21 document what the background monitoring data represent to the extent possible, including any

22 information that may be available from the state or other agency responsible for siting and

Public Review Draft 03/04/2013


maintaining the monitor.16 It is also worth noting that the relative 1 makeup of PM2.5 components

2 and temporal patterns associated with the highest 24-hour PM2.5 levels may differ considerably

3 from the relative amounts of PM2.5 components associated with annual average PM2.5 levels,

4 especially in western states.

5 The determination of representative background monitored concentrations of PM2.5 to

6 include in the PM2.5 cumulative impact assessment may entail different considerations from

7 those for other criteria pollutants and may also depend on whether the application involves full

8 quantitative photochemical grid modeling. An important aspect of the monitored background

9 concentration for PM2.5 is that the monitored data should, in most cases account for the

10 contribution of secondary PM2.5 formation associated with existing sources impacting the

11 modeling domain in addition to the background levels of primary PM2.5 associated with

12 background sources that are not included in the modeled inventory. As with other criteria

13 pollutants, consideration should also be given to the potential for some double-counting of the

14 impacts from modeled emissions that may be contributing to the background monitored

15 concentrations, but this should generally be of less importance for PM2.5 than the

16 representativeness of the monitor for secondary contributions, unless the monitor is located

17 relatively close to nearby sources of primary PM2.5 that could be impacting the monitor. Also,


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