On October 1~(st), 2015, EPA announced that the 8-hour ozone NAAQS was being reduced from 75 ppb to 70 ppb. One particularly onerous development with regard to ozone impacts is the recent revision to Appendix W, finalized on December 20th, 2016, that requires major sources applying for a PSD permit to assess the impacts on ozone formation. EPA now contends that advances in photochemical models such as CAMx and CMAQ, as well as the widespread availability of more powerful computers, have made it feasible to assess the impact of individual sources on the formation of these secondary pollutants. This is a significant change in approach to these pollutants that have long been thought of as regional and handled by addressing the sum of regional and upwind emissions rather than studying the potential contribution of a single facility. EPA has incorporated a two-tiered approach for assessing the impacts of ozone from a project: 1. Tier 1 involves using known relationships between precursor emissions and a source's impacts to qualitatively assess the impact on ozone formation. 2. Tier 2 requires a more detailed analysis and could involve application of a photochemical grid model to determine the impacts. As one possible Tier 1 approach, EPA has published initial guidance to establish both a Significant Impact Level (SIL) for ozone and Model Emission Rates for Precursors (MERPs). A MERP would represent a level of emissions of precursors that is not expected to contribute significantly to concentrations of ozone or secondarily-formed PM_(2.5). Impacts in excess of the ozone MERPs would likely require a Tier 2 analysis that would start with an analysis against the SIL and could result in potentially time consuming and expensive photochemical modeling.
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