October 25, 2007 : EPA is extending the comment period for the Emissions Factor Uncertainty Assessment. Comments on the technical analysis contained in the Assessment are welcome and were accepted until November 16, 2007. Contact Barrett Parker by email at parker.barrett@epa.gov or by telephone, 919.541.5635, with specific questions or comments.

Since the posting of the Draft Emissions Factor Uncertainty Assessment for comment on April 24, 2007, we have received numerous inquiries from interested stakeholders questioning whether the Agency intends to include the effect of uncertainty on a source's applicable emission-related requirements, or whether states or local authorities need to apply the effect of uncertainty to determine area compliance with NAAQS standards. As originally stated in the Draft Assessment, we do not attempt to evaluate or provide guidance on the application of emissions factor uncertainty in making environmental decisions. Moreover, any such decisions would be made only after a formal notice and comment rulemaking process. However, EPA did not intend any change in current practices that would warrant rulemaking. Rather, our intent is to educate sources and regulators about the accuracy of emission factors and to improve such accuracy through the incorporation of the results of direct emissions testing into the estimation of future emission factors.

The peer-reviewed Assessment describes the statistical analyses used to characterize the uncertainties for categories of emissions factors and provides a procedure to calculate an estimate of the uncertainty associated with emissions factor use. As discussed in the Assessment, uncertainty is dependent on the kind of emissions released, the number of tests used to determine the emissions factor, the appropriate decision level (or percentile) within the distribution range, and the number of similar emissions units within a specific area.

The draft report and appendices below are available in PDF.

Emissions Factor Uncertainty Assessment (PDF) - Review Draft February 2007(108 pp, 1 M)

Appendix A Statistical Approach (PDF)(11 pp, 65 K)
Appendix B Emissions Factor Data Set Records (PDF)(74 pp, 175 K)
Appendix C Emission Factor Data Plots (PDF)(56 pp, 315 K)
Appendix D Emissions Factor Uncertainty Ratios (PDF)(56 pp, 125 K)
Appendix E Normalized Monte Carlo Sampling Distributions of the Mean (PDF)(56 pp, 135 K)
Appendix F Examination of Alternative Procedures (PDF)(15 pp, 40 K)
Appendix G Emission Factor Uncertainty Ratios Based on the Difference of Means (PDF)(10 pp, 30 K)
Appendix H Relative Accuracy Assessment (PDF)(12 pp, 45 K)