Plumes of inert gas are rising above Manhattan this week as part of a research programme to improve the US response to terrorist attacks. Researchers will track the fate of seven different gases to test and refine models of how chemical, biological or radioactive material might spread through the city.

Emergency services responding to attacks in built-up areas already have to balance helping the injured, evacuating survivors and sealing the site for forensic examination. But responding to an explosive release of harmful airborne material would be even more difficult. To decide how far to move people from the scene and in which direction, authorities need some idea of how material will spread, and how that is affected by factors such as the weather.

Data from the New York experiments, principally funded by the Department of Homeland Security (DHS), will be fed into computer models that can be run in an emergency, to predict how a plume of material will disperse over the following hours and to inform emergency services about how to respond.

It is impossible to test every material, in every location and in all conditions. But Tom Keiss, acting programme manager for radiological countermeasures in the DHS Office of Research and Development, says the results from such experiments could lead to models accurate enough for use in an emergency.

Researchers will monitor the movement of test gases across New York. Credit: EPA

“These models could tell us what sort of area first-responders should evacuate, and even the information you would give the public,” he says, such as safe routes out of a city. “We consider this to be a sound investment that can save lives.”

The gases are being released from an area of two square kilometres south of Central Park on six days (see graphic), determined by the weather, between 6 and 26 August. Using nearly 180 samplers and 35 weather stations, researchers from government and university labs will track the gases' movement through the streets and into buildings and the subway system.

The research is part of the four-year Urban Dispersion Programme, which began in 2003. Earlier tracer experiments took place in Oklahoma City and Salt Lake City, and a small-scale preliminary to the Manhattan experiments was carried out around Madison Square Garden in March.

Across the Atlantic, the British government is interested in tracer releases in London. It tested its models with data from a tracer experiment to examine air pollution in the city, and is deciding whether to fund a large-scale tracer-release programme in London.

“Experiments in New York are not much use to London or Paris. Our cities are very different physically, and have different climates,” says Alan Robins, a specialist in environmental fluid mechanics at the University of Surrey, Guildford, and chair of the London project.