The graph shows the significant pollution reduction of new technologies and new emission standards. Incredibly, in its argument before the Minnesota Public Utility Commission, NSP defies this logic. However, even if no net increase in pollution occurs, this scenario creates a tradeoff of pollution between one geographical region and another. The Twin Cities Metro area would experience a net increase in emissions while the Chicago area would experience a net decrease. Such a tradeoff is not compatible with Minnesotans' interest in clean air.
Several factors can explain the cause of such variation.
First, the capacity factor strongly influences plant emissions. Simply put, the more often a plant runs, the more pollution it will emit. Second, the method used to calculate emissions can cause wide variation, especially if the same method is not used annually. According to the Minnesota Pollution Control Agency, the only significant change in control equipment on metro plants was an improvement at Black Dog during 1995. Despite these explanations, there remain serious unexplained anomalies in the data record, especially as it relates to particulates less than 10 microns in size.