Because stationary sources of fine particles are also
declining at the same time and the overall change from year
to year is modest, those interested in near roadway trends
might want to consider targeting tracking programs to areas
with the best “signal to noise” for such sources. A number of
areas are implementing or planning interventions that reduce
existing diesel emissions faster than the national new source
regulations. They involve school bus and fleet retrofits and
programs to address existing marine emissions. Examples
include the West Coast Diesel Emissions Reductions
Collaborative, the Midwest Clean Diesel Initiative, and
the Rocky Mountain Clean Diesel Collaborative (EPA
2008b). Areas affected by these programs should have
faster than average improvements in diesel PM and related
gas emissions. The patterns of emissions and reductions
will vary in port cities, such as Seattle, as compared to
areas with programs that address only school buses and
fleets. It may be easier to detect the effect of heavy-duty
diesel emissions in such areas, and a comparison of areas
with and without interventions, as well as comparing
eastern (with additional regional PM reductions) to western
areas. From a within-city point of view, there may be
advantages for a cross-sectional study in a port city with
major near road and near port gradients in exposures to
marine and terrestrial emissions.