The simulation, which will use colorless, odorless and harmless "tracer" gases, follows a smaller effort in March that focused on the Madison Square Garden area. This time, a team of more than 150 researchers and volunteers working on the Urban Dispersion Program's second field study will fan out over a much larger section of midtown, ranging from 37th to 59th streets and from 10th to Third avenues, and including stations along the Broadway subway line and other lines in the area.

Scientists will release the gases at four of eight possible locations above ground, depending on the wind, within an office building and on a subway platform of the Broadway line at 50th Street and Seventh Avenue.

Six separate experiments are planned over the next three weeks, with specific dates dependent on the weather, and the subway portion planned for the final three release dates.

"We'll be looking at the interaction of the subways in how the gases are dispersed," said Paul Kalb, a senior research engineer at Brookhaven National Laboratory. "If we release on the ground level, the street level, some of the material may be drawn into the subways and moved around that way."

Likewise, releasing a gas in the subway will tell researchers whether that gas remains within the tunnel system or drifts up to street level, and how it disperses.

One of the substances is known as PFT, or perfluorocarbon tracer gas, a safe and extensively used labeling compound. Jerry Allwine, the project's lead scientist and an engineer with the Pacific Northwest National Laboratory in Richland, Wash., said the second type of benign tracer gas, known as sulfur hexafluoride, also has been used for years. With it, scientists can measure exposure rates at one-second intervals.

To collect the reams of data expected from the experiments, electronic monitors will be positioned on rooftops, in subway stations, in baskets hanging from lampposts, in a half-dozen unmarked vans, and in the pockets of volunteers simulating the movements of pedestrians and subway riders.

Another experiment in a midtown office building will analyze how air flows inside and exchanges with the air outside.

For more information, see http://urbandispersion.pnl.gov.