On March 7, 2024, the Board proposed amendments to keep Illinois’ ambient air quality standards “identical in substance” to the National Ambient Air Quality Standards (NAAQS). The amendments reflect two actions taken by the United States Environmental Protection Agency (USEPA) during the second half of 2023. First, USEPA updated the current ozone absorption cross-section to the recommended consensus-based cross-section value. Second, USEPA updated its List of Designated Reference and Equivalent Method. This update modified existing method designations and designated a new Federal Equivalent Method (FEM) for measuring concentrations of fine particulate matter (PM10) in ambient air.
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