EM February 2004 23 EM EM Feature INTRODUCTION The U.S. Environmental Protection Agency’s Office of Research and Development (EPA-ORD) was requested by EPA’s Region 2 office in New York on September 12, 2001, to assist with air quality monitoring in response to the collapse of the World Trade Center (WTC) twin towers. Scientists at EPA-ORD’s National Exposure Research Laboratory (NERL) in Research Triangle Park, NC, quickly assembled a field team and ar- rived in New York on September 16. There, in coordination with the on-site Emergency Response Team (ERT) headed by the U.S. Coast Guard, EPA-ORD outlined a monitoring strategy to support the efforts of the ERT. The ERT also worked closely with EPA’s Office of Air Quality Planning and Standards to set up an asbestos monitoring network in lower Manhattan. Because the full extent of the hazards associated with 9/11 were not yet known, EPA-ORD set out to characterize the on- going emissions associated with the fires and the materials generated during debris removal, especially particulate matter (PM). The initial collapse of the WTC towers produced in ex- cess of one million tons of pulverized debris and combusted materials—both respirable and nonrespirable PM, including asbestos fibers and other potentially irritating materials—that blanketed the lower Manhattan area. Debris removal was likely to resuspend respirable-sized PM (<10 micrometers, PM 10 ), which could migrate into inhabited portions of the city and, therefore, it was important to characterize the levels of pollut- ants in the settled dust as well. The smoldering fires generated products of incomplete combustion, including partially oxy- genated organic compounds and polycyclic aromatic hydro- carbons (PAHs), as well as fine PM (PM 2.5 ), which can penetrate deep into the lungs. This article summarizes the efforts under- taken by EPA-ORD in response to the WTC disaster and pre- sents results on the monitored air pollutants. APPROACH EPA-ORD established an air pollution monitoring network in conjunction with EPA Region 2 to identify and characterize the levels of pollutants emitted by the underground fires burning at the site. Site selection and monitoring occurred in three phases to determine the levels of air pollutants: (1) in close proximity to the WTC site, (2) further from the WTC in an area that could represent community exposures, and (3) addi- tional and more sophisticated monitoring equipment in the outlying community to estimate the public’s exposure to specific hazardous pollutants. Three monitoring sites were chosen that triangulated the WTC complex, commonly referred to as “Ground Zero,” to characterize emissions under various wind directions (see Figure 1). Monitoring began at these sites, which were located approximately 100 m from Ground Zero, on September 21, 2001. The locations close to Ground Zero were chosen to better characterize the concentrations of pollutants emitted from the source, but also to determine the source “signature” (i.e., the relative amounts of specific pollutants that can be attributed to a single source), so that results from monitors further downwind could be compared with this signature to determine the potential downwind impact of emissions from Ground Zero. The air in every city contains a mix of gaseous and particulate pollutants derived from day-to-day activities, including motor vehicles and local or regional industrial emissions, therefore, differ- entiating the contribution of WTC pollutants from urban background pollutants required an accurate assessment of the relative abundances of a wide variety of air pollutants derived from the source. Additionally, the monitoring data could be used as inputs to transport and dispersion models to estimate downwind concentrations and estimates of com- munity exposures. At the sites closest to Ground Zero, EPA-ORD had to rely on battery-powered equipment since the power grid to lower Manhattan was shut down in the days following 9/11. This greatly limited EPA-ORD’s capabilities, but the field team was able to identify equipment that could collect sufficient vol- umes of air to measure the pollutants necessary to characterize the fires (see Table 1 of Vette et al. 1 ). On September 22, a site 0.5 km from Ground Zero was established at 290 Broadway, the building where EPA Region 2’s offices are located. The Air Pollution Measurements in the Vicinity of the World Trade Center Summary of Measurements Conducted by EPA–ORD by Alan Vette, Robert Seila, Erick Swartz, Joachim Pleil, Laura Webb, Matthew Landis, Alan Huber, and Daniel Vallero