VOC sampling locations table for Ground Zero area, Jan 2002

NYC 9/11 Public Portal Document

gravimetric and aerodynamic methods) of particles settled in dust collected September 16-17 at weather-protected sites just east of the WTC, Lioy et al. (2002) reported the largest mass concentrations in the settled dust to be due mostly to particles > 53 pm diameter (~ 51-64% of total mass) and 10-53 pm (~ 35-45% of total mass), followed by lesser percentages for 2.5-10 pm (0.3-0.4% of total mass), and < 2.5 pm (~ 0.9-1.3% of total mass) particles. Given the tendency of large coarse particles to settle out of the atmosphere closer to their emission source(s) than smaller fine particles, it is likely that higher percentages of small coarse particles (> 2.5 but <10 pm) and fine particles (< 2.5 pm) were more widely dispersed in the plume of dust and smoke that spread primarily to the southeast over Brooklyn and to the south over New York Harbor during the first 18 to 24 hours after the collapse of the WTC buildings on September 11.

Figure 5 shows the results of initial plume dispersion modeling for September 11. The predominant wind direction was to the south-southeast; and this direction continued well into the next day (September 12). As seen in Figure 5, the predominant direction of the modeled WTC plume flow is to the southeast, based on wind directions and speeds indicated by black arrows in this figure. This is consistent with photo images, such as the one shown in Figure 6.

Although no direct measurements of PM concentrations are available for nearby lower Manhattan areas during the collapse of the WTC, some rough estimates can nevertheless be made of what concentrations may have been reached. The dust cloud was optically dense, as can be seen fi-om the airborne images. Under such conditions, sunlight does not reach the surface, and visibilities are greatly restricted. Conditions such as these have been encountered in dust storms and in the London smog episodes of 1952 and 1962 (Elsom, 1992). During such conditions, PM concentrations could have been several milligrams per cubic meter (mg/m’), i.e., thousands of micrograms per cubic meter (pg/m’).

Particles smaller than 2.5 micrometers (fine particles) limit visibility much more effectively than larger (coarse) particles and under conditions usually found in the eastern United States, ambient air concentrations of fine particles are typically higher than those of coarse particles. As a result, under these conditions, visibility reductions are caused mainly by fine particles. There are a number of simple formulas that relate visibility to the concentration of PM2 5, such as one from Stevens, et al. (1984):

0.5 (km- mg/m’) = Vis (km)* C (mg/m’) (1)

where V is the visibility range (km) and C is the concentration of PM2 5 (mg/m^). During the collapse of the WTC towers, visibilities were reduced to less than 100 m (about 1 city block) on many streets. If we assume that visibility on streets in lower Manhattan affected by the dust cloud (see Figure 3) was controlled by fine particles, then application of the above formula indicates that PM2.5 concentrations could have been about 5 mg/m’ (5,000 |ig/m’). However, the collapse of the World Trade Center towers mainly produced coarse particles (Lioy et al., 2002), that, as mentioned above, are less effective than fine particles in controlling visibility. Thus, the values given above represent lower limits on the abundance of total PM. It should also be noted that the above estimate of visibility is based on the loss of contrast between light and dark objects. In many streets, sunlight was blocked and, hence, total PM concentrations could have

DRAFT-DO NOT QUOTE OR CITE 29 October 2002

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