There are presently 47 areas that violate
the current PM-10 standard. The form
of the PM-2.5 standard requires that the 3-year average (rounded
to the nearest 0.1 ug/m3) of the annual means from single monitors
or the average of multiple monitors must be at or below the level
of the annual standard and the 3-year average (rounded to the
nearest 1 ug/m3) of the ninety-eighth percentile values at each
monitor cannot exceed the level of the daily standard. In determining
attainment of the annual average standard, an area may choose
to use either the spatially averaged concentrations across all
population-oriented monitors or it may use the highest 3-year
average based on individual monitors. The annual standard is
15 ug/m3. The 24-hour PM-2.5 standard announced in 1997 was 65
ug/m3. Based on estimated PM-2.5 data for the period 1993-1995,
the percentile-based 24-h standard had approximately 41 counties
that violated the standard compared to the 228 counties that
violated the spatially averaged annual PM-2.5 standard. Thus,
based on the 1997 PM-2.5 standards, the 15 ug/m3 annual average
PM-2.5 standard will determine many of the violating areas in
the United States. On
September 21, 2006, EPA announced that it revised the level of
the 24-hour PM-2.5 standard to 35 micrograms per cubic meter
(µg/m3) and retained the level of the annual PM-2.5 standard
at 15 µg/m3. The 24-hour PM-2.5 standard will determine
many of the violating areas in the United States versus the annual
PM-2.5 standard. Investigate the violation
areas that A.S.L. & Associates has identified for
the spatially averaged annual PM-2.5
15 ug/m3 standard.
