Heading Off Terrorism, Scientists Model Airflow Transport

July 24, 1998

By Paul Preuss, [email protected]

Chemical and biological terrorism is quickly moving off the paperback-thriller page and the movie screen into the real world. Last May, only four days after President Clinton announced a presidential directive to strengthen the government's management of chemical and biological crises, The New York Times revealed that the Aum Shinrikyo cult, which in 1995 killed a dozen people by releasing nerve gas in Tokyo subways, had on nine previous occasions sprayed deadly organisms, including anthrax, over wide areas of Tokyo and nearby U.S. military bases. Bad weather and weak strains of germs apparently blunted those attacks.

EETD's Helmut Feustel, Joan Daisey and Rich Sextro -- shown here holding a gas mask -- are working to mitigate the effects of terrorist attacks before they happen.
Photo by Don Fike

"So far our domestic terrorists seem to prefer bombs," says Joan Daisey of Berkeley Lab's Environmental Energy Technologies Division (EETD), "but an effective chemical or biological attack, although a rare event, would have terrible consequences. We can't ignore the possibility."

Since 1996 the Department of Energy has mounted a multi-laboratory effort to improve response to terrorist attack through its Chemical and Biological Nonproliferation Program. One area of study is known as "Transport and Fate" -- an innocent (if ominous-sounding) term for what happens to gases and particles as they travel through buildings, subways and urban areas.

"Our goal is to support incident-response teams," Daisey explains, "not only by helping them plan what to do in real time, but by seeing what modifications can be made to mitigate possible effects in advance."

Transport and Fate team members began by surveying virtually all existing computer models that could simulate chemical and biological releases in an urban environment -- even though none of the models were designed with terrorism in mind.

The aim is "to provide rules of thumb to first responders," says Daisey. "Because we need to get something out there right away, on the first cut we assume only air flow matters, that there are no losses of the chemical or biological agent."

The next step is to refine the models. "On the next cut, we need to know where the contaminants go. For one thing, that tells us where to put detectors."

Of the four DOE national laboratories in the Transport and Fate collaboration, says Daisey, Los Alamos and Livermore are concentrating on outdoor and urban areas, Argonne is studying subways, and Berkeley Lab studies buildings.

All the labs are working to develop state-of-the-art modeling capabilities, which will be tested in case studies.

Subways have peculiar problems, including the piston effects of trains moving in tunnels and the sometimes subtle interconnections of subways, buildings, and streets. As for outdoor spaces, "even though pollution studies have driven outdoor models for 50 years, there are still unanswered questions," Daisey says.

Energy-efficient buildings have been a major research area for EETD since the oil crises of the 1970s. Over a period of eight years, an international team led by Helmut Feustel of EETD developed a computer model dubbed COMIS (Conjunction of Multizone Infiltration Specialists), which treats buildings as interlacing systems of paths along which air masses flow among hundreds of separately defined zones. COMIS is the basis for DOE's Transport and Fate work on interiors.

Despite many years of work, the COMIS model must be extended, although, says Daisey, "we know where many of the holes are. We haven't done well with stairwells, for example, because of the temperature gradient, and as yet we can't predict dispersion in large rooms where the air is not well mixed." Modeling only a minute of real time with adequate computational fluid dynamics can take a long time; new techniques are needed -- specifically "better and faster lumped parameter modules" -- to extend the COMIS model.

When it comes to modeling, "accounting for deposition losses of chemicals is relatively straightforward; we have to study chemical interactions, including absorption and desorption," Daisey says, but of the chemicals and gases used by terrorists, "a lot of them are just glorified pesticides." She notes that although chemicals are typically quick acting, if released inside a building they may reach the outside only very slowly.

Biological agents are a different matter. "In the current COMIS model we treat bio-aerosols as collections of particles with a single average deposition rate, but we are now integrating the MIAQ4 model developed by Bill Nazaroff and Glenn Cass to account for particle deposition by size, surface orientation and air turbulence. Particle size, air movement, and air filtration all affect deposition losses in buildings." Nazaroff is with Berkeley Lab's EETD and UC Berkeley, and Cass is with Cal Tech.

Since air in most buildings is recirculated, germs may be widely transported within minutes. Yet depending on how quickly the disease organisms take effect, it may be days before a biological attack is detected, if ever. Daisey points out, however, that "filtration systems don't just protect against terrorism; we always have aerosols with us. Good filtration can reduce the incidence of colds and flu."

Accurately modeling the transport of contaminants in a building is one challenge; just as important is correctly characterizing the building.

"Complex buildings change from season to season. If you want to evacuate the building, do you shut off the HVAC systems? For how long? If there are two systems slopping over, it matters," Daisey says.

EETD's Rich Sextro and Helmut Feustel are leading the effort to improve methods for characterizing complex buildings in order to make accurate modeling of airflow and pollutant transport possible.

Meanwhile, in an effort to get useful information to the quick-response teams as soon as possible, Daisey and her colleagues have concentrated their efforts on the kinds of buildings that offer the most attractive targets. "In terms of airflow and other variables, we're trying to generalize office buildings, shopping malls, auditoriums, hospitals, and so on," she explains. "Terrorists are not likely to strike a residential suburb. The targets are places with lots of people."

Attempts -- so far inept -- to carry chemical and biological terrorism to the U.S. homeland have already been made. The Lab's EETD researchers hope that their efforts, in partnership with Livermore, Los Alamos and Argonne, will help to mitigate any such attempts in the future.

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