What are the differences between the summer and winter models and what is their justification?

The equations, extrapolations, and normal-to-high emitter ratios remain the same when one switches from the summer version of the Complex Model to the winter version. However, four changes do take place. First, the baseline fuel is changed from the statutory summer values to the winter values. Second, the baseline emissions are changed from summer to winter values.

The baseline emissions were derived from the MOBILE model with scenarios representing typical summer and winter conditions, and using the RVPs associated with the summer and winter baseline fuels. Third, the difference in non-exhaust emissions between the baseline fuel and the candidate fuel is given a value of zero under the winter complex model.

As explained in Section IV.E of the Regulatory Impact Analysis for the RFG program, EPA determined that vapor generation rates under summer conditions are substantially higher than under winter conditions, and that the mechanisms involved in non-exhaust emission production are highly temperature dependent. Since the data on which the Complex Model was based was collected entirely under summer conditions, it was deemed unfit to represent non-exhaust winter emissions.

Last, under the winter complex model the RVP for both the baseline fuel and target (candidate) fuels must be set to 8.7 psi when calculating emission performances with the Complex Model. (See § 80.45(c)(2), for example.) This last condition is designed to remove the effect of RVP on exhaust emissions, again since RVP effects are highly temperature dependent, and all the data on which the Complex Model was based was collected under summer conditions. See 59 FR 7716, 7731 (February 16, 1994) (11/10/97)

This question and answer was posted at Consolidated List of Reformulated Gasoline and Anti-Dumping Questions and Answers: July 1, 1994 through November 10, 1997 (pdf) (333 pp, 18.17 MB, EPA420-R-03-009, July 2003)