The Screen model only produces 1-hour average results (with the exception of 24-hour estimates for complex terrain impacts), however, scaling factors can be used to estimate other averaging periods.
To obtain worst-case impacts for longer averaging periods, such as 3-hour, 8-hour, 24-hour, or annual, multiply the 1-hour maximum concentration from SCREEN3 by the appropriate scaling factor.
For example, for an annual averaging period, you multiply by a scaling factor of 0.08 (+/- 0.02) following the EPA document "Screening Procedures for Estimating the Air Quality Impact of Stationary Sources, Revised" (EPA-454/R-92-019, October 1992).
The suggested scaling factors are as follows:
Averaging Period --> Scaling Factor
3-hour --> 0.9 (+/- 0.1)
8-hour --> 0.7 (+/- 0.2)
24-hour --> 0.4 (+/- 0.2)
Annual --> 0.08 (+/- 0.02)
Note: These factors are designed to be a rough guide and are based on elevated point sources