Mass Fraction Spatiotemporal Geostatistics
and its Application to Map Atmospheric Polycyclic Aromatic Hydrocarbons after 9/11


Ben Allshouse, Joachim Pleil, Stephen Rappaport, Marc Serre*

University of North Carolina at Chapel Hill, NC 27599-7431, USA

* Corresponding author: marc_serre(at)


The tragic destruction of the World Trade Centre towers on 9/11 and the ensuing fires lead to thick black smoke containing particle-bound polycyclic aromatic hydrocarbons (PAHs). PAHs are suspected to be carcinogenic and may lead to low birth weight for infants of women exposed to these compounds. PAHs were measured in archived PM2.5 samples at four locations around Ground Zero from September 22, 2001 until March 27, 2002. The EPA AIRS monitoring system provided additional PM2.5 data at sites located across the New York City area. Using the PM2.5 data, and the small subset of archived PM2.5 samples that were re-analysed for PAH, we developed a non-parametric, probabilistic model predicting the PAH mass fraction in the neighbourhood of the WTC. Bayesian Maximum Entropy data integration of the few directly measured PAH data with the more numerous PM2.5 data provided maps showing the distribution of PAH across New York City at any day of interest after 9/11. These concentration maps are used to construct the time integrated exposure and population burden maps. The time integrated exposure is calculated as the mass of PAH toxicologically equivalent to benzo(a)pyrene that one would have inhaled at some location s for the duration between 9/11 until any given time of interest. The population burden is then obtained by multiplying the exposure with population density. For a given PAH, the time integrated individual-level exposure up to time T can be calculated at location s as:


Exposure (ng) = *DAI*TEF (1)


where (ng*day/m3) is the time integrated PAH concentration at location s from 9/11 until the time T of interest, DAI is the daily air intake (assumed to be 11m3/day for a person at rest), and TEF (unitless) is the toxicity equivalency factor relative to benzo(a)pyrene. The time integrated PAH population burden up to time T is then simply calculated as


Population Burden (ng*persons/mi2) = Exposure *PD (2)


where PD (persons/mi2) is the population density. Movies of the maps obtained for benzo(a)pyrene are shown below.



Time integrated benzo(a)pyrene exposure (ng inhaled) after 9/11 (Eq. 1)

New York Area

Lower Manhattan

Time integrated benzo(a)pyrene population burden (ng*persons/mi2) after 9/11 (Eq. 2)

New York Area

Lower Manhattan