NEW YORK -- Government scientists released colorless, harmless gas in four Manhattan locations yesterday as part of an effort to find out how fast and far a toxic substance could spread if released in the city.
''It went very well," said Susan Bauer, a spokeswoman for the Urban Dispersion Program, which aims to produce a computerized model of airflow patterns that could help authorities decide how to evacuate people after a chemical or biological attack.
The project started five years ago with pilot programs in Salt Lake City and Oklahoma City.
In March, gas was released in sections of Manhattan to allow scientists to learn about outdoor air patterns in the city. This month's tests will track how gases move in and out of structures.
''You can use those models to say, 'What if something happened here?' " said Jerry Allwine, an engineer with Pacific Northwest National Laboratory in Richland, Wash., who oversees the project.
The gases were released yesterday in an office building and at three outdoor locations in midtown, Bauer said. There will be five more test days over the next three weeks, depending on the weather.
Air samples will be taken by tracer boxes fastened to light poles and stationed on subway platforms and from smaller boxes clipped to the belts of volunteers.
The scientists will conduct a third set of tests in New York next March and plan to complete their research by 2007.
The $10 million project is sponsored by the federal departments of homeland security, defense, and energy.