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Fall 2001

ACD's new director eyes the division's future

by Bob Henson

Daniel McKenna. (Photo by Carlye Calvin.)

False facts are highly injurious to the progress of science, for they often endure long; but false views, if supported by some evidence, do little harm, for every one takes a salutary pleasure in proving their falseness.

This quote from Charles Darwin's The Descent of Man is a favorite of Daniel McKenna's. The new director of NCAR's Atmospheric Chemistry Division stresses the importance of rock- solid measurement technique. "Most of our work is in observations, so there is an asymmetric responsibility. The observations really have to nail down the numbers."

ACD specializes in measuring aspects of our air that can't be seen easily, if at all. Its fleet of instruments is one of the world's best at ferreting out trace species. With these data in hand, scientists can deduce unseen, often lightning-quick processes that help shape the atmosphere's evolution both locally and globally. McKenna is now working with the division's 80-plus staff (and with 20 to 40 visitors at any time) to figure out where to focus ACD's instrumental and numerical eyes in the next several years. The process will include observations from field studies and satellites, as well as modeling tools such as MOZART, the Model for Ozone and Related Chemical Tracers.

McKenna, a native of Scotland, came to NCAR last year from the Forschungszentrum Jülich in Germany, where he headed its institute for stratospheric chemistry. Before that, McKenna led the atmospheric chemistry section of the United Kingdom's Meteorological Research Flight, operated by the UK Met Office. Since he arrived at ACD, McKenna and the division have been considering several potential scientific themes.

Biogeochemistry is particularly hot right now. Already, says McKenna, "we do a lot of fundamental work in making measurements of biogenic emissions, which are a very important aspect of the climate system." ACD plans to collaborate with NCAR's Climate and Global Dynamics Division to bring more detailed chemistry into the NCAR Community Climate System Model. (An updated CCSM is due for release this winter; watch for more details in an upcoming UCAR Quarterly.)

The reactive side of carbon

While global warming has put carbon dioxide and other industrial emissions in the spotlight, much of ACD's focus is on carbon in its organic state. "We tend to say in ACD that CO2 isn't really a chemical. It is, of course, but it's so inert that there's not much chemistry of interest to us." However, he adds, "because of the coupling between nitrogen and carbon, there are huge biological impacts" from CO2.

The question marks swirling around organic carbon (those forms bound to hydrogen) have drawn keen interest from ACD and colleagues elsewhere. Because carbon is "four-valent"—it can form up to four chemical bonds—"the possible number of chemical species that can be made is almost infinite," McKenna says. "The basic stability of the CH [carbon-hydrogen] bond means that many of these compounds will also persist to some lesser or greater degree in the atmosphere." In aerosol form alone, McKenna suspects, there may be 10 to 20 different species of organic carbon. "These are hypothetical in the sense that they're reasonable products of organic chemical oxidation. Some of them have been measured in the atmosphere, but most have not. I think we're beginning to get the capabilities to see whether some of these hypothetical molecules really exist—or, if not, why they don't."

Over the past few years, atmospheric chemists have confirmed the importance of organic carbon in the form of isoprene (C5H8), the primary nonmethane hydrocarbon in the atmosphere. Emitted en masse by forests, isoprene is so prevalent (some 500 teragrams in the global atmosphere) that it's an ideal starting point for developing models of other types of organic carbon. To see how isoprene and other reactive carbon species behave on the small scale, ACD researchers are using a "master mechanism" box model developed by Sasha Madronich.

In the box model, "you release isoprene in the atmosphere, degrade it with OH [the hydroxyl radical], and let all the other chemistry work." The results can be bewilderingly complex, says McKenna. "An isoprene scheme can have over a thousand chemical species. Certainly that's not suitable for climate models. The complexity of this chemistry is much greater than for all of the species we've tackled up to now." Yet, he notes, "it's the basic fuel for ozone production in the troposphere—burning carbon and oxidizing it into carbon dioxide. We're thinking about what we can do in this area."

