Development of a RIOPA database and evaluation of the effect of proximity on the potential residential exposure to VOCs from ambient sources

Ambient volatile organic compound concentrations around residences were measured in Elizabeth, NJ during the Relationship among Indoor, Outdoor, and Personal Air (RIOPA) study to assess the influence of proximity of the residences to known ambient emissions sources. National emission inventory data for the year 1999 were used to identify the potential sources on or near the study area. The closest distances between the sampler locations and mobile and point sources were determined using Geographic Information Systems (GIS) techniques after inputting the exact longitude and latitude measured at the sampler location of each home, and after correction of the location of the gas stations and the 2000 TIGER/Line (Topologically Integrated Geographic Encoding and Referencing system) data to follow the exact centerline of roadways based on the 1997 digital orthoquarter quadrangles available from New Jersey Department of Environmental Protection (NJ DEP). Meteorological data for Newark International Airport for individual sampling days from NOAA/NCDC (National Oceanic and Atmospheric Administration/National Climatic Data Center) and proximity to emission sources data were integrated into the RIOPA database. Distributional properties of and correlations among the variables were examined prior to conducting multiple linear regression analyses of the concentration of aromatic hydrocarbons, Methyl tert Butyl Ether (MTBE) and tetrachloroethylene (PCE) to verify that model assumptions were not violated. The best-fitting multiple regression models were determined for the selected VOCs and validated through post diagnosis on equal variances and multicollinearity. The ambient concentrations of the aromatic hydrocarbons (BTEX) were inversely associated with distances from the sampler to major roadways with high traffic densities and gasoline stations, atmospheric stability, temperature, and wind speed. Ambient MTBE was more strongly associated with inverse distance to gas stations compared to BTEX, while MTBE was also inversely associated with interstate highways. Residential ambient PCE concentration was inversely associated with distance to dry cleaning facilities, atmospheric stability, temperature, wind speed and relative humidity. The relative increase in air concentration with distance to sources was estimated from the mathematical model for each compound. The data support 1.5∼4 times higher air concentration above urban background levels around homes very close (<200 meters) to ambient sources. Proximity and the meteorological variables explained 16 to 45% of overall variations of ambient VOC concentrations near homes in Elizabeth, NJ. Meteorological conditions were more important, explaining 60 to 90% of the model predicted variations of the model-predicted VOC concentrations than proximity variables.