On December 4, 2017, the Wisconsin Department of Natural Resources (DNR) amended its National Ambient Air Quality Standards (NAAQS) for fine particulate matter (PM_2.5). The rule reduces the primary annual standard for PM_2.5 from 15.0 to 12.0 micrograms per cubic meter (µg/m³). DNR was required to promulgate this rule to be consistent with the U.S. EPA NAAQS for PM_2.5, published in January, 2013. The DNR rule is scheduled to be effective January 1, 2018, and will be submitted to the EPA as a revision to the Wisconsin state implementation plan.

Sources of direct PM_2.5 emissions include fossil fuel combustion, vegetation burning, agricultural drying operations such as large grain storage facilities, and the smelting or processing of metals. However, PM_2.5 is most commonly created by atmospheric reactions of other pollutants including sulfur dioxide (SO₂), nitrogen oxides (NOx), and certain organic compounds. Fine particles, including PM_2.5, present a human health concern because they have long atmospheric lifetimes and can penetrate deep into the lungs. Fortunately, the entire state of Wisconsin is below the revised PM2.5 standard. In future air permitting, however, DNR will be required to determine that the permit application source will not cause the new standard to be violated.

What is the practical effect of this new annual PM2.5 emissions standard?

The new standard will affect the air quality assessment portion of the permit review process for Wisconsin facilities seeking a major construction permit under the federal Prevention of Significant Deterioration (PSD) program. This new standard will also impact facilities which are obtaining a minor source construction permit, obtaining an operating permit for the first time, or renewing a Title V operating permit.

In February 2016, DNR issued a technical support document (TSD) on PM_2.5 that examines the science behind the formation of PM_2.5, its distribution and lifetime in the atmosphere, statewide monitoring results of PM_2.5, and the appropriateness of using refined air dispersion models to estimate concentrations of PM_2.5 in the ambient air. View the TSD here. The TSD provides guidance on how and when PM_2.5 emissions are estimated in permit applications, including when to expect PM_2.5 emission limits in permits. The TSD implies that PM_2.5 emissions may be addressed primarily by regulation of SO₂ and NO_X emissions. Impacted sources seeking permits should consult the TSD in preparing permit applications.