The current NDEQ air dispersion modeling guidance for both major and minor sources (as revised by NDEQ in December 2017) states that modeling is required when the net emission increase equals or exceeds the Significant Emission Rate (SER). Thus, both PSD and minor sources must compare the net emissions increase for a project to the SERs to evaluate whether modeling is triggered. The guidance implies that the net emissions increase should be calculated by determining the difference in Baseline Actual Emissions (BAEs) and Projected Actual Emissions (PAEs)/Potential to Emit (PTE), in line with PSD regulations found in Title 129 Chapter 19, Sections 005 through 008. It is not clear whether this calculation method which applies for new major sources or modification to existing major sources should also be applied to minor sources.
For both major and minor sources, NDEQ stated that facilities have found it difficult to determine BAEs. NDEQ has also had challenges with confirming the calculated BAEs. Thus, in recent communication, NDEQ stated that sources can use the pre-project PTE to represent BAEs. However, per PSD regulatory guidance, a net emissions increase should be based on actual emissions for major source projects. Thus, to be in line with the Title 129 regulations, major source projects should calculate a net emissions increase using actual emissions, while minor source projects should use the change in potential emissions. NDEQ stated that they are in the process of installing a new emissions inventory software that would make it easier to determine BAEs and thus more practical to use in determining the net emissions increase. The software is expected to be functional in early 2019. NDEQ is also reviewing their current modeling guidance to determine if clarity can be added on calculation approaches.