ACD's James Smith is working on spectrometers that can analyze some of the more oxygenated species of organic carbon, which can be as small as 4 to 20 nanometers in diameter. According to McKenna, "If you can measure the organic content of these nanosized particles, then you've got an extremely powerful [tool] for measuring carbon species in the atmosphere at much smaller concentrations."

Pollution beyond city limits

Along with reactive carbon, ACD also hopes to mount a long-term initiative on regional to global scales to understand the influence of megacities, urban clusters with over ten million residents. The idea is to go beyond municipal borders in analyzing air quality. Urban-area studies are already being performed by the U.S. Environmental Protection Agency, among others, and "we wouldn't want to try to duplicate that work," says McKenna. Instead, ACD is proposing to examine areas hundreds to thousands of kilometers downwind from megacities, where surprising interactions may take place.

The Megacity Impact on Regional and Global Environments (MIRAGE) experiment will examine whether emissions released from megacities produce greater amounts of ground-level ozone far away from the source area than occurs if the same emissions are distributed over a greater geographical area. "If you have a dense aerosol cloud, it tends to shut off the sunlight and slow down the photochemical processes. It's going to wait till it's farther out of the city before it starts to cook." Also, very high emissions within a city can put a stopper on ozone production because a needed catalyst—OH—gets caught up in creating nitric acid. Once the air blows out of town and gets diluted, "the OH concentration becomes somewhat higher and the chemical processes can go that much more quickly."

The proposal for a MIRAGE project is now being examined. The challenge, says McKenna, is finding a megacity that's roughly representative of others around the globe. The capital of Mexico played host to a pilot study in 1999, but "Mexico City's a fairly unusual city," with its high altitude, low latitude, and nearby volcanoes.

"One of the things we're asking is how relevant the results will be from one or two cities when we translate them forward. At some point you want to scale up what you've learned. What fuel do the residents burn? Is it biofuel for cooking that creates the pollution, or is it more [due to] combustion engines? If your test city is very different from a typical one—if there is such a thing as a typical megacity—you're on weaker ground in arguing about the relevance of the results."

ACD is enlisting NCAR's Environmental and Societal Impacts Group, led by Robert Harriss, for help in its analysis of candidate megacities for MIRAGE. Madronich, the project lead, is seeking external input as well. A community workshop may be held in Boulder within the next few months, although any field phase is at least a couple of years away, says McKenna.

The path to HIRDLS

With ACD's instrumentation in perennially high demand, the division continues to play a role in many other field projects and satellite ventures (see links below and the ( President's Corner). ACD's John Gille (also with the University of Colorado) is chief U.S. scientist for the High Resolution Dynamics Limb Sounder (HIRDLS), which is scheduled for launch in 2003 aboard the Aura mission of NASA's Earth Observing System. Later in the decade, the NSF/NCAR High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) is expected to be in service, providing another new window on upper-atmosphere processes.

McKenna expects that HIRDLS, HIAPER, and new theoretical approaches could help to revive research in lower statosphere–upper troposphere interactions pioneered at NCAR in the 1970s by Melvyn Shapiro (now at NOAA) and the late Edwin Danielson. This research may help dynamicists explain how the lower stratosphere influences weather events at the surface days later. "Knowing the chemical composition of air masses in these complex mesoscale features gives you an idea of where the air masses have actually come from, and so it helps to verify what your model is telling you about the air motions and trajectories," McKenna says.

Here and elsewhere, the importance of solid data may be taken for granted, but it's never forgotten by McKenna. "We all live and die by the observations, and if they're wrong, then we're barking up the wrong tree."

On the Web:
ACD
HIRDLS
HIAPER


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UCAR > Communications > UCAR Quarterly > Fall 2001 Search

Edited by Bob Henson, bhenson@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Tue Oct 23 11:26:05 MDT 2001