A large degree of 24-hour PM variability exists.
In its June 2001 draft, the EPA Staff Paper stated that for base case risk estimates, the Agency would select the midpoint of the appropriate ranges of annual average estimates for PM background levels presented in the PM Criteria Document.
The use of an annual average smoothes the episodic, natural PM events, such that it is impossible to take into consideration these events when accumulating the daily PM values in a risk analyses.
The variability of 24-hour average PM for clean western PM sites should be presented in Chapter 3. It is not appropriate that the Staff Paper use annual average estimates for PM background levels.