INDUSTRY

By Oliver Baker for NCSA -- CHAMPAIGN, IL--As high-school chemistry may have taught you, it's easy to calculate the odds that a molecule you've just inhaled came from the dying breath of Julius Caesar. But imagine you need to account for the molecule's experiences in the meantime. Say you have to weigh the chances of each chemical transformation it might have undergone -- as it ascended from ancient Rome, drifted through the dirt and droplets of clouds, and endured temperatures, solar rays, and collisions with gaseous chemicals in changing combinations. Now for extra credit, explain how the global distribution of gases shifts and evolves. Include all life, pollution, inanimate matter, and climate change. You might raise your hand and ask to use a supercomputer. In fact, Don Wuebbles and Ken Patten of the University of Illinois at Urbana-Champaign and collaborator Rao Kotamarthi of Argonne National Laboratory did just that. Through simulations on an NCSA Origin2000 supercomputer, they are examining the lives of molecules in Earth's atmosphere and contemplating the planet's fate. Most closely they are watching ozone. At stratospheric heights, ozone shields Earth from solar radiation. But lower down it is a toxin, produced when benign gases react with car and factory pollution or with lightning. In collaboration with the U.S. Environmental Protection Agency (EPA), the researchers are evaluating alternatives to such notorious ozone-destroying compounds as chlorofluorocarbons (CFCs). With other simulations, they are pinpointing the sources of plumes of pollution above the supposedly pristine south Pacific. Ultimately, they want to predict the pace at which the protective ozone layer will recover under different international emission control agreements. The researchers comprise one of a few teams charged with testing new software that simulates the chemistry and migration of gases around the globe. This software -- a revamping of the Model for OZone And Related chemical Tracers, or MOZART-2 for short -- feeds on output from a global climate simulation and takes twice as much computer power as the climate simulation. The chemical transport model is a project of the National Center for Atmospheric Research (NCAR), which intends to release MOZART-2 by the end of this year. Testing how well the model works and using it to answer questions are intertwined activities, says Kotamarthi. Because the complexity of global processes involves so many variables and uncertainties, a "validated model" is rarely an option. "The preferred word around here now is 'evaluated' model," he says. Wuebbles, Kotamarthi, and their collaborators recently finished "probably one of the harshest tests MOZART was ever subjected to," says Kotamarthi. It took him aboard a NASA jet on missions across the south Pacific as an "onsite modeler." A modified DC-8 passenger plane with air inlets and instrument stations for 30 to 40 scientists, the plane flew up to 40,000 feet and made spiral-staircase descents to 1000 feet above the ocean at sites along its route. "The idea is you are trying to sample the vertical profile of trace gases at various altitudes," Kotamarthi says. The air of the remote south Pacific is supposed to be squeaky clean, and it generally was.ÊBut the scientists were surprised by fingers of polluted air -- "dirtier than maybe in the LA city area on a bad day," says Kotamarthi. These fingers seemed to extend thousands of miles in strips only a few thousand feet thick. On the ground, Kotamarthi and fellow modelers compared the flight measurements to MOZART-2 predictions for the same region and time of year. The concentrations of ozone differed by about 30 percent, Kotamarthi says. But the model tracked seasonal changes. It also predicted dirty fingers like those the scientists encountered, identifying them as the listing plumes of fires in subequatorial Africa or South America that farmers light seasonally to clear rainforests and grassland for cultivation. To see the simulated plumes, the modelers had to double the burning emissions they had first assumed. But scientists' best estimates of the dirtiness and extent of such fires are just rough averages, says Kotamarthi, and they relate to 1991. The fires that produced the plumes NASA detected on its 1996 and 1999 south Pacific flights may have generated more pollution, he says. The model was further handicapped by having to work from generic seasonal winds created by a computer. While MOZART-2 can't compute global chemistry from weather measurements alone, the modelers have begun using artificial weather steered by real data for specific years. A precise assessment of how MOZART-2 and the Pacific measurements disagree awaits the results of these runs, Kotamarthi says. But he says the team has already acquired confidence in using the model to answer basic questions. Some of these questions have immediate practical importance. For example, Wuebbles worries that many proposed alternatives to CFCs are not so benign as scientists have supposed. Using MOZART-2, he has set his team to examining them more rigorously before an international timetable requires countries to choose which ones to allow. Many of the new compounds that chemists have engineered are like CFCs, yet flimsier, so the molecules degrade before they reach the stratospheric ozone layer almost 100 percent of the time. Nevertheless some kinds of degradation reactions stall, Wuebbles warns, generating not-yet-neutralized compounds that are heartier than their progenitors. Wuebbles says such scenarios need to be considered. To make his case, he points to results his team reported in the July 16 Journal of Geophysical Research. From manuals and chemistry reports, the researchers culled constants for calculating how fast and in what way n-propyl-bromide (a CFC substitute) and its breakdown products react with different molecules in the atmosphere. Feeding the numbers to MOZART-2, team members then simulated the release of this compound from the equator and points north and south. The high and low latitude launches took n-propyl-bromide on a slow journey to the stratosphere that few molecules survived in any form. But trips from the tropics were fast and delivered many molecules in a merely maimed state -- in which they were as dangerous as CFCs. The results are surprising, says Jeff Cohen, chief of the EPA branch formulating U.S. recommendations on CFC alternatives for the United Nations. He says the inventors of n-propyl-bromide are already manufacturing and selling it in anticipation of a multinational thumbs up. "I think some countries will be inclined not to allow its use," he says. Wuebbles next wants to simulate how the ozone layer will recover under alternate schedules for the phase-out of CFCs and their successors. "It's important for us to know just what it's going to take for ozone to recover," he says. Because the repair is liable to last decades, the simulations will need to account for climate change -- both how warming impacts ozone and how ozone impacts warming. No doubt exists that ozone and the climate interact powerfully, says Wuebbles. Antarctica would have no hole were its winters slightly balmier, he says, citing the comparative health of Earth's opposite pole as illustration. Meanwhile, as the planet's ultraviolet shield, ozone controls how much sunlight penetrates the atmosphere; and as the third most influential green-house gas after water and carbon dioxide, it regulates how much of that energy the Earth retains for warmth. Finally, the climate and ozone are connected through their interdependence on the ecology carpeting Earth's surface. The time is ripe for learning how such interactions play out, Wuebbles says. Already NCAR scientists are fitting a stratosphere around MOZART-2's naked troposphere (the layer that holds the clouds and weather) so that the ozone layer and hole may be part of future simulations. Patten is preparing to extend the study of CFC substitutes into the unique chemical environment high above the south pole, which he will do on NCSA computers next year. And NCAR, Wuebbles team, and other collaborators are arranging a duet between MOZART and BACHa Biosphere-Atmosphere-CHemistry model that will include ecology. Harnessed to a computer, this fused application would be the most gargantuan ever to draw the Earthand not every student of the atmosphere will be able to run it. Wuebbles predicts that he and his collaborators will soon be raising their hands again for supercomputing time from NCSA.