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The 1997 PM-2.5 Standard

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.

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