June 2009
Front Matter
Training
30
Air Toxics Emissions
Source Type Characteristics
•Understanding the emission source type of a particular air toxic can help the analyst begin to develop a conceptual model of concentration patterns and gradients that might be expected. 
•Major source emissions, for example, are a localized source of toxics and may show steep concentration gradients.
•Area source emissions are typically well-distributed emissions sources because there are 
multiple sources in an area.
•Mobile source air toxics exhibit 
both point source and area 
source characteristics.
Steep concentration gradients of primarily emitted toxics around point sources are typical, especially if there are no other nearby sources of the pollutants.
Area source emissions can lead to relatively homogeneous concentrations of toxics on the urban scale.   However, if a monitor is placed close to any source type, gradients may be observed.
Very close to a roadway or near a construction site, mobile source air toxics may be seen in higher concentrations.  A few hundred meters
away from the roadway, for example, concentrations typically fall to more normal levels.