year-2000] field project," says Sue. The module may be used to plan flight trajectories and carry out adaptive sampling during the primary MIRAGE field campaign.

Meanwhile, ACD's Sasha Madronich will be modeling photolysis (sun-produced chemical changes) across Mexico City and comparing his results to Rick Shetter's measurements of actinic flux, which serves as a tracer of photolysis. "If we can show an agreement," says Sasha, "then we can trust the models to give us actinic flux levels and photolysis rates everywhere in the region."

Sasha also plans to do some single-column modeling of the vertical variations of radiation and chemical composition. Results from Project Azteca indicated that the heating rate near the top of the mixed layer varied greatly depending on how the layer's pollutants were distributed vertically. To Sasha, this indicates that pollution isn't a passive bystander in the evolution of Mexico City's atmosphere.

"None of the current models allow pollutants to modify the radiative and dynamic environment. [These findings show that] you don't have polllution sitting in a neutral environment; it appears to be changing its own environment. It's an interesting problem that couples radiation, chemistry and dynamics." •Carol Rasmussen and Bob Henson

On the Web:
A Web site on the MIRAGE field work is located at
Megacity Impact on Regional And Global Environment.

Details on Sue Durlak's aerosol module are at
NCAR Aerosol Box Model Homepage.

A book tailor-made by ACD

It isn't just any book on chemistry and global change. For now, at least, it's the book. Published by Oxford University Press in February, Atmospheric Chemistry and Global Change is the first comprehensive, textbook-style guide to the field. The 672-page volume reflects the wide-ranging experience and hard work of its editors: Guy Brasseur, John Orlando, and Geoff Tyndall.

John and Geoff are both part of ACD's Laboratory Kinetics group, where they examine the behavior of small groups of molecules in detail. Their specialty is "tiger-team" studies with an emphasis on organic chemistry: tackling a particular project over two to six months, often in response to the needs of one or more ACD scientists. This approach lends itself well to postdocs and other visitors who want to gain lab experience with a single, time-limited project. "In that sense we've got a broad perspective on the whole atmosphere, just because of the many things we do," says group leader Geoff.

Guy asked his co-editors to join the book project in 1993. Over the ensuing five years, the volume demanded almost every molecule of the team's patience and energy. "It was a huge effort getting contributions and then trying to homogenize them," recalls Geoff. Since individual chapters involved as many as 12 authors, "even within chapters, there were huge mismatches in style." Out of roughly 45 authors altogether, about two-thirds were from ACD. Chapter authors from outside NCAR were Shaw Liu (NOAA Aeronomy Laboratory, now at the Georgia Institute of Technology); Sonia Kreidenweis (Colorado State University), Suzanne Paulson (University of California, Los Angeles), Mike Mozurkewich (York University, Canada) and Frank Dentener and Jos Lelieveld (Wageningen University, Netherlands).

John Orlando, Guy Brasseur, and Geoff Tyndall. (Photo by Carlye Calvin.)

The editors solicited a brief essay for each chapter from a leader on the topic covered by that chapter, to provide "a little bit of perspective," notes Guy. The book also includes more than 300 line illustrations. Ronna Bailey was the production coordinator for ACD, inputting and editing most of the copy. She did "an amazing job," Geoff says.

Atmospheric Chemistry and Global Change is intended as a primary or supplementary textbook for graduate students in atmospheric science. It could also serve as an invaluable resource for dissertations or other research projects. In the eyes of Geoff, that is reward enough for five years of sweat: "We thought it was a great thing to be involved in." •BH

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