U.S. EPA Finalizes Key PM2.5 New Source Permitting Concentration Limits

U.S. EPA has finalized ambient PM_2.5concentration limits that apply to projects undergoing Prevention of Significant Deterioration (PSD) permitting. The following thresholds have now been established for Class II areas (i.e., any area that is not considered a national park or other protected area): 

• PSD Increment: the allowable incremental increase in ambient concentrations from a selected baseline date that may be caused by a project. The PSD increments were finalized at 4 and 9 µg/m³ on an annual and 24-hour basis, respectively. 

• Significant Impact Levels (SILs): screening ambient concentrations that are used to determine whether a full National Ambient Air Quality Standard (NAAQS) evaluation is required as part of a permit application. The SILs are finalized at 0.3 and 1.2 µg/m3 on an annual and 24-hour basis, respectively. 

• Significant Monitoring Concentration (SMC): an ambient concentration resulting from an emissions increase that, if exceeded, would result in the requirement to perform pre-construction ambient monitoring for PM_2.5. The SMC is finalized at 4 µg/m³ on a 24-hour basis (there is no annual SMC). 

The final ambient thresholds are extremely stringent and will make it difficult to avoid conducting a full NAAQS assessment and performing ambient monitoring for any project that triggers PSD requirements for PM_2.5. Similar to the recently promulgated NAAQS for NO₂ and SO₂, these PM_2.5 ambient concentration thresholds underscore the importance of evaluating ambient requirements early in the planning phases of any air quality permitting project.