RAPID ASSESSMENT OF AN URBAN HAZARD: SPECTROSCOPY OF THE WORLD TRADE CENTER DUST

On September 16th, 2001, five days following the collapse of the World Trade Center Towers, hyperspectral data was collected over ground zero with the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS). Our intent was to rapidly assess the asbestos hazards of the dust that blanketed a large portion of lower Manhattan. Within two days of the overflight, a two person team had collected ground samples of the dust and airfall debris from 35 sites within a 1- km radius of the collapse site including samples from two indoor locations unaffected by rainfall and samples of insulation from two steel beams at ground zero. Spectral measurements of dust-free cement pavement on the top level of a parking garage in New Jersey, located 3 km west of ground zero, were used to calibrate the AVIRIS data prior to mapping with the USGS Tetracorder spectral identification algorithm. The dust and beam-insulation samples were analyzed for a variety of mineralogical and chemical parameters using reflectance spectroscopy, scanning electron microscopy, X-ray diffraction analysis (XRD), and chemical leach tests.

AVIRIS mineral maps do not show widespread distribution of chrysotile or amphibole asbestos above the few-percent detection limit of the instrument at the ground surface, but do show a few isolated pixels of potentially asbestiform minerals. AVIRIS images were also used to locate hot spots in the debris pile hidden from view by smoke, thus allowing firefighters to more effectively battle the fires. Delivery of this information to emergency managers within two weeks of the attack demonstrated how rapidly these urban hazards could be assessed on a spatially-comprehensive scale with imaging spectroscopy. Spectral and XRD analysis of the field samples took several weeks longer and revealed that trace levels of chrysotile were present in 2/3 of the dust samples but at concentrations lower than 1wt%, well below the sensitivity level of AVIRIS. The field data suggest that trace levels of chrysotile were distributed with the dust radially in west, north, and easterly directions perhaps to distances greater than 3/4 km from ground zero. The lack of chrysotile at levels above the detection limits of both methods in all but one sample collected south of ground zero may indicate that chrysotile was not distributed uniformly during the collapse. More info at http://speclab.cr.usgs.gov