On October 20, 2010, U.S. EPA published final rules for controlling air emissions of particulates less then 2.5 microns in diameter (PM 2.5) in areas that meet federal air quality standards. The new rules will be implemented through the Prevention of Significant Deterioration (PSD) new source review program. Any source of PM emissions proposing to locate or expand in an attainment area within the next 12 months, or soon thereafter, should determine the potential impact of the new PM 2.5 rules right away.

The PSD program prevents the significant deterioration of existing air quality by requiring pre-construction review. The mechanism PSD uses to estimate significant deterioration is the “increment,” which is the maximum allowable increase in ambient concentration of a pollutant in an area. Increases above that level are presumed to cause significant deterioration. A source that will cause an increase in ambient concentrations greater than the increment must prevent that impact by installing controls or accepting limitations on its operations. Otherwise, the proposed construction will not be allowed.

Previously, there was no federal increment for PM 2.5. The increment for PM10 was used instead. The new rule sets an annual PM 2.5 increment of 15 micrograms per cubic meter (µg/m³) and a 24-hour PM 2.5 increment of 35 µg/m³. In other words, a new or expanded source of PM 2.5 may not cause an increase in the annual or daily average air concentrations of PM 2.5 greater than these levels.

The new rules also set Significant Impact Levels (SILs) for PM 2.5. SILs are screening tools used by permitting authorities to determine whether a proposed source’s emissions will have a “significant” impact on air quality. If a proposed project’s impacts for a pollutant will be less than the corresponding SIL, the impact is de minimis. The source will not be required to perform complicated and expensive computer modeling of air quality impacts from all existing sources in the area. However, if the SIL will be exceeded, the source must use computer modeling to demonstrate it will not cause or contribute to violations of the federal air quality standard or increment.

The new PSD rules for PM 2.5 create another new screening tool for fine particulates, the Significant Monitoring Concentration or SMC. The SMC is used to determine whether a proposed source or project must perform 12 months of air monitoring before in can commence construction. If the projected impact is less than the SMC, the impact is considered de minimis for monitoring purposes. The new SMC for PM 2.5 is 4 µg/m³.

For PSD permits issued by EPA, the new PM 2.5 SIL and SMC become effective on December 20, 2010. EPA will begin using the new increments and screening tools in reviewing PSD permit applications after that date. The SILs are optional for State administered PSD programs, but this merely means that in states electing not to adopt the SIL and SMC it will not be possible to use the screening tools to escape extensive modeling or air monitoring.

What does all this mean as a practical matter? Previously, the PSD program included no PM 2.5 increments, SILs or SMCs. Sources were allowed to use PM10 as a surrogate. Now, sources of PM 2.5 proposing to locate in areas that meet federal air quality standards for fine particulate must either perform their modeling and monitoring for PM 2.5 as well as PM 10, or keep their PM 2.5 emissions and impacts below the newly established levels.

In other words, life has again become more complicated for industries and combustion sources. Since states that run their own PSD programs will need to determine how to implement the new Federal rules, affected sources must become familiar with and participate in their states’ PM 2.5 rulemaking efforts.

Affected sources must determine how to measure and predict PM 2.5 emissions. This task has proven to be complicated indeed, since certain materials such as NOx and SOx are considered precursors to PM 2.5 formation. The need to consider such “condensable” particulates will result in more stringent regulatory determinations.

All potentially affected companies planning expansions need to determine their PM 2.5 impacts under the new rules. The outcome of such reviews could include decisions to locate new projects in areas where the impacts will be less than PM 2.5 increments, SILs and SMC. In some cases, new projects in areas significantly affected by proposed new PM 2.5 emissions may not be allowed at all.