58.30—Special considerations for data comparisons to the NAAQS.
(a)
Comparability of PM
2.5
data. (1) There are two forms of the PM2.5 NAAQS described in part 50 of this chapter. The PM2.5 monitoring site characteristics (see appendix D to this part, section 4.7.1) impact how the resulting PM2.5 data can be compared to the annual PM2.5 NAAQS form. PM2.5 data that are representative, not of areawide but rather, of relatively unique population-oriented microscale, or localized hot spot, or unique population-oriented middle-scale impact sites are only eligible for comparison to the 24-hour PM2.5 NAAQS. For example, if the PM2.5 monitoring site is adjacent to a unique dominating local PM2.5 source or can be shown to have average 24-hour concentrations representative of a smaller than neighborhood spatial scale, then data from a monitor at the site would only be eligible for comparison to the 24-hour PM2.5 NAAQS.
(2)
There are cases where certain population-oriented microscale or middle scale PM2.5 monitoring sites are determined by the Regional Administrator to collectively identify a larger region of localized high ambient PM2.5 concentrations. In those cases, data from these population-oriented sites would be eligible for comparison to the annual PM2.5 NAAQS.