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The collapse of the World Trade Center on September 11, 2001, and the
fires that followed produced a noxious smoke plume, a complex mixture of
tiny airborne particles and gases. Determining where humans may have been
exposed to these contaminants and the amount of their exposure is very
difficult. But recently, scientists from the Environmental and
Occupational Health Science Institute Robert Wood Johnson Medical School
and Rutgers University, in partnership with the Environmental Protection
Agency and NASA's Jet Propulsion Laboratory have created a detailed
numerical model that shows the pollutant dispersion from "Ground Zero" to
the surrounding New York - New Jersey region.
Air pollution in the urban atmosphere can damage human health, biological
systems, and vegetation. A team of science assessment experts is using
this new computer model to analyze the environmental and health impacts of
this extreme event.
For example, researchers are using model results in studies of the birth
weights of infants whose mothers were near the World Trade Center on Sept.
11, 2001, when they were pregnant and of the incidence of asthma during
the first weeks after the attack. The model helps them estimate human
exposure levels based on where the plume was located, how much material
it contained, and how long it remained.
To map the path of the plume of pollution from the World Trade Center, the
researchers used mathematical models of micrometeorological atmospheric
circulation and tracer transport, surface measurements, and space-based
observations from the high-resolution Landsat imager and the Multi-angle
Imaging SpectroRadiometer (MISR) on NASA's Terra satellite.
While some airborne material circulated within the urban "canyons," much
of the material was lifted above the buildings and transported great
distances. As a result, surface level exposures were not as frequent and
intense as they could have been under different meteorological conditions.
To simulate the transport of pollutants in the plume, the researchers had
to understand its behavior on scales ranging from tens of meters to
several hundred kilometers. They calculated atmospheric motions using a
multi-grid regional atmospheric modeling system covering scales from 250
meters to 300 kilometers.
To calculate pollutant transport, they used a hybrid particle and
concentration transport model. They evaluated their model's simulated
pollutant concentrations, transport, direction, and timing, by comparing
the results with fine-scale aerosol measurements routinely acquired from
the roofs of New York City public school buildings.
The researchers calculated the evolution of the plume using a "Lagrangian"
transport model, which considers the plume to be a collection of parcels
that flow downwind. They found that for the model results to match the
observations, the peak aerosol emissions in the World Trade Center fire
must reach about 35 to 350 kg of particles per hour -- thus, the
concentration of aerosols within the core of the WTC plume was higher
than the routine aerosol pollution in the world's most polluted cities.
Since wind speed and direction can change dramatically at different levels
in the atmosphere, knowing the height of the aerosol plume was a crucial
part of ensuring that the model produced realistic results. JPL scientists
used stereo images of the region acquired by the MISR instrument at about
noon on September 12, 2001, combined with ground-based photographs of the
plume, to determine the plume height. A natural color MISR image appears
here (acquired by MISR's 70ยบ forward-viewing camera on September 12) along
with histograms of stereo-derived elevations at four points (P1, P2, P3, P4)
progressing from the World Trade Center source to about 70 kilometers
downwind.
In addition to plume altitude and direction, MISR also provided
information about plume evolution. Researchers could trace the plume's
development by comparing the combination of airborne particles observed
near the plume's source with particles that appear downwind in this MISR
snapshot. The researchers also use MISR observations to check how well
their model estimates the amount of particulate material the plume
contained.
The scientists published their findings in the July issue of the journal
Environmental Fluid Mechanics. The full citation is: Stenchikov, G., N.
Lahoti, D.J. Diner, R. Kahn, P. Lioy, and P. Georgopoulos (2006).
Multiscale plume transport from the collapse of the World Trade Center on
September 11, 2001. Environmental Fluid Mech., doi
10.1007/s10652-006-9001-8.
MISR was built and is managed by NASA's Jet Propulsion Laboratory,
Pasadena, CA, for NASA's Office of Earth Science, Washington, DC. The
Terra satellite is managed by NASA's Goddard Space Flight Center,
Greenbelt, MD. JPL is a division of the California Institute of
Technology.