Table 1. GIS-based source density indicators used for LUR modeling.

Source category	Covariates examined (50–1000 m concentric radial buffers)	Data source
Traffic density indicators	Mean density traffic (primary roads) Mean density traffic (primary and secondary roads) Number of signaled intersections	Pennsylvania Department of Transportation (PADOT)
Road-specific measures	Average daily traffic on nearest primary road Distance to nearest major road Summed length of primary roadways Summed length of primary and secondary roadways	PADOT
Truck, bus, and diesel	Mean density of bus traffic Distance to nearest bus route Outbound and inbound trip frequency per week summed by route Mean density of heavy truck traffic on nearest primary roadway	Google Transit (11/11 −3/12) PADOT
Population	Census population density (blockgroup)	US Census Bureau (2010)
Land use/built environment	Total area of industrial parcels Total area of commercial parcels Total area of industrial and commercial parcels Percent developed imperviousness Land use/land cover (LULC) urban built-up total area from orthophotography	Allegheny County Assessment Data, by parcel (2011) National Land Cover Dataset (NLCD, 2006) Southwestern Pennsylvania Commission (SPC, 2011)
Industrial emissions	Mean density of total PM2.5 emitted per meter Mean density of total SO2 emitted per meter Mean density of total NOx emitted per meter Mean density of total VOCs emitted per meter	National Emissions Inventory (NEI, 2011)
Transportation facilities	Distance to nearest active railroad Summed line length of active railroads Distance to nearest bus depot	SPC, 2011
Potential modifying factors
Topography	Average elevation	National Elevation Dataset (NED, 2011)
Meteorology	Temperature/relative humidity Frequency of inversions Wind direction and wind speed	Obtained from sampler University of Wyoming, Department of Atmospheric Science (2011–2012) National Oceanic and Atmospheric Association (NOAA, 2011–2012)