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In the weeks following the World Trade Center (WTC) Disaster in New York City (NYC), it was presumed by many that particulate matter (PM) air pollution in Lower Manhattan was dominated by pollution from Ground Zero. But NYC is a city with numerous pollution sources. So we sought to determine the nature and mass contribution of fine particulate matter (PM2.5) measured at a community-oriented site in Lower Manhattan that was associated with the WTC dust and combustion plume. We analyzed PM2.5 samples collected once or twice daily by our monitor at the NYU Downtown Hospital, located some 5 blocks east of Ground Zero, from Sept 14 through December, 2001. Each sample was analyzed for trace elements via X-ray Fluorescence and for elemental carbon (EC) via reflectance. Positive Matrix Factorization (PMF) source apportionment analysis was applied to determine the PM components contributing to the ambient PM2.5 measured in September-December, 2001. The PMF identified 5 PM2.5 source components: 1) the WTC fire's plume (S, Cl, K, Cu, Zn, Pb, EC); 2) the WTC Collapse-related Dust (Mn, Cr, EC); 3) WTC Demolition-related Dust (S, Si, Ca, Ti, Fe); 4) oil combustion particles (S, V, EC); and, 5) soil (Al, Fe). Time-series plots of the source factor impacts indicate that the WTC fire plume impact were maximum during September, diminishing greatly during October. In mid-October, the demolition operations dust increased as the rescue operations ended and the clean-up began, and then decreased greatly during November. Oil combustion was a large contributor to PM2.5 throughout the entire study period, regardless of WTC emissions. PM2.5 concentrations at this site averaged 35 ug/m3 during late September, and some 23 ug/m3 during the month of October, while during November and December levels returned to more usual NYC levels (18 and 14 ug/m3, respectively). Analyses of the source contributions indicated that the WTC-related sources (Factors 1-3) contributed some 54% of the PM2.5 pollution at this site in Lower Manhattan during September 14-30th, 29% in October, 16% in November, and only 5% in December. While the WTC pollution added greatly to the PM2.5 levels in lower Manhattan in September, it had a diminishing impact on this pollutant in the following months, and non-WTC sources of PM2.5 (such as fuel oil burning) were also major contributors to pollution in Lower Manhattan throughout the entire period. While the WTC pollution added significantly to the PM2.5 impacts in Lower Manhattan in the weeks following 9/11, any analysis of fine PM outdoor exposures seeking to assess the impacts of the WTC Disaster must also consider non-WTC PM2.5 sources. Research Supported by: The NYU-NIEHS Environmental Health Center (ES00260) and the NYU-EPA Particulate Matter Health Research Center (R827351). [ABSTRACT FROM AUTHOR]