18 due to the nature of secondary PM2.5, background monitored concentrations of PM2.5 are likely to

19 be more homogeneous across the modeling domain in most cases, compared to other pollutants,

20 although this will also depend on the potential for local sources of primary PM2.5 to be

21 contributing to the monitored concentrations.

16 Please note in the case of an existing source seeking a permit for modification, there is potential overlap across

secondary contributions from monitored background and from precursor emission from the existing source. In such

cases, recommendations for excluding monitored values when the source in question is impacting the monitor in

Section 8.2.2.b may need to be modified to avoid overcompensating in cases where the monitored concentrations are

also intended to account for the existing project source’s contributions to secondary PM2.5.

Depending on the nature of local PM2.5 levels within 1 the modeling domain, it may be

2 appropriate to account for seasonal variations in background PM2.5 levels which may not be

3 correlated with seasonal patterns of the modeled primary PM2.5 levels. For example, maximum

4 modeled primary PM2.5 impacts associated with fugitive or other low-level emission sources are

5 likely to occur during winter months due to longer periods of stable atmospheric conditions,

6 whereas maximum ambient levels of secondary PM2.5 in the eastern United States typically

7 occur during spring and summer months due to high levels of sulfates. The use of temporally8

varying background concentrations in a cumulative impact analysis is discussed in more detail in

9 Section IV.3.


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