Which way would airborne toxic gases blow during a terrorist release?

In the event of a terrorist release of a harmful chemical, biological or radiological (CBR) agent as a weapon in an urban area, the motion of the gas and particles is unpredictable.  Factors that contribute to the complexity of the prediction problem include the corrugated building landscape, local surface heating (e.g., asphalt and building sides) and evolving meteorological conditions.  Much can be done to reduce this predictability gap.

Computer simulation of midtown NYC with ~6 m resolution showing contaminant dispersion from an instantaneous release in Times Square, New York City as predicted by the Naval Research Laboratory FAST3D-CT model. The frames show concentrations at 3, 5, 7, and 15 minutes after release. The simulation illustrates how a gas moves alongside a tall building, drawing the gas into higher, swifter air where it can rapidly materialize at distances downwind and impact the population many blocks away. (Courtesy of Jay Boris)

Downtown Manhattan remains a significant terrorist target

Almost ten years after 9/11, continued progress on the new WTC site heralds the revitalization of the downtown commercial and residential sectors.  In addition, leisure, tourist and commuter traffic is projected to soar with the transportation hub (Calatrava-designed PATH and subway station), 9/11 memorial and urban park collocated at the WTC site.  The NYPD is increasing its presence in the area by establishing a WTC post at the 9/11 memorial.  Over 600 officers will staff the command post.  With the Goldman Sachs headquarters now located across the street from the WTC site and other major financial entities housed in the adjacent World Financial Center, the threat profile of this section of the NYC continues to be raised.

Significant security concerns exist for both the WTC downtown area and the Wall Street area a few blocks away that contains the Federal Reserve and New York Stock Exchange. A network of surveillance cameras has been deployed as part of the Lower Manhattan Security Initiative.  And at the WTC site itself, NYPD Commissioner Ray Kelly noted that as part of the June 2010 World Trade Center Strategic Security Plan:

“…the Port Authority has agreed to integrate all of the security technologies deployed throughout the site (including more than 3,000 closed-circuit TV cameras) into the NYPD’s state-of-the art Coordination Center, where they can be monitored 24/7 by police personnel.”

Through the Department of Homeland Security (DHS) Securing the Cities program radiation detectors on NYPD and fire department boats and helicopters, along with chem/bio detectors, bridge the technology gap and facilitate detection.  But what happens if the terrorist, despite cameras and sensors, manages to commit an attack with a chemical/biological/radiological weapon? Can we effectively warn and evacuate the most severely impacted parts of the city?

A pressing need for airflow studies of downtown Manhattan

I was a principal investigator and member of the management team of the DHS Urban Dispersion Program.  By releasing harmless gas in midtown Manhattan in 2005 and tracking where it went, we were able to discern much about the patterns of transport in a complicated urban environment.  What we learned is nicely summarized here.  The upshot is that there is so much that we do not know about how airflow around particular buildings, changing surface heating conditions, and evolving meteorology conspire to drive the movement of a potentially harmful gas.

Also, different locations can have very different weather regimes, and thus have different air transport patterns.  Downtown Manhattan has a different “microclimate” than midtown Manhattan.  So although there are some basic principles that will apply at both sites, there are many surprises waiting for us.  For instance, the concentration of very tall buildings is much denser in lower Manhattan compared with midtown.  This could have a channeling effect on the gas, causing it to flow more quickly down the narrow, tall urban canyons.  In addition, downtown Manhattan is much more exposed to the river and ocean environment that often generates strong and fluctuating winds, such as sea breezes.

By conducting a field experiment in downtown Manhattan to release tracer gas in multiple locations at different times of the day and night and under different weather conditions, we can build a database of information.  This information would consist of not just the concentration of the gas at different locations, but also the air temperature and winds throughout the city.  Fortunately, as a legacy of the Urban Dispersion Program, we have a network of meteorological sensors that can be employed for this task.

We can then use this information to improve airflow models configured for local city areas, such as the one shown above.  And we can continue to make these models more sophisticated, such as linking them with a weather forecasting model, in order to enhance the realism of the predictions.  We need to be prepared with the detailed knowledge of how a CBR contaminant would move through the unique urban environment of downtown Manhattan.  And we need to be prepared in advance so we can save lives in the event of a terrorist CBR attack on lower Manhattan.

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