The Phase 2 Volatility Standards (55 FR 23658 (June 11, 1990)) provided the basis for the different RVP standards, depending on VOC Control Region, for reformulated gasoline under the Simple Model. The standards for VOC emissions performance for Phase I RFG under the Complex Model were based on the Simple Model standards, which were translated into equivalent VOC emission performance standards under the Complex Model.
The Phase II RFG standards for VOC emissions performance, however, were derived using the Complex Model, which takes into account RVP and several other factors in determining VOC emissions performance. Since the Complex Model already evaluates the effect of RVP on VOC emissions performance, there was no need to make any additional RVP distinction between the two regions. (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, About PDF)