Figure 4 (EPA 2008a) shows recent and projected
improvements in PM and ozone air quality resulting
from the Clean Air Rules and other baseline emissions
reductions programs. Recent significant improvements
observed in these pollutants between 1999 and 2001
and 2003–2005 are primarily due to the acid rain
program, NOx SIP Call, and mobile source programs
implemented during this period. The pollutant emission
reductions forecast by 2020 from these programs would
result in still fewer projected non-attainment areas for
ozone and PM2.5
. Ozone and PM2.5
exceedances are
projected to continue in Southern and Central California.
Ozone is a persistent issue for the Northeast corridor and
Houston area, while PM2.5
issues will continue in
midwestern cities such as Chicago, IL; Detroit, MI; and
Cleveland, OH; as well as Birmingham, AL. These maps
overstate non-attainment to the extent that local or subregional programs are adopted to attain the standards. On
the other hand, these figures do not include the 2006
PM2.5 standard or the revised 8-h ozone standard of
0.75 ppm. Figure 5 (EPA 2006, 2007b) shows areas with
monitors not meeting the most recently revised O3 and
PM2.5 NAAQS.
Some implications for tracking and accountability
Recognizing their inherent limitations, these national level
forecasts can provide some indication of the extent of potential
emissions and air quality improvements in particular areas
expected over the next decade. This might provide some guidance to those seeking to track areas with the most
significant reductions. In particular, those areas forecast to
have continued non-attainment for ozone and PM in 2020 are
those with a responsibility to develop additional control
programs that attain the relevant standards. More area-specific
forecasts will be produced by areas as they develop their
control strategies and implementation plans.
These forecasts can be broken according to specific source
categories and particular areas. An examination of the
historical trends and the time course of the emissions
reductions indicate we are currently in a period with the
highest rate of anticipated emissions reductions. Figure 6
(Sommers 2007; EPA 2007a) shows forecasted national
changes in mobile source emissions from on- and off-road
vehicles for direct PM and NOx
. The later year projections
are more uncertain, but it appears that tracking programs over
the next several years have a better chance for detecting
trends.
Trends, timing, and relative importance of local sources
will, of course, vary with location. In addition, uncertainties in these emissions inventories may be significant, especially for mobile source PM emissions. The relative change
forecast for mobile emissions is large for both particles and
gases. Given increasing evidence of increased health risk
with proximity to traffic, it would be of some interest to
examine the feasibility of detecting trends in areas with greater
than average reductions forecast for direct PM and pollutant
gases. The national forecasts suggest that such tracking
programs should begin soon, if they are not already underway,
as we are already moving into a period of maximum change
for both on and off-road sources. The absolute PM reductions
forecast appear small but recent comparisons of emissions
data with air quality data suggest the contribution of direct
mobile source PM is substantially larger than implied by the
emissions data (EPA 2006). While overall mass will decline,
the trends for near-roadway ultrafine particles are less clear.
Reductions in fine particle mass can increase ultrafine
particles, but some technologies reduce both and the
reduction in roadway SO2 and NOx will also affect
secondary ultrafine particle production.