Problem Statement

Some diesel retrofit technologies greatly lower the mass concentrations of PM2.5 (fine particulate matter ≤2.5 microns (µm) in diameter) in diesel exhaust. They may, however, increase ultrafine particle (UFP; liquid or solid particles <0.1 µm in diameter) number concentrations under some conditions along roadways and other sites where diesel engines operate. This is a public health concern because UFP in high number (count) concentrations may have adverse health effects. At the same time, efforts to reduce mass-based PM2.5 are needed because of the known health effects that are significantly associated with PM2.5 mass concentrations. Along roadways, vehicles powered by diesel engines produce PM2.5 that includes a UFP fraction.1 While arising from the same diesel engine sources, potential exposures to PM2.5 and UFPs differ with location. UFP counts are highest at and near roadways, and drop off within several hundred meters at a much more rapid rate than PM2.5 mass levels (Levy et al., 2003; Zhang et al., 2004). Because of this rapid drop off, on-road commuters can have their greatest exposure to UFPs during commute times, even if the time spent in traffic is a relatively small part of the day. Those living, working, or otherwise spending time near major roadways or other sources of diesel emissions will also be exposed to higher concentrations of UFPs than th