in 40CFR Part 50, Appendix N
In some cases, two nearby sites can be spatially averaged to determine the annual average design
value for both sites. For details on spatial averaging requirements see 40 CFR Part 58, Appendix
D, Section 2.8.
24-Hour Average PM2.5
The 24-hr NAAQS for PM2.5 is met if the 98th percentile 24-hour average concentration of
particulate matter with aerodynamic diameter < 2.5 micrometers, averaged over three
consecutive years, is < 65:g/m3.14 The test applies at all monitoring sites--spatial averaging is
not allowed for the 24-hour NAAQS. The “98th percentile” concentration (and, thus, the design
value concentration) depends on the number of days on which PM2.5 is monitored during a year.
See 40CFR Part 50, Appendix N for details. Attainment is reached if the design value at the site
is < 65:g/m3.
How is a “site-specific” baseline design value ((DVB)I) calculated?
The baseline measured concentrations at each monitoring site is the anchor point for
future year projected concentrations. The baseline design values are projected to the future using
RRFs. In practice, the choice of the baseline design value can be critical to the determination of
the estimated future year design values. Therefore, careful consideration should be given to the
calculation of baseline values. The baseline design values should have the following attributes:
1) Should be consistent with the form of the applicable NAAQS.
2) Should be easy to calculate.
3) Should represent the baseline inventory year.
4) Should take into account the year-to-year variability of meteorology.
5) Should take into account the year-to-year variability of emissions.
Several possible methodologies to calculate baseline design values are:
1) The designation design value period (i.e. 2001-2003).
2) The design value period that straddles the baseline inventory year (e.g., the 2001-2003 design
value period for a 2002 baseline inventory year).
3) The highest (of the three) design value periods which include the baseline inventory year (e.g.
the 2000-2002, 2001-2003, and 2002-2004 design value periods for a 2002 baseline inventory
4) The average (of the three) design value periods which straddle the baseline inventory year.
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