The Variability of 24-Hour Average Background PM
Estimating background PM concentrations is important for the EPA’s health risk analyses.
It is important for the risk assessment to use the quantified 24-hour background PM levels at clean sites.
Without an adequate characterization in Chapter 3 of the clean western sites, there will be insufficient information in the CD from which the Staff Paper can draw for the Agency’s risk assessment.
Chapter 3 states that peak 24-hour average natural background concentrations may be substantially higher than the annual or seasonal average natural background concentrations